linux-kernel - Re: [PATCH] [RFC] drivers: dma-coherent: use MEMREMAP_WB instead of MEMREMAP

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Date:   Wed, 09 Nov 2016 18:47:26 +0900
From:   Jaewon Kim <jaewon31.kim@...sung.com>
To:     Brian Starkey <brian.starkey@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] drivers: dma-coherent: use MEMREMAP_WB instead of
 MEMREMAP_WC



On 2016년 11월 09일 18:27, Brian Starkey wrote:
> 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
>
Hi Brian

Thank you for your comment.
If allocated memory will be used by TZ side, however, I think cacheable
also can be used to be fast on memset in dma_alloc_from_coherent.
How do you think to add another flag to distinguish this case?
>> Signed-off-by: Jaewon Kim <jaewon31.kim@...sung.com>
>> ---
>> 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
>>
>
>
>