dma-buf: Update cpu access documentation
- Again move the information relevant for driver writers next to the callbacks. - Put the overview and userspace interface documentation into a DOC: section within the code. - Remove the text that mmap needs to be coherent - since the DMA_BUF_IOCTL_SYNC landed that's no longer the case. But keep the text that for pte zapping exporters need to adjust the address space. - Add a FIXME that kmap and the new begin/end stuff used by the SYNC ioctl don't really mix correctly. That's something I just realized while doing this doc rework. - Augment function and structure docs like usual. Cc: linux-doc@vger.kernel.org Cc: Jonathan Corbet <corbet@lwn.net> Cc: Sumit Semwal <sumit.semwal@linaro.org> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org> [sumits: fix cosmetic issues] Link: http://patchwork.freedesktop.org/patch/msgid/20161209185309.1682-5-daniel.vetter@ffwll.ch
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@ -6,205 +6,6 @@
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<sumit dot semwal at ti dot com>
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Kernel cpu access to a dma-buf buffer object
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--------------------------------------------
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The motivation to allow cpu access from the kernel to a dma-buf object from the
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importers side are:
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- fallback operations, e.g. if the devices is connected to a usb bus and the
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kernel needs to shuffle the data around first before sending it away.
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- full transparency for existing users on the importer side, i.e. userspace
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should not notice the difference between a normal object from that subsystem
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and an imported one backed by a dma-buf. This is really important for drm
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opengl drivers that expect to still use all the existing upload/download
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paths.
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Access to a dma_buf from the kernel context involves three steps:
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1. Prepare access, which invalidate any necessary caches and make the object
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available for cpu access.
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2. Access the object page-by-page with the dma_buf map apis
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3. Finish access, which will flush any necessary cpu caches and free reserved
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resources.
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1. Prepare access
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Before an importer can access a dma_buf object with the cpu from the kernel
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context, it needs to notify the exporter of the access that is about to
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happen.
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Interface:
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int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
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enum dma_data_direction direction)
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This allows the exporter to ensure that the memory is actually available for
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cpu access - the exporter might need to allocate or swap-in and pin the
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backing storage. The exporter also needs to ensure that cpu access is
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coherent for the access direction. The direction can be used by the exporter
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to optimize the cache flushing, i.e. access with a different direction (read
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instead of write) might return stale or even bogus data (e.g. when the
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exporter needs to copy the data to temporary storage).
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This step might fail, e.g. in oom conditions.
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2. Accessing the buffer
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To support dma_buf objects residing in highmem cpu access is page-based using
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an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
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PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
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a pointer in kernel virtual address space. Afterwards the chunk needs to be
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unmapped again. There is no limit on how often a given chunk can be mapped
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and unmapped, i.e. the importer does not need to call begin_cpu_access again
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before mapping the same chunk again.
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Interfaces:
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void *dma_buf_kmap(struct dma_buf *, unsigned long);
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void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
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There are also atomic variants of these interfaces. Like for kmap they
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facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
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the callback) is allowed to block when using these.
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Interfaces:
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void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
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void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
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For importers all the restrictions of using kmap apply, like the limited
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supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
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atomic dma_buf kmaps at the same time (in any given process context).
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dma_buf kmap calls outside of the range specified in begin_cpu_access are
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undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
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the partial chunks at the beginning and end but may return stale or bogus
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data outside of the range (in these partial chunks).
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Note that these calls need to always succeed. The exporter needs to complete
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any preparations that might fail in begin_cpu_access.
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For some cases the overhead of kmap can be too high, a vmap interface
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is introduced. This interface should be used very carefully, as vmalloc
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space is a limited resources on many architectures.
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Interfaces:
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void *dma_buf_vmap(struct dma_buf *dmabuf)
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void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
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The vmap call can fail if there is no vmap support in the exporter, or if it
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runs out of vmalloc space. Fallback to kmap should be implemented. Note that
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the dma-buf layer keeps a reference count for all vmap access and calls down
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into the exporter's vmap function only when no vmapping exists, and only
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unmaps it once. Protection against concurrent vmap/vunmap calls is provided
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by taking the dma_buf->lock mutex.
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3. Finish access
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When the importer is done accessing the CPU, it needs to announce this to
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the exporter (to facilitate cache flushing and unpinning of any pinned
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resources). The result of any dma_buf kmap calls after end_cpu_access is
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undefined.
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Interface:
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void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
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enum dma_data_direction dir);
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Direct Userspace Access/mmap Support
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------------------------------------
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Being able to mmap an export dma-buf buffer object has 2 main use-cases:
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- CPU fallback processing in a pipeline and
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- supporting existing mmap interfaces in importers.
