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

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Date:   Tue, 28 Jul 2020 02:25:47 -0700
From:   Joe Perches <joe@...ches.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        mchehab@...nel.org, akpm@...ux-foundation.org, rppt@...nel.org,
        hverkuil-cisco@...all.nl
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 2/2] media: bt8xx: avoid a useless memset

On Tue, 2020-07-28 at 10:05 +0200, Christophe JAILLET wrote:
> 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().

Just a suggestion.

As it's dependent on your first patch, perhaps
you could make the appropriate change.