From: Stuart Menefy Date: Mon, 24 Aug 2009 08:36:24 +0000 (+0900) Subject: sh: Improve comments int SH4 cache flushing code X-Git-Tag: v2.6.32-rc1~640^2~35^2~14^2~7 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=a5cf9e2444ec15de5407696ff21c32dd21ca0a8d;p=karo-tx-linux.git sh: Improve comments int SH4 cache flushing code This is a pure documentation, to try to explain why the cache flushing code for the SH4 is implemented the way it is. Signed-off-by: Stuart Menefy Signed-off-by: Paul Mundt --- diff --git a/arch/sh/mm/cache-sh4.c b/arch/sh/mm/cache-sh4.c index 5cfe08dbb59e..7ce816188313 100644 --- a/arch/sh/mm/cache-sh4.c +++ b/arch/sh/mm/cache-sh4.c @@ -581,6 +581,17 @@ static void __flush_cache_4096(unsigned long addr, unsigned long phys, * Break the 1, 2 and 4 way variants of this out into separate functions to * avoid nearly all the overhead of having the conditional stuff in the function * bodies (+ the 1 and 2 way cases avoid saving any registers too). + * + * We want to eliminate unnecessary bus transactions, so this code uses + * a non-obvious technique. + * + * Loop over a cache way sized block of, one cache line at a time. For each + * line, use movca.a to cause the current cache line contents to be written + * back, but without reading anything from main memory. However this has the + * side effect that the cache is now caching that memory location. So follow + * this with a cache invalidate to mark the cache line invalid. And do all + * this with interrupts disabled, to avoid the cache line being accidently + * evicted while it is holding garbage. */ static void __flush_dcache_segment_1way(unsigned long start, unsigned long extent_per_way)