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1. CPU fallback processing in a pipeline
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In many processing pipelines it is sometimes required that the cpu can access
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the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
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the need to handle this specially in userspace frameworks for buffer sharing
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it's ideal if the dma_buf fd itself can be used to access the backing storage
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from userspace using mmap.
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Furthermore Android's ION framework already supports this (and is otherwise
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rather similar to dma-buf from a userspace consumer side with using fds as
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handles, too). So it's beneficial to support this in a similar fashion on
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dma-buf to have a good transition path for existing Android userspace.
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No special interfaces, userspace simply calls mmap on the dma-buf fd, making
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sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
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used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
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-EAGAIN or -EINTR, in which case it must be restarted.
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Some systems might need some sort of cache coherency management e.g. when
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CPU and GPU domains are being accessed through dma-buf at the same time. To
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circumvent this problem there are begin/end coherency markers, that forward
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directly to existing dma-buf device drivers vfunc hooks. Userspace can make
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use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
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would be used like following:
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- mmap dma-buf fd
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- for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
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to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
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want (with the new data being consumed by the GPU or say scanout device)
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- munmap once you don't need the buffer any more
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For correctness and optimal performance, it is always required to use
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SYNC_START and SYNC_END before and after, respectively, when accessing the
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mapped address. Userspace cannot rely on coherent access, even when there
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are systems where it just works without calling these ioctls.
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2. Supporting existing mmap interfaces in importers
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Similar to the motivation for kernel cpu access it is again important that
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the userspace code of a given importing subsystem can use the same interfaces
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with a imported dma-buf buffer object as with a native buffer object. This is
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especially important for drm where the userspace part of contemporary OpenGL,
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X, and other drivers is huge, and reworking them to use a different way to
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mmap a buffer rather invasive.
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The assumption in the current dma-buf interfaces is that redirecting the
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initial mmap is all that's needed. A survey of some of the existing
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subsystems shows that no driver seems to do any nefarious thing like syncing
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up with outstanding asynchronous processing on the device or allocating
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special resources at fault time. So hopefully this is good enough, since
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adding interfaces to intercept pagefaults and allow pte shootdowns would
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increase the complexity quite a bit.
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Interface:
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int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
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unsigned long);
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If the importing subsystem simply provides a special-purpose mmap call to set
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up a mapping in userspace, calling do_mmap with dma_buf->file will equally
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achieve that for a dma-buf object.
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3. Implementation notes for exporters
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Because dma-buf buffers have invariant size over their lifetime, the dma-buf
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core checks whether a vma is too large and rejects such mappings. The
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exporter hence does not need to duplicate this check.
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Because existing importing subsystems might presume coherent mappings for
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userspace, the exporter needs to set up a coherent mapping. If that's not
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possible, it needs to fake coherency by manually shooting down ptes when
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leaving the cpu domain and flushing caches at fault time. Note that all the
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dma_buf files share the same anon inode, hence the exporter needs to replace
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the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
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required. This is because the kernel uses the underlying inode's address_space
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for vma tracking (and hence pte tracking at shootdown time with
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unmap_mapping_range).
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If the above shootdown dance turns out to be too expensive in certain
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scenarios, we can extend dma-buf with a more explicit cache tracking scheme
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for userspace mappings. But the current assumption is that using mmap is
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always a slower path, so some inefficiencies should be acceptable.
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Exporters that shoot down mappings (for any reasons) shall not do any
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synchronization at fault time with outstanding device operations.
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Synchronization is an orthogonal issue to sharing the backing storage of a
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buffer and hence should not be handled by dma-buf itself. This is explicitly
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mentioned here because many people seem to want something like this, but if
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different exporters handle this differently, buffer sharing can fail in
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interesting ways depending upong the exporter (if userspace starts depending
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upon this implicit synchronization).
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Other Interfaces Exposed to Userspace on the dma-buf FD
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------------------------------------------------------
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@ -240,20 +41,6 @@ Miscellaneous notes
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the exporting driver to create a dmabuf fd must provide a way to let
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userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
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- If an exporter needs to manually flush caches and hence needs to fake
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coherency for mmap support, it needs to be able to zap all the ptes pointing
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at the backing storage. Now linux mm needs a struct address_space associated
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with the struct file stored in vma->vm_file to do that with the function
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unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
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with the anon_file struct file, i.e. all dma_bufs share the same file.
