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Commit 8d5c6603 authored by Keith Packard's avatar Keith Packard Committed by Ingo Molnar
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io mapping: improve documentation 



Impact: add documentation

Signed-off-by: default avatarKeith Packard <keithp@keithp.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 0839ccb8

Documentation/io-mapping.txt

+9 −3
Original line number Diff line number Diff line
@@ -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.