linux-kernel - Re: Why does ion_cma_allocate need GFP_HIGHUSER

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Message-ID: <546559B2.4090003@codeaurora.org>
Date:	Thu, 13 Nov 2014 17:24:02 -0800
From:	Laura Abbott <lauraa@...eaurora.org>
To:	Gioh Kim <gioh.kim@....com>, Colin Cross <ccross@...roid.com>,
	Joonsoo Kim <js1304@...il.com>
CC:	John Stultz <john.stultz@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	"이건호/책임연구원/SW Platform(연)AOT팀(gunho.lee@....com)" <gunho.lee@....com>
Subject: Re: Why does ion_cma_allocate need GFP_HIGHUSER | __GFP_ZERO flags?

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

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