Re: [PATCH] [RFC] drivers: dma-coherent: use MEMREMAP_WB instead of MEMREMAP_WC

[Brian Starkey]

Hi Jaewon,

On Wed, Nov 09, 2016 at 06:10:09PM +0900, Jaewon Kim wrote:
> Commit 6b03ae0d42bf (drivers: dma-coherent: use MEMREMAP_WC for DMA_MEMORY_MA)
> added MEMREMAP_WC for DMA_MEMORY_MAP. If, however, CPU cache can be used on
> DMA_MEMORY_MAP, I think MEMREMAP_WC can be changed to MEMREMAP_WB. On my local
> ARM device, memset in dma_alloc_from_coherent sometimes takes much longer with
> MEMREMAP_WC compared to MEMREMAP_WB.
>
> Test results on AArch64 by allocating 4MB with putting trace_printk right
> before and after memset.
> 	MEMREMAP_WC : 11.0ms, 5.7ms, 4.2ms, 4.9ms, 5.4ms, 4.3ms, 3.5ms
> 	MEMREMAP_WB : 0.7ms, 0.6ms, 0.6ms, 0.6ms, 0.6ms, 0.5ms, 0.4 ms

This doesn't look like a good idea to me. The point of coherent memory
is to have it non-cached, however WB will make writes hit the cache.

Writing to the cache is of course faster than writing to RAM, but
that's not what we want to do here.

-Brian

> Signed-off-by: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
> ---
> drivers/base/dma-coherent.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
> index 640a7e6..0512a1d 100644
> --- a/drivers/base/dma-coherent.c
> +++ b/drivers/base/dma-coherent.c
> @@ -33,7 +33,7 @@ static bool dma_init_coherent_memory(
> 		goto out;
>
> 	if (flags & DMA_MEMORY_MAP)
> -		mem_base = memremap(phys_addr, size, MEMREMAP_WC);
> +		mem_base = memremap(phys_addr, size, MEMREMAP_WB);
> 	else
> 		mem_base = ioremap(phys_addr, size);
> 	if (!mem_base)
> --
> 1.9.1