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io mapping: improve documentation 

Impact: add documentation

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
hifive-unleashed-5.1
Keith Packard 2008-11-03 18:21:44 +01:00 committed by Ingo Molnar
parent 0839ccb8ac
commit 8d5c6603c4
1 changed files with 9 additions and 3 deletions
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12
Documentation/io-mapping.txt
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@ -71,6 +71,12 @@ range, creating a permanent kernel-visible mapping to the resource. The
map_atomic and map functions add the requested offset to the base of the
virtual address returned by ioremap_wc.
On 32-bit processors, io_mapping_map_atomic_wc uses io_map_atomic_prot_pfn,
which uses the fixmaps to get us a mapping to a page using an atomic fashion.
For io_mapping_map_wc, ioremap_wc() is used to get a mapping of the region.
On 32-bit processors with HIGHMEM defined, io_mapping_map_atomic_wc uses
kmap_atomic_pfn to map the specified page in an atomic fashion;
kmap_atomic_pfn isn't really supposed to be used with device pages, but it
provides an efficient mapping for this usage.
On 32-bit processors without HIGHMEM defined, io_mapping_map_atomic_wc and
io_mapping_map_wc both use ioremap_wc, a terribly inefficient function which
performs an IPI to inform all processors about the new mapping. This results
in a significant performance penalty.