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Hence exporters need to setup their own file (and address_space) association
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by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
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callback. In the specific case of a gem driver the exporter could use the
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shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
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zap ptes by unmapping the corresponding range of the struct address_space
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associated with their own file.
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References:
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[1] struct dma_buf_ops in include/linux/dma-buf.h
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[2] All interfaces mentioned above defined in include/linux/dma-buf.h
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@ -52,6 +52,12 @@ Basic Operation and Device DMA Access
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:doc: dma buf device access
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CPU Access to DMA Buffer Objects
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:doc: cpu access
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Kernel Functions and Structures Reference
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -640,6 +640,122 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
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}
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EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
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/**
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* DOC: cpu access
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*
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* There are mutliple reasons for supporting CPU access to a dma buffer object:
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*
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* - Fallback operations in the kernel, for example when a device is connected
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* over USB and the kernel needs to shuffle the data around first before
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* sending it away. Cache coherency is handled by braketing any transactions
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* with calls to dma_buf_begin_cpu_access() and dma_buf_end_cpu_access()
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* access.
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*
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* To support dma_buf objects residing in highmem cpu access is page-based
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* using an api similar to kmap. Accessing a dma_buf is done in aligned chunks
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* of PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which
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* returns a pointer in kernel virtual address space. Afterwards the chunk
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* needs to be unmapped again. There is no limit on how often a given chunk
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* can be mapped and unmapped, i.e. the importer does not need to call
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* begin_cpu_access again before mapping the same chunk again.
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*
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* Interfaces::
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* void \*dma_buf_kmap(struct dma_buf \*, unsigned long);
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* void dma_buf_kunmap(struct dma_buf \*, unsigned long, void \*);
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*
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* There are also atomic variants of these interfaces. Like for kmap they
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* facilitate non-blocking fast-paths. Neither the importer nor the exporter
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* (in the callback) is allowed to block when using these.
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*
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* Interfaces::
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* void \*dma_buf_kmap_atomic(struct dma_buf \*, unsigned long);
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* void dma_buf_kunmap_atomic(struct dma_buf \*, unsigned long, void \*);
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*
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* For importers all the restrictions of using kmap apply, like the limited
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* supply of kmap_atomic slots. Hence an importer shall only hold onto at
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* max 2 atomic dma_buf kmaps at the same time (in any given process context).
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*
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* dma_buf kmap calls outside of the range specified in begin_cpu_access are
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* undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
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* the partial chunks at the beginning and end but may return stale or bogus
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* data outside of the range (in these partial chunks).
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*
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* Note that these calls need to always succeed. The exporter needs to
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* complete any preparations that might fail in begin_cpu_access.
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*
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* For some cases the overhead of kmap can be too high, a vmap interface
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* is introduced. This interface should be used very carefully, as vmalloc
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* space is a limited resources on many architectures.
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*
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* Interfaces::
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* void \*dma_buf_vmap(struct dma_buf \*dmabuf)
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* void dma_buf_vunmap(struct dma_buf \*dmabuf, void \*vaddr)
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*
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* The vmap call can fail if there is no vmap support in the exporter, or if
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* it runs out of vmalloc space. Fallback to kmap should be implemented. Note
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* that the dma-buf layer keeps a reference count for all vmap access and
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* calls down into the exporter's vmap function only when no vmapping exists,
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* and only unmaps it once. Protection against concurrent vmap/vunmap calls is
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* provided by taking the dma_buf->lock mutex.
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*
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* - For full compatibility on the importer side with existing userspace
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* interfaces, which might already support mmap'ing buffers. This is needed in
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* many processing pipelines (e.g. feeding a software rendered image into a
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* hardware pipeline, thumbnail creation, snapshots, ...). Also, Android's ION
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* framework already supported this and for DMA buffer file descriptors to
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* replace ION buffers mmap support was needed.
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*
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* There is no special interfaces, userspace simply calls mmap on the dma-buf
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* fd. But like for CPU access there's a need to braket the actual access,
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* which is handled by the ioctl (DMA_BUF_IOCTL_SYNC). Note that
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* DMA_BUF_IOCTL_SYNC can fail with -EAGAIN or -EINTR, in which case it must
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* be restarted.
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*
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* Some systems might need some sort of cache coherency management e.g. when
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* CPU and GPU domains are being accessed through dma-buf at the same time.
