![Zenghui Yu](/assets/img/avatar_default.png)
Documentation/PCI/msi-howto.rst referred to pci_irq_alloc_vectors() when it should refer to pci_alloc_irq_vectors(). Fix the typo. Link: https://lore.kernel.org/r/20191230131428.1200-1-yuzenghui@huawei.com Signed-off-by: Zenghui Yu <yuzenghui@huawei.com> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> Reviewed-by: Andrew Murray <andrew.murray@arm.com>
288 lines
12 KiB
ReStructuredText
288 lines
12 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
.. include:: <isonum.txt>
|
|
|
|
==========================
|
|
The MSI Driver Guide HOWTO
|
|
==========================
|
|
|
|
:Authors: Tom L Nguyen; Martine Silbermann; Matthew Wilcox
|
|
|
|
:Copyright: 2003, 2008 Intel Corporation
|
|
|
|
About this guide
|
|
================
|
|
|
|
This guide describes the basics of Message Signaled Interrupts (MSIs),
|
|
the advantages of using MSI over traditional interrupt mechanisms, how
|
|
to change your driver to use MSI or MSI-X and some basic diagnostics to
|
|
try if a device doesn't support MSIs.
|
|
|
|
|
|
What are MSIs?
|
|
==============
|
|
|
|
A Message Signaled Interrupt is a write from the device to a special
|
|
address which causes an interrupt to be received by the CPU.
|
|
|
|
The MSI capability was first specified in PCI 2.2 and was later enhanced
|
|
in PCI 3.0 to allow each interrupt to be masked individually. The MSI-X
|
|
capability was also introduced with PCI 3.0. It supports more interrupts
|
|
per device than MSI and allows interrupts to be independently configured.
|
|
|
|
Devices may support both MSI and MSI-X, but only one can be enabled at
|
|
a time.
|
|
|
|
|
|
Why use MSIs?
|
|
=============
|
|
|
|
There are three reasons why using MSIs can give an advantage over
|
|
traditional pin-based interrupts.
|
|
|
|
Pin-based PCI interrupts are often shared amongst several devices.
|
|
To support this, the kernel must call each interrupt handler associated
|
|
with an interrupt, which leads to reduced performance for the system as
|
|
a whole. MSIs are never shared, so this problem cannot arise.
|
|
|
|
When a device writes data to memory, then raises a pin-based interrupt,
|
|
it is possible that the interrupt may arrive before all the data has
|
|
arrived in memory (this becomes more likely with devices behind PCI-PCI
|
|
bridges). In order to ensure that all the data has arrived in memory,
|
|
the interrupt handler must read a register on the device which raised
|
|
the interrupt. PCI transaction ordering rules require that all the data
|
|
arrive in memory before the value may be returned from the register.
|
|
Using MSIs avoids this problem as the interrupt-generating write cannot
|
|
pass the data writes, so by the time the interrupt is raised, the driver
|
|
knows that all the data has arrived in memory.
|
|
|
|
PCI devices can only support a single pin-based interrupt per function.
|
|
Often drivers have to query the device to find out what event has
|
|
occurred, slowing down interrupt handling for the common case. With
|
|
MSIs, a device can support more interrupts, allowing each interrupt
|
|
to be specialised to a different purpose. One possible design gives
|
|
infrequent conditions (such as errors) their own interrupt which allows
|
|
the driver to handle the normal interrupt handling path more efficiently.
|
|
Other possible designs include giving one interrupt to each packet queue
|
|
in a network card or each port in a storage controller.
|
|
|
|
|
|
How to use MSIs
|
|
===============
|
|
|
|
PCI devices are initialised to use pin-based interrupts. The device
|
|
driver has to set up the device to use MSI or MSI-X. Not all machines
|
|
support MSIs correctly, and for those machines, the APIs described below
|
|
will simply fail and the device will continue to use pin-based interrupts.
|
|
|
|
Include kernel support for MSIs
|
|
-------------------------------
|
|
|
|
To support MSI or MSI-X, the kernel must be built with the CONFIG_PCI_MSI
|
|
option enabled. This option is only available on some architectures,
|
|
and it may depend on some other options also being set. For example,
|
|
on x86, you must also enable X86_UP_APIC or SMP in order to see the
|
|
CONFIG_PCI_MSI option.
|
|
|
|
Using MSI
|
|
---------
|
|
|
|
Most of the hard work is done for the driver in the PCI layer. The driver
|
|
simply has to request that the PCI layer set up the MSI capability for this
|
|
device.
