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authorStuart Menefy <stuart.menefy@st.com>2009-08-24 17:36:24 +0900
committerPaul Mundt <lethal@linux-sh.org>2009-08-24 17:36:24 +0900
commita5cf9e2444ec15de5407696ff21c32dd21ca0a8d (patch)
tree6835770b0361cffc204d0491156a76408f03719b /arch/sh/mm
parent5e9377ec6f84e5334e9347e84e77d34e9a089ca7 (diff)
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 <stuart.menefy@st.com> Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Diffstat (limited to 'arch/sh/mm')
-rw-r--r--arch/sh/mm/cache-sh4.c11
1 files changed, 11 insertions, 0 deletions
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)