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* To circumvent this problem there are begin/end coherency markers, that
|
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* forward directly to existing dma-buf device drivers vfunc hooks. Userspace
|
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* can make use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The
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* sequence would be used like following:
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*
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* - mmap dma-buf fd
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* - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
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* to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
|
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* want (with the new data being consumed by say the GPU or the scanout
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* device)
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* - munmap once you don't need the buffer any more
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*
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* For correctness and optimal performance, it is always required to use
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* SYNC_START and SYNC_END before and after, respectively, when accessing the
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* mapped address. Userspace cannot rely on coherent access, even when there
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* are systems where it just works without calling these ioctls.
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*
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* - And as a CPU fallback in userspace processing pipelines.
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*
|
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* Similar to the motivation for kernel cpu access it is again important that
|
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* the userspace code of a given importing subsystem can use the same
|
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* interfaces with a imported dma-buf buffer object as with a native buffer
|
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* object. This is especially important for drm where the userspace part of
|
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* contemporary OpenGL, X, and other drivers is huge, and reworking them to
|
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* use a different way to mmap a buffer rather invasive.
|
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*
|
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* The assumption in the current dma-buf interfaces is that redirecting the
|
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* initial mmap is all that's needed. A survey of some of the existing
|
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* subsystems shows that no driver seems to do any nefarious thing like
|
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* syncing up with outstanding asynchronous processing on the device or
|
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* allocating special resources at fault time. So hopefully this is good
|
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* enough, since adding interfaces to intercept pagefaults and allow pte
|
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* shootdowns would increase the complexity quite a bit.
|
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*
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* Interface::
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* int dma_buf_mmap(struct dma_buf \*, struct vm_area_struct \*,
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* unsigned long);
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*
|
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* If the importing subsystem simply provides a special-purpose mmap call to
|
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* set up a mapping in userspace, calling do_mmap with dma_buf->file will
|
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* equally achieve that for a dma-buf object.
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*/
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static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
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enum dma_data_direction direction)
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{
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|
@ -665,6 +781,10 @@ static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
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* @dmabuf: [in] buffer to prepare cpu access for.
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* @direction: [in] length of range for cpu access.
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*
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* After the cpu access is complete the caller should call
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* dma_buf_end_cpu_access(). Only when cpu access is braketed by both calls is
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* it guaranteed to be coherent with other DMA access.
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*
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* Can return negative error values, returns 0 on success.
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*/
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int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
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|
@ -697,6 +817,8 @@ EXPORT_SYMBOL_GPL(dma_buf_begin_cpu_access);
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* @dmabuf: [in] buffer to complete cpu access for.
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* @direction: [in] length of range for cpu access.
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*
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* This terminates CPU access started with dma_buf_begin_cpu_access().
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*
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* Can return negative error values, returns 0 on success.
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*/
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int dma_buf_end_cpu_access(struct dma_buf *dmabuf,
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@ -39,10 +39,6 @@ struct dma_buf_attachment;
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/**
|
||||
* struct dma_buf_ops - operations possible on struct dma_buf
|
||||
* @begin_cpu_access: [optional] called before cpu access to invalidate cpu
|
||||
* caches and allocate backing storage (if not yet done)
|
||||
* respectively pin the object into memory.
|
||||
* @end_cpu_access: [optional] called after cpu access to flush caches.
|
||||
* @kmap_atomic: maps a page from the buffer into kernel address
|
||||
* space, users may not block until the subsequent unmap call.
|
||||
* This callback must not sleep.
|
||||
|
@ -50,10 +46,6 @@ struct dma_buf_attachment;
|
|||
* This Callback must not sleep.
|
||||
* @kmap: maps a page from the buffer into kernel address space.
|
||||
* @kunmap: [optional] unmaps a page from the buffer.
|
||||
* @mmap: used to expose the backing storage to userspace. Note that the
|
||||
* mapping needs to be coherent - if the exporter doesn't directly
|
||||
* support this, it needs to fake coherency by shooting down any ptes
|
||||
* when transitioning away from the cpu domain.
|
||||
* @vmap: [optional] creates a virtual mapping for the buffer into kernel
|
||||
* address space. Same restrictions as for vmap and friends apply.
|
||||
* @vunmap: [optional] unmaps a vmap from the buffer
|
||||
|
@ -164,13 +156,96 @@ struct dma_buf_ops {
|
|||
*/
|
||||
void (*release)(struct dma_buf *);
|
||||
|
||||
/**
|
||||
* @begin_cpu_access:
|
||||
*
|
||||
* This is called from dma_buf_begin_cpu_access() and allows the
|
||||
* exporter to ensure that the memory is actually available for cpu
|
||||
* access - the exporter might need to allocate or swap-in and pin the
|
||||
* backing storage. The exporter also needs to ensure that cpu access is
|
||||
* coherent for the access direction. The direction can be used by the
|
||||
* exporter to optimize the cache flushing, i.e. access with a different
|
||||
* direction (read instead of write) might return stale or even bogus
|
||||
* data (e.g. when the exporter needs to copy the data to temporary
|
||||
* storage).
