linux-kernel - Re: [Question] devm

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Message-ID: <20170308195901.GD21222@n2100.armlinux.org.uk>
Date:   Wed, 8 Mar 2017 19:59:01 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Lars-Peter Clausen <lars@...afoo.de>
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 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_*().)

Updates that are made by the CPU on overlapping cache lines while the
memory is mapped for DMA may end up doing one of two things: either
overwriting the newly DMA'd data, or being lost altogether.

In the case of devm_* list manipulations, these should only ever happen
during device probe and tear down, and if DMA is active at those times,
the driver is seriously buggy.

-- 
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