linux-kernel - Re: [PATCH v4 1/3] videobuf2-dma-contig: user can specify GFP flags

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Date:	Tue, 08 Jan 2013 11:15:28 +0100
From:	Federico Vaga <federico.vaga@...il.com>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Pawel Osciak <pawel@...iak.com>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Giancarlo Asnaghi <giancarlo.asnaghi@...com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v4 1/3] videobuf2-dma-contig: user can specify GFP flags

> > @@ -165,7 +161,8 @@ static void *vb2_dc_alloc(void *alloc_ctx, unsigned
> > long size)> 
> >   	/* align image size to PAGE_SIZE */
> >   	size = PAGE_ALIGN(size);
> > 
> > -	buf->vaddr = dma_alloc_coherent(dev, size, &buf->dma_addr, 
GFP_KERNEL);
> > +	buf->vaddr = dma_alloc_coherent(dev, size, &buf->dma_addr,
> > +									
GFP_KERNEL | conf->mem_flags);
> 
> I think we can add GFP_DMA flag unconditionally to the vb2_dc_contig
> allocator.
> It won't hurt existing clients as most of nowadays platforms doesn't
> have DMA
> zone (GFP_DMA is ignored in such case), but it should fix the issues
> with some
> older and non-standard systems.

I did not set GFP_DMA fixed in the allocator because I do not want to brake 
something in the future. On x86 platform GFP_DMA allocates under 16MB and this 
limit can be too strict. When many other drivers use GFP_DMA we can saturate 
this tiny zone.
As you said, this fix the issue with _older_ and _non-standard_ (like sta2x11) 
systems. But this fix has effect on every other standard and new systems. 
That's why I preferred to set the flag optionally.

-- 
Federico Vaga
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