|
|
|
|
To automatically use MSI or MSI-X interrupt vectors, use the following
|
|
function::
|
|
|
|
int pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
|
|
unsigned int max_vecs, unsigned int flags);
|
|
|
|
which allocates up to max_vecs interrupt vectors for a PCI device. It
|
|
returns the number of vectors allocated or a negative error. If the device
|
|
has a requirements for a minimum number of vectors the driver can pass a
|
|
min_vecs argument set to this limit, and the PCI core will return -ENOSPC
|
|
if it can't meet the minimum number of vectors.
|
|
|
|
The flags argument is used to specify which type of interrupt can be used
|
|
by the device and the driver (PCI_IRQ_LEGACY, PCI_IRQ_MSI, PCI_IRQ_MSIX).
|
|
A convenient short-hand (PCI_IRQ_ALL_TYPES) is also available to ask for
|
|
any possible kind of interrupt. If the PCI_IRQ_AFFINITY flag is set,
|
|
pci_alloc_irq_vectors() will spread the interrupts around the available CPUs.
|
|
|
|
To get the Linux IRQ numbers passed to request_irq() and free_irq() and the
|
|
vectors, use the following function::
|
|
|
|
int pci_irq_vector(struct pci_dev *dev, unsigned int nr);
|
|
|
|
Any allocated resources should be freed before removing the device using
|
|
the following function::
|
|
|
|
void pci_free_irq_vectors(struct pci_dev *dev);
|
|
|
|
If a device supports both MSI-X and MSI capabilities, this API will use the
|
|
MSI-X facilities in preference to the MSI facilities. MSI-X supports any
|
|
number of interrupts between 1 and 2048. In contrast, MSI is restricted to
|
|
a maximum of 32 interrupts (and must be a power of two). In addition, the
|
|
MSI interrupt vectors must be allocated consecutively, so the system might
|
|
not be able to allocate as many vectors for MSI as it could for MSI-X. On
|
|
some platforms, MSI interrupts must all be targeted at the same set of CPUs
|
|
whereas MSI-X interrupts can all be targeted at different CPUs.
|
|
|
|
If a device supports neither MSI-X or MSI it will fall back to a single
|
|
legacy IRQ vector.
|
|
|
|
The typical usage of MSI or MSI-X interrupts is to allocate as many vectors
|
|
as possible, likely up to the limit supported by the device. If nvec is
|
|
larger than the number supported by the device it will automatically be
|
|
capped to the supported limit, so there is no need to query the number of
|
|
vectors supported beforehand::
|
|
|
|
nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_ALL_TYPES)
|
|
if (nvec < 0)
|
|
goto out_err;
|
|
|
|
If a driver is unable or unwilling to deal with a variable number of MSI
|
|
interrupts it can request a particular number of interrupts by passing that
|
|
number to pci_alloc_irq_vectors() function as both 'min_vecs' and
|
|
'max_vecs' parameters::
|
|
|
|
ret = pci_alloc_irq_vectors(pdev, nvec, nvec, PCI_IRQ_ALL_TYPES);
|
|
if (ret < 0)
|
|
goto out_err;
|
|
|
|
The most notorious example of the request type described above is enabling
|
|
the single MSI mode for a device. It could be done by passing two 1s as
|
|
'min_vecs' and 'max_vecs'::
|
|
|
|
ret = pci_alloc_irq_vectors(pdev, 1, 1, PCI_IRQ_ALL_TYPES);
|
|
if (ret < 0)
|
|
goto out_err;
|
|
|
|
Some devices might not support using legacy line interrupts, in which case
|
|
the driver can specify that only MSI or MSI-X is acceptable::
|
|
|
|
nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_MSI | PCI_IRQ_MSIX);
|
|
if (nvec < 0)
|
|
goto out_err;
|
|
|
|
Legacy APIs
|
|
-----------
|
|
|
|
The following old APIs to enable and disable MSI or MSI-X interrupts should
|
|
not be used in new code::
|
|
|
|
pci_enable_msi() /* deprecated */
|
|
pci_disable_msi() /* deprecated */
|
|
pci_enable_msix_range() /* deprecated */
|
|
pci_enable_msix_exact() /* deprecated */
|
|
pci_disable_msix() /* deprecated */
|
|
|
|
Additionally there are APIs to provide the number of supported MSI or MSI-X
|
|
vectors: pci_msi_vec_count() and pci_msix_vec_count(). In general these
|
|
should be avoided in favor of letting pci_alloc_irq_vectors() cap the
|
|
number of vectors. If you have a legitimate special use case for the count
|
|
of vectors we might have to revisit that decision and add a
|
|
pci_nr_irq_vectors() helper that handles MSI and MSI-X transparently.
