linux-kernel - Re: using DMA-API on ARM

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Message-ID: <20141205192544.GY11285@n2100.arm.linux.org.uk>
Date:	Fri, 5 Dec 2014 19:25:44 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arend van Spriel <arend@...adcom.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Hante Meuleman <meuleman@...adcom.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	brcm80211-dev-list <brcm80211-dev-list@...adcom.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>,
	David Miller <davem@...emloft.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: using DMA-API on ARM

On Fri, Dec 05, 2014 at 08:22:05PM +0100, Arend van Spriel wrote:
> On 12/05/14 19:28, Catalin Marinas wrote:
> >This is solved by using a pre-allocated, pre-mapped atomic_pool which
> >avoids any further mapping. __dma_alloc() calls __alloc_from_pool() when
> >!__GFP_WAIT.
> 
> So we are actually calling dma_alloc_coherent() with GFP_KERNEL during
> device probe. That last paragraph Russell pointed out seems to suggest this
> is not allowed.

device probe is a schedulable, sleepable context, so dma_alloc_coherent()
is fine there.  As Catalin points out, and as I realised after sending
them ail, it does check for __GFP_WAIT and uses a smaller atomic pool
for those allocations.  This explains why no one has hit any warnings in
map_vm_area.

So, it's safe from atomic contexts after all.

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