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

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Message-ID: <CAC5umygcr0a_BfDJ9tB1EDry4UEwThG7WvkA8Dn27zO9RQab8Q@mail.gmail.com>
Date:	Fri, 17 Jan 2014 08:34:38 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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

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.

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.
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