From: Keith Packard Date: Mon, 3 Nov 2008 17:21:44 +0000 (+0100) Subject: io mapping: improve documentation X-Git-Tag: v2.6.28-rc4~91^2~1 X-Git-Url: http://www.pilppa.org/gitweb/gitweb.cgi?a=commitdiff_plain;h=8d5c6603c408d91ecf543f244f10ccb8b500ad95;p=linux-2.6-omap-h63xx.git io mapping: improve documentation Impact: add documentation Signed-off-by: Keith Packard Signed-off-by: Ingo Molnar --- diff --git a/Documentation/io-mapping.txt b/Documentation/io-mapping.txt index cd2f726becc..473e43b2d58 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.