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>

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 */