Commit 3f091387 authored by Helge Deller's avatar Helge Deller
Browse files

parisc: shmparam.h: Document aliasing requirements of PA-RISC 



Add some documentation why PA-RISC uses SHMLBA and SHM_COLOUR.

Signed-off-by: default avatarHelge Deller <deller@gmx.de>
parent b1bef138

arch/parisc/include/asm/shmparam.h

+15 −0
Original line number Diff line number Diff line
@@ -2,6 +2,21 @@ 
#ifndef _ASMPARISC_SHMPARAM_H

#define _ASMPARISC_SHMPARAM_H



/*

 * PA-RISC uses virtually indexed & physically tagged (VIPT) caches

 * which has strict requirements when two pages to the same physical

 * address are accessed through different mappings. Read the section

 * "Address Aliasing" in the arch docs for more detail:

 * PA-RISC 1.1 (page 3-6):

 * https://parisc.wiki.kernel.org/images-parisc/6/68/Pa11_acd.pdf

 * PA-RISC 2.0 (page F-5):

 * https://parisc.wiki.kernel.org/images-parisc/7/73/Parisc2.0.pdf

 *

 * For Linux we allow kernel and userspace to map pages on page size

 * granularity (SHMLBA) but have to ensure that, if two pages are

 * mapped to the same physical address, the virtual and physical

 * addresses modulo SHM_COLOUR are identical.

 */

#define SHMLBA	   PAGE_SIZE	/* attach addr a multiple of this */

#define SHM_COLOUR 0x00400000	/* shared mappings colouring */