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

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

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?

my test code is

         ring->dma = dma_zalloc_coherent(eth->dev,
                                          MTK_DMA_SIZE * sz,
                                          &ring->phys,
                                          GFP_ATOMIC);
        if (!ring->dma)
                goto no_tx_mem;

        print_hex_dump(KERN_INFO, "mtk_tx_alloc:",
                       DUMP_PREFIX_OFFSET, 16, 1,
                       ring->dma, MTK_DMA_SIZE * sz, false);

        memset(ring->dma, 0, MTK_DMA_SIZE * sz);

        print_hex_dump(KERN_INFO, "mtk_tx_alloc2:",
                       DUMP_PREFIX_OFFSET, 16, 1,
                       ring->dma, MTK_DMA_SIZE * sz, false);


and the output 

...

[  259.610413] mtk_tx_alloc:00000f40: 00 00 00 00 50 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.617934] mtk_tx_alloc:00000f50: 00 00 00 00 60 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.625470] mtk_tx_alloc:00000f60: 00 00 00 00 70 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.633005] mtk_tx_alloc:00000f70: 00 00 00 00 80 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.640539] mtk_tx_alloc:00000f80: 00 00 00 00 90 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.648073] mtk_tx_alloc:00000f90: 00 00 00 00 a0 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.655590] mtk_tx_alloc:00000fa0: 00 00 00 00 b0 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.663124] mtk_tx_alloc:00000fb0: 00 00 00 00 c0 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.670660] mtk_tx_alloc:00000fc0: 00 00 00 00 d0 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.678196] mtk_tx_alloc:00000fd0: 00 00 00 00 e0 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.685713] mtk_tx_alloc:00000fe0: 00 00 00 00 f0 1f 00 bc 00 00 00 c0 00 00 00 00
[  259.693247] mtk_tx_alloc:00000ff0: 02 c0 90 ad 00 10 00 bc 00 40 5a c0 00 00 00 02
[  259.700782] mtk_tx_alloc2:00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.708405] mtk_tx_alloc2:00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.716013] mtk_tx_alloc2:00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.723634] mtk_tx_alloc2:00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.731253] mtk_tx_alloc2:00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.738875] mtk_tx_alloc2:00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.746481] mtk_tx_alloc2:00000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.754103] mtk_tx_alloc2:00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.761723] mtk_tx_alloc2:00000080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.769344] mtk_tx_alloc2:00000090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[  259.776951] mtk_tx_alloc2:000000a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

....


	Sean



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