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@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 >> > > > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>