Re: [SPAM]Re: [PATCH] net: mediatek: use dma_zalloc_coherent instead of allocator/memset

On 2018/7/20 16:13, Russell King - ARM Linux wrote:

quoted

On Fri, Jul 20, 2018 at 02:30:53PM +0800, YueHaibing wrote:

quoted

On 2018/7/20 1:02, Sean Wang wrote:

quoted

On Thu, 2018-07-19 at 15:17 +0100, Russell King - ARM Linux wrote:

quoted

On Thu, Jul 19, 2018 at 10:09:55PM +0800, YueHaibing wrote:

quoted

Use dma_zalloc_coherent instead of dma_alloc_coherent
followed by memset 0.

Signed-off-by: YueHaibing <redacted>
---
 drivers/net/ethernet/mediatek/mtk_eth_soc.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
index d8ebf0a..fbdb3e3 100644
--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -1221,14 +1221,11 @@ static int mtk_tx_alloc(struct mtk_eth *eth)
 	if (!ring->buf)
 		goto no_tx_mem;
 
-	ring->dma = dma_alloc_coherent(eth->dev,
-					  MTK_DMA_SIZE * sz,
-					  &ring->phys,
-					  GFP_ATOMIC | __GFP_ZERO);
+	ring->dma = dma_zalloc_coherent(eth->dev, MTK_DMA_SIZE * sz,
+					&ring->phys, GFP_ATOMIC | __GFP_ZERO);
 	if (!ring->dma)
 		goto no_tx_mem;
 
-	memset(ring->dma, 0, MTK_DMA_SIZE * sz);

I have to wonder whether this code needs two forms of zeroing... in
the original code, __GFP_ZERO _and_ a call to memset() just in case
__GFP_ZERO failed to do its job, and in the replacement code, just
in case dma_zalloc_coherent() hasn't got the idea...

I think you can drop the __GFP_ZERO. ;)

Just now I did an experiment on 4.14.56 on armv7. I found that
dma_zalloc_coherent does not guarantee that the buffer we get
is all filled with 0.


I really think it's a little bit weird OR what was I missing something
for enabling dma_zalloc_coherent ? The result seems to tell that we
can't remove freely the memset with 0 at this moment until we get a
cause.

That means dma_zalloc_coherent doesn't work as expect on armv7?

Can someone work out which underlying allocator is being used - the
possibilities are:

- dma_alloc_from_dev_coherent
- cma
- simple
- remap
- pool

Looking at the code, I'd guess it's the pool allocator, as I don't see
anything which zeros memory there, and it doesn't honor the __GFP_ZERO
flag.  This is definitely an allocator bug.

Sean,

can you test bellow patch：

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index be0fa7e..52029bb 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c

@@ -564,6 +564,7 @@ static void *__alloc_from_pool(size_t size, struct page **ret_page)

*ret_page = phys_to_page(phys);
ptr = (void *)val;
+ memset(ptr, 0, size);
}