summaryrefslogtreecommitdiff
path: root/arch
diff options
context:
space:
mode:
authorHelge Deller <deller@gmx.de>2023-09-01 09:04:31 +0300
committerHelge Deller <deller@gmx.de>2023-09-07 09:00:32 +0300
commit3f091387a39795812aab4303949bbc9baa22c077 (patch)
tree698cc18fd6ece70318b1a8b7d628a76695f3b323 /arch
parentb1bef1388c427cdad7331a9c8eb4ebbbe5b954b0 (diff)
downloadlinux-3f091387a39795812aab4303949bbc9baa22c077.tar.xz
parisc: shmparam.h: Document aliasing requirements of PA-RISC
Add some documentation why PA-RISC uses SHMLBA and SHM_COLOUR. Signed-off-by: Helge Deller <deller@gmx.de>
Diffstat (limited to 'arch')
-rw-r--r--arch/parisc/include/asm/shmparam.h15
1 files changed, 15 insertions, 0 deletions
diff --git a/arch/parisc/include/asm/shmparam.h b/arch/parisc/include/asm/shmparam.h
index 74f74e4d35b7..5a95b0f62b87 100644
--- a/arch/parisc/include/asm/shmparam.h
+++ b/arch/parisc/include/asm/shmparam.h
@@ -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 */