path: root/Documentation/io-mapping.txt
diff options
authorKeith Packard <keithp@keithp.com>2008-11-03 18:21:44 +0100
committerIngo Molnar <mingo@elte.hu>2008-11-03 18:21:44 +0100
commit8d5c6603c408d91ecf543f244f10ccb8b500ad95 (patch)
tree733e70b2e7fd5e5251e2c813f6cdc2605e20bb40 /Documentation/io-mapping.txt
parent0839ccb8ac6a9e2d5e175a4ae9c82b5c574d510d (diff)
io mapping: improve documentation
Impact: add documentation Signed-off-by: Keith Packard <keithp@keithp.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'Documentation/io-mapping.txt')
1 files changed, 9 insertions, 3 deletions
diff --git a/Documentation/io-mapping.txt b/Documentation/io-mapping.txt
index cd2f726becc8..473e43b2d588 100644
--- a/Documentation/io-mapping.txt
+++ b/Documentation/io-mapping.txt
@@ -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.

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