Re: [PATCH 2/2] media: bt8xx: avoid a useless memset

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Le 27/07/2020 à 18:16, Joe Perches a écrit :
On Mon, 2020-07-27 at 09:09 -0700, Joe Perches wrote:
On Mon, 2020-07-27 at 15:51 +0200, Christophe JAILLET wrote:
Avoid a memset after a call to 'dma_alloc_coherent()'.
This is useless since
commit 518a2f1925c3 ("dma-mapping: zero memory returned from dma_alloc_*")
[]
diff --git a/drivers/media/pci/bt8xx/btcx-risc.c b/drivers/media/pci/bt8xx/btcx-risc.c
[]
@@ -73,7 +73,6 @@ int btcx_riscmem_alloc(struct pci_dev *pci,
  		dprintk("btcx: riscmem alloc [%d] dma=%lx cpu=%p size=%d\n",
  			memcnt, (unsigned long)dma, cpu, size);
  	}
-	memset(risc->cpu,0,risc->size);
  	return 0;
  }

Likely NAK.

This is not useless as risc->cpu may be reused
and the alloc may not have been done.

Perhaps a little rewrite for clarity:
---
  drivers/media/pci/bt8xx/btcx-risc.c | 24 +++++++++++++-----------
  1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/drivers/media/pci/bt8xx/btcx-risc.c b/drivers/media/pci/bt8xx/btcx-risc.c
index 51257980f539..311f4ca2a108 100644
--- a/drivers/media/pci/bt8xx/btcx-risc.c
+++ b/drivers/media/pci/bt8xx/btcx-risc.c
@@ -56,24 +56,26 @@ int btcx_riscmem_alloc(struct pci_dev *pci,
  		       struct btcx_riscmem *risc,
  		       unsigned int size)
  {
-	__le32 *cpu;
-	dma_addr_t dma = 0;
-
-	if (NULL != risc->cpu && risc->size < size)
-		btcx_riscmem_free(pci,risc);
-	if (NULL == risc->cpu) {
-		cpu = pci_alloc_consistent(pci, size, &dma);
-		if (NULL == cpu)
+	if (risc->cpu && risc->size < size)
+		btcx_riscmem_free(pci, risc);
+
+	if (risc->cpu) {
+		memset(risc->cpu, 0, risc->size);
+	} else {
+		dma_addr_t dma = 0;
+
+		risc->cpu = pci_alloc_consistent(pci, size, &dma);
+		if (!risc->cpu)
  			return -ENOMEM;
-		risc->cpu  = cpu;
+
  		risc->dma  = dma;
  		risc->size = size;
memcnt++;
  		dprintk("btcx: riscmem alloc [%d] dma=%lx cpu=%p size=%d\n",
-			memcnt, (unsigned long)dma, cpu, size);
+			memcnt, (unsigned long)dma, risc->cpu, size);
  	}
-	memset(risc->cpu,0,risc->size);
+
  	return 0;
  }

Looks good to me.

Just note, that this will not apply after patch 1/2 is applied, because it turns pci_alloc_consistent() into dma_alloc_coherent().

CJ



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