|
||||
*
|
||||
* This callback is optional.
|
||||
*
|
||||
* FIXME: This is both called through the DMA_BUF_IOCTL_SYNC command
|
||||
* from userspace (where storage shouldn't be pinned to avoid handing
|
||||
* de-factor mlock rights to userspace) and for the kernel-internal
|
||||
* users of the various kmap interfaces, where the backing storage must
|
||||
* be pinned to guarantee that the atomic kmap calls can succeed. Since
|
||||
* there's no in-kernel users of the kmap interfaces yet this isn't a
|
||||
* real problem.
|
||||
*
|
||||
* Returns:
|
||||
*
|
||||
* 0 on success or a negative error code on failure. This can for
|
||||
* example fail when the backing storage can't be allocated. Can also
|
||||
* return -ERESTARTSYS or -EINTR when the call has been interrupted and
|
||||
* needs to be restarted.
|
||||
*/
|
||||
int (*begin_cpu_access)(struct dma_buf *, enum dma_data_direction);
|
||||
|
||||
/**
|
||||
* @end_cpu_access:
|
||||
*
|
||||
* This is called from dma_buf_end_cpu_access() when the importer is
|
||||
* done accessing the CPU. The exporter can use this to flush caches and
|
||||
* unpin any resources pinned in @begin_cpu_access.
|
||||
* The result of any dma_buf kmap calls after end_cpu_access is
|
||||
* undefined.
|
||||
*
|
||||
* This callback is optional.
|
||||
*
|
||||
* Returns:
|
||||
*
|
||||
* 0 on success or a negative error code on failure. Can return
|
||||
* -ERESTARTSYS or -EINTR when the call has been interrupted and needs
|
||||
* to be restarted.
|
||||
*/
|
||||
int (*end_cpu_access)(struct dma_buf *, enum dma_data_direction);
|
||||
void *(*kmap_atomic)(struct dma_buf *, unsigned long);
|
||||
void (*kunmap_atomic)(struct dma_buf *, unsigned long, void *);
|
||||
void *(*kmap)(struct dma_buf *, unsigned long);
|
||||
void (*kunmap)(struct dma_buf *, unsigned long, void *);
|
||||
|
||||
/**
|
||||
* @mmap:
|
||||
*
|
||||
* This callback is used by the dma_buf_mmap() function
|
||||
*
|
||||
* Note that the mapping needs to be incoherent, userspace is expected
|
||||
* to braket CPU access using the DMA_BUF_IOCTL_SYNC interface.
|
||||
*
|
||||
* Because dma-buf buffers have invariant size over their lifetime, the
|
||||
* dma-buf core checks whether a vma is too large and rejects such
|
||||
* mappings. The exporter hence does not need to duplicate this check.
|
||||
* Drivers do not need to check this themselves.
|
||||
*
|
||||
* If an exporter needs to manually flush caches and hence needs to fake
|
||||
* coherency for mmap support, it needs to be able to zap all the ptes
|
||||
* pointing at the backing storage. Now linux mm needs a struct
|
||||
* address_space associated with the struct file stored in vma->vm_file
|
||||
* to do that with the function unmap_mapping_range. But the dma_buf
|
||||
* framework only backs every dma_buf fd with the anon_file struct file,
|
||||
* i.e. all dma_bufs share the same file.
|
||||
*
|
||||
* Hence exporters need to setup their own file (and address_space)
|
||||
* association by setting vma->vm_file and adjusting vma->vm_pgoff in
|
||||
* the dma_buf mmap callback. In the specific case of a gem driver the
|
||||
* exporter could use the shmem file already provided by gem (and set
|
||||
* vm_pgoff = 0). Exporters can then zap ptes by unmapping the
|
||||
* corresponding range of the struct address_space associated with their
|
||||
* own file.
|
||||
*
|
||||
* This callback is optional.
|
||||
*
|
||||
* Returns:
|
||||
*
|
||||
* 0 on success or a negative error code on failure.
|
||||
*/
|
||||
int (*mmap)(struct dma_buf *, struct vm_area_struct *vma);
|
||||
|
||||
void *(*vmap)(struct dma_buf *);
|
||||
|
|
Loading…
Reference in a new issue