linux-kernel - Re: [PATCH v2 1/5] x86: make dma_alloc_coherent() return zeroed memory if CMA is enabled

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Date:	Fri, 24 Jan 2014 13:07:41 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Don Dutile <ddutile@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/5] x86: make dma_alloc_coherent() return zeroed
 memory if CMA is enabled

On Fri, Jan 17, 2014 at 08:34:38AM +0900, Akinobu Mita wrote:
> 2014/1/16 Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>:
> > On Tue, Jan 14, 2014 at 11:13:46PM +0900, Akinobu Mita wrote:
> >> Calling dma_alloc_coherent() with __GFP_ZERO must return zeroed memory.
> >>
> >> But when the contiguous memory allocator (CMA) is enabled on x86 and
> >> the memory region is allocated by dma_alloc_from_contiguous(), it
> >> doesn't return zeroed memory.  Because dma_generic_alloc_coherent()
> >
> > So why not fix it there to return zeroed out memory?
> 
> I thought it looked nicer than this patch as we can remove memset
> from all caller of dma_alloc_from_contiguous().  But if I look at
> the caller on arm, we can't simply remove the memset because
> __dma_clear_buffer() is used there for ensuring cache flushing and
> it is used in many places.

OK, that should be part of the commit description.
> 
> Of course we can do redundant memset in dma_alloc_from_contiguous(),
> but now I think this patch is less impact for fixing this problem.

OK.
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