[PATCH v2] arm: mm: refactor v7 cache cleaning ops to use way/index sequence

[Lorenzo Pieralisi]

Set-associative caches on all v7 implementations map the index bits
to physical addresses LSBs and tag bits to MSBs. As the last level
of cache on current and upcoming ARM systems grows in size,
this means that under normal DRAM controller configurations, the
current v7 cache flush routine using set/way operations triggers a
DRAM memory controller precharge/activate for every cache line
writeback since the cache routine cleans lines by first fixing the
index and then looping through ways (index bits are mapped to lower
physical addresses on all v7 cache implementations; this means that,
with last level cache sizes in the order of MBytes, lines belonging
to the same set but different ways map to different DRAM pages).

Given the random content of cache tags, swapping the order between
indexes and ways loops do not prevent DRAM pages precharge and
activate cycles but at least, on average, improves the chances that
either multiple lines hit the same page or multiple lines belong to
different DRAM banks, improving throughput significantly.

This patch swaps the inner loops in the v7 cache flushing routine
to carry out the clean operations first on all sets belonging to
a given way (looping through sets) and then decrementing the way.

Benchmarks showed that by swapping the ordering in which sets and
ways are decremented in the v7 cache flushing routine, that uses
set/way operations, time required to flush caches is reduced
significantly, owing to improved writebacks throughput to the DRAM
controller.

Benchmarks results vary and depend heavily on the last level of
cache tag RAM content when cache is cleaned and invalidated, ranging
from 2x throughput when all tag RAM entries contain dirty lines
mapping to sequential pages of RAM to 1x (ie no improvement) when
all tag RAM accesses trigger a DRAM precharge/activate cycle, as the
current code implies on most DRAM controller configurations.

Acked-by: Catalin Marinas <catalin.marinas at arm.com>
Acked-by: Nicolas Pitre <nico at linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar at ti.com>
Reviewed-by: Dave Martin <Dave.Martin at arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
---
 arch/arm/mm/cache-v7.S | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/arm/mm/cache-v7.S b/arch/arm/mm/cache-v7.S
index b5c467a..778bcf8 100644
--- a/arch/arm/mm/cache-v7.S
+++ b/arch/arm/mm/cache-v7.S
@@ -146,18 +146,18 @@ flush_levels:
 	ldr	r7, =0x7fff
 	ands	r7, r7, r1, lsr #13		@ extract max number of the index size
 loop1:
-	mov	r9, r4				@ create working copy of max way size
+	mov	r9, r7				@ create working copy of max index
 loop2:
- ARM(	orr	r11, r10, r9, lsl r5	)	@ factor way and cache number into r11
- THUMB(	lsl	r6, r9, r5		)
+ ARM(	orr	r11, r10, r4, lsl r5	)	@ factor way and cache number into r11
+ THUMB(	lsl	r6, r4, r5		)
  THUMB(	orr	r11, r10, r6		)	@ factor way and cache number into r11
- ARM(	orr	r11, r11, r7, lsl r2	)	@ factor index number into r11
- THUMB(	lsl	r6, r7, r2		)
+ ARM(	orr	r11, r11, r9, lsl r2	)	@ factor index number into r11
+ THUMB(	lsl	r6, r9, r2		)
  THUMB(	orr	r11, r11, r6		)	@ factor index number into r11
 	mcr	p15, 0, r11, c7, c14, 2		@ clean & invalidate by set/way
-	subs	r9, r9, #1			@ decrement the way
+	subs	r9, r9, #1			@ decrement the index
 	bge	loop2
-	subs	r7, r7, #1			@ decrement the index
+	subs	r4, r4, #1			@ decrement the way
 	bge	loop1
 skip:
 	add	r10, r10, #2			@ increment cache number
-- 
1.8.2.2