linux-kernel - Re: [PATCH] au1200fb: don't use DMA_ATTR_NON

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Message-ID: <CAOLZvyF0NsB6_e0=Vat48Mq1r-Qy45z509jAgQczG7RoaOqjBA@mail.gmail.com>
Date:   Wed, 26 Jun 2019 09:16:46 +0200
From:   Manuel Lauss <manuel.lauss@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Linux-MIPS <linux-mips@...r.kernel.org>,
        linux-fbdev <linux-fbdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] au1200fb: don't use DMA_ATTR_NON_CONSISTENT

On Tue, Jun 25, 2019 at 2:13 PM Christoph Hellwig <hch@....de> wrote:
>
> au1200fb allocates DMA memory using DMA_ATTR_NON_CONSISTENT, but never
> calls dma_cache_sync to synchronize the memory between the CPU and the
> device.  If it was use on a not cache coherent bus that would be fatal,
> but as far as I can tell from the naming and the mips platform
> implementation it always is used in cache coherent systems.  Remove
> the DMA_ATTR_NON_CONSISTENT flag, which is a no-op in that case.

Very early au1200 chips, on which this driver apparently was developed on,
had issues with cache coherency, but this was fixed in a later step,
none of the 3 steppings I have access to exhibit any problems
with this patch applied.

> Signed-off-by: Christoph Hellwig <hch@....de>

Acked-By: Manuel Lauss <manuel.lauss@...il.com>