|
|
|
|
Considerations when using MSIs
|
|
------------------------------
|
|
|
|
Spinlocks
|
|
~~~~~~~~~
|
|
|
|
Most device drivers have a per-device spinlock which is taken in the
|
|
interrupt handler. With pin-based interrupts or a single MSI, it is not
|
|
necessary to disable interrupts (Linux guarantees the same interrupt will
|
|
not be re-entered). If a device uses multiple interrupts, the driver
|
|
must disable interrupts while the lock is held. If the device sends
|
|
a different interrupt, the driver will deadlock trying to recursively
|
|
acquire the spinlock. Such deadlocks can be avoided by using
|
|
spin_lock_irqsave() or spin_lock_irq() which disable local interrupts
|
|
and acquire the lock (see Documentation/kernel-hacking/locking.rst).
|
|
|
|
How to tell whether MSI/MSI-X is enabled on a device
|
|
----------------------------------------------------
|
|
|
|
Using 'lspci -v' (as root) may show some devices with "MSI", "Message
|
|
Signalled Interrupts" or "MSI-X" capabilities. Each of these capabilities
|
|
has an 'Enable' flag which is followed with either "+" (enabled)
|
|
or "-" (disabled).
|
|
|
|
|
|
MSI quirks
|
|
==========
|
|
|
|
Several PCI chipsets or devices are known not to support MSIs.
|
|
The PCI stack provides three ways to disable MSIs:
|
|
|
|
1. globally
|
|
2. on all devices behind a specific bridge
|
|
3. on a single device
|
|
|
|
Disabling MSIs globally
|
|
-----------------------
|
|
|
|
Some host chipsets simply don't support MSIs properly. If we're
|
|
lucky, the manufacturer knows this and has indicated it in the ACPI
|
|
FADT table. In this case, Linux automatically disables MSIs.
|
|
Some boards don't include this information in the table and so we have
|
|
to detect them ourselves. The complete list of these is found near the
|
|
quirk_disable_all_msi() function in drivers/pci/quirks.c.
|
|
|
|
If you have a board which has problems with MSIs, you can pass pci=nomsi
|
|
on the kernel command line to disable MSIs on all devices. It would be
|
|
in your best interests to report the problem to linux-pci@vger.kernel.org
|
|
including a full 'lspci -v' so we can add the quirks to the kernel.
|
|
|
|
Disabling MSIs below a bridge
|
|
-----------------------------
|
|
|
|
Some PCI bridges are not able to route MSIs between busses properly.
|
|
In this case, MSIs must be disabled on all devices behind the bridge.
|
|
|
|
Some bridges allow you to enable MSIs by changing some bits in their
|
|
PCI configuration space (especially the Hypertransport chipsets such
|
|
as the nVidia nForce and Serverworks HT2000). As with host chipsets,
|
|
Linux mostly knows about them and automatically enables MSIs if it can.
|
|
If you have a bridge unknown to Linux, you can enable
|
|
MSIs in configuration space using whatever method you know works, then
|
|
enable MSIs on that bridge by doing::
|
|
|
|
echo 1 > /sys/bus/pci/devices/$bridge/msi_bus
|
|
|
|
where $bridge is the PCI address of the bridge you've enabled (eg
|
|
0000:00:0e.0).
|
|
|
|
To disable MSIs, echo 0 instead of 1. Changing this value should be
|
|
done with caution as it could break interrupt handling for all devices
|
|
below this bridge.
|
|
|
|
Again, please notify linux-pci@vger.kernel.org of any bridges that need
|
|
special handling.
|
|
|
|
Disabling MSIs on a single device
|
|
---------------------------------
|
|
|
|
Some devices are known to have faulty MSI implementations. Usually this
|
|
is handled in the individual device driver, but occasionally it's necessary
|
|
to handle this with a quirk. Some drivers have an option to disable use
|
|
of MSI. While this is a convenient workaround for the driver author,
|
|
it is not good practice, and should not be emulated.
|
|
|
|
Finding why MSIs are disabled on a device
|
|
-----------------------------------------
|
|
|
|
From the above three sections, you can see that there are many reasons
|
|
why MSIs may not be enabled for a given device. Your first step should
|
|
be to examine your dmesg carefully to determine whether MSIs are enabled
|
|
for your machine. You should also check your .config to be sure you
|
|
have enabled CONFIG_PCI_MSI.
|
|
|
|
Then, 'lspci -t' gives the list of bridges above a device. Reading
|
|
`/sys/bus/pci/devices/*/msi_bus` will tell you whether MSIs are enabled (1)
|
|
or disabled (0). If 0 is found in any of the msi_bus files belonging
|
|
to bridges between the PCI root and the device, MSIs are disabled.
|
|
|
|
It is also worth checking the device driver to see whether it supports MSIs.
|
|
For example, it may contain calls to pci_alloc_irq_vectors() with the
|
|
PCI_IRQ_MSI or PCI_IRQ_MSIX flags.
|