From mboxrd@z Thu Jan  1 00:00:00 1970
From: arnd@arndb.de (Arnd Bergmann)
Date: Tue, 10 Nov 2015 11:13:24 +0100
Subject: [PATCH V3 3/4] dmaselftest: add memcpy selftest support functions
In-Reply-To: <56417772.7050905@codeaurora.org>
References: <1446958380-23298-1-git-send-email-okaya@codeaurora.org>
 <5640A429.6090302@codeaurora.org> <56417772.7050905@codeaurora.org>
Message-ID: <3824948.EldtE7Hrx3@wuerfel>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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