linux-kernel - Re: [Question] devm

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Date:   Wed, 8 Mar 2017 21:44:17 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Robin Murphy <robin.murphy@....com>, dmaengine@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Tejun Heo <tj@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [Question] devm_kmalloc() for DMA ?

On 03/08/2017 08:59 PM, Russell King - ARM Linux wrote:
> On Wed, Mar 08, 2017 at 08:48:31PM +0100, Lars-Peter Clausen wrote:
>> When the DMA memory is mapped for reading from the device the associated
>> cachelines are invalidated without writeback. There is no guarantee that
>> the changes made to the devres_node have made it to main memory yet, or
>> is there?
> 
> That is incorrect.
> 
> Overlapping cache lines are always written back on transitions from CPU
> to device ownership of the buffer (eg, dma_map_*().)

On ARM. But my understanding is that this is not a universal requirement
according to DMA-API.txt. It says that mappings must be cache line aligned
and otherwise behavior is undefined.