linux-kernel - Re: [PATCH V3 3/4] dmaselftest: add memcpy selftest support functions

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Date:	Tue, 10 Nov 2015 11:13:24 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Sinan Kaya <okaya@...eaurora.org>,
	Timur Tabi <timur@...eaurora.org>, dmaengine@...r.kernel.org,
	cov@...eaurora.org, jcm@...hat.com,
	Vinod Koul <vinod.koul@...el.com>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	agross@...eaurora.org, Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH V3 3/4] dmaselftest: add memcpy selftest support functions

On Monday 09 November 2015 23:49:54 Sinan Kaya wrote:
> On 11/9/2015 8:48 AM, Timur Tabi wrote:
> > Sinan Kaya wrote:
> >>>
> >>> And why kmalloc anyway?  Why not leave it on the stack?
> >>>
> >>>      char src[] = "hello world";
> >>>
> >>> ?
> >>
> >> I need to call dma_map_single on this address to convert it to a DMA
> >> address. That's why.
> >
> > And you can't do that with an object that's on the stack?
> >
> 
> no, pasting from here.
> 
> https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt
> 
> under 'What memory is DMA'able?'
> 
> This rule also means that you may use neither kernel image addresses
> (items in data/text/bss segments), nor module image addresses, nor
> stack addresses for DMA.

Correct. I think this is just because of cache line alignment that
is guaranteed for kmalloc but not for anything on the stack.

	Arnd
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