netdev - Re: [net-next PATCH RFC 04/26] arch/arm: Add option to skip sync on DMA map and unmap

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Message-ID: <20161031102057.GZ1041@n2100.armlinux.org.uk>
Date:   Mon, 31 Oct 2016 10:20:57 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Alexander Duyck <alexander.h.duyck@...el.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, brouer@...hat.com, davem@...emloft.net
Subject: Re: [net-next PATCH RFC 04/26] arch/arm: Add option to skip sync on
 DMA map and unmap

On Mon, Oct 24, 2016 at 08:04:47AM -0400, Alexander Duyck wrote:
> The use of DMA_ATTR_SKIP_CPU_SYNC was not consistent across all of the DMA
> APIs in the arch/arm folder.  This change is meant to correct that so that
> we get consistent behavior.

I'm really not convinced that this is anywhere close to correct behaviour.

If we're DMA-ing to a buffer, and we unmap it or sync_for_cpu, then we
will want to access the DMA'd data - especially in the sync_for_cpu case,
it's pointless to call sync_for_cpu if we're not going to access the
data.

So the idea of skipping the CPU copy when DMA_ATTR_SKIP_CPU_SYNC is set
seems to be completely wrong - it means we end up reading the stale data
that was in the buffer, completely ignoring whatever was DMA'd to it.

What's the use case for DMA_ATTR_SKIP_CPU_SYNC ?

-- 
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