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

[Dave Martin]

On Tue, Nov 19, 2013 at 03:29:53PM +0000, Lorenzo Pieralisi wrote:
> Set-associative caches on all v7 implementations map the index bits
> to physical addresses LSBs and tag bits to MSBs. On most systems with
> sane DRAM controller configurations, this means that 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.
> 
> 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.

For the correctness of this patch:

Reviewed-by: Dave Martin <Dave.Martin at arm.com>

My understanding of the performance implications is more limited, so
I'm happy to defer to others on that.

Cheers
---Dave

> 
> 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
> 
> 
> 
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