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From: Laura Abbott <lauraa@codeaurora.org>
To: Gioh Kim <gioh.kim@lge.com>, Colin Cross <ccross@android.com>,
	Joonsoo Kim <js1304@gmail.com>
Cc: "John Stultz" <john.stultz@linaro.org>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org,
	"\"이건호/책임연구원/SW Platform(연)AOT팀(gunho.lee@lge.com)\""
	<gunho.lee@lge.com>
Subject: Re: Why does ion_cma_allocate need GFP_HIGHUSER | __GFP_ZERO flags?
Date: Thu, 13 Nov 2014 17:24:02 -0800	[thread overview]
Message-ID: <546559B2.4090003@codeaurora.org> (raw)
In-Reply-To: <54645B38.10603@lge.com>

On 11/12/2014 11:18 PM, Gioh Kim wrote:
> Hi,
> 
> I'm wondering why dma_alloc_coherent needs GFP_HIGHUSER | __GFP_ZERO flags in ion_cma_allocate().
> 
> I think dma_alloc_coherent can go to __dma_alloc if the system has CMA area.
> If __GFP_WAIT flag is set, it allocates memory from atomic pool.
> So I think calling dma_alloc_coherent with __GFP_WAIT is enough.
> 
> And can I get zero-filled-memory from CMA if I set __GFP_ZERO?
> 
> 

If we are actually using CMA, the GFP flags don't make much of a difference. If
we didn't have CMA, using GFP_HIGHUSER would still be appropriate. The memory
is going to userspace so the user part is necessary and there should be no need
for a lowmem mapping so highmem can be used. __GFP_ZERO might be redundant since
the dma layer zeros memory anyway but it's probably safe.

I think the flags should be fine as is.

Thanks,
Laura

-- 
Qualcomm Innovation Center, Inc. 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

  reply	other threads:[~2014-11-14  1:24 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-11-13  7:18 Gioh Kim
2014-11-14  1:24 ` Laura Abbott [this message]
2014-11-14  1:38   ` Gioh Kim

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