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From:   Paul Burton <paul.burton@mips.com>
To:     Christoph Hellwig <hch@lst.de>
CC:     Ralf Baechle <ralf@linux-mips.org>,
        James Hogan <jhogan@kernel.org>,
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        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>
Subject: Re: [PATCH 4/7] dma-direct: provide generic support for uncached
 kernel segments
Thread-Topic: [PATCH 4/7] dma-direct: provide generic support for uncached
 kernel segments
Thread-Index: AQHU/0QKKKiJlPSrw0CLkNp0FmAaxqZWhWOAgAAC3QCAAAMrgA==
Date:   Wed, 1 May 2019 17:40:34 +0000
Message-ID: <20190501174033.6rj5aiopdeo4uqpw@pburton-laptop>
References: <20190430110032.25301-1-hch@lst.de>
 <20190430110032.25301-5-hch@lst.de>
 <20190501171857.chfxqntvm6r4xrr4@pburton-laptop>
 <20190501172912.GA19375@lst.de>
In-Reply-To: <20190501172912.GA19375@lst.de>
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Hi Christoph,

On Wed, May 01, 2019 at 07:29:12PM +0200, Christoph Hellwig wrote:
> On Wed, May 01, 2019 at 05:18:59PM +0000, Paul Burton wrote:
> > I'm not so sure about this part though.
> >=20
> > On MIPS we currently don't clear the allocated memory with memset. Is
> > doing that really necessary?
>=20
> We are clearling it on mips, it is inside dma_direct_alloc_pages.

Ah, of course, I clearly require more caffeine :)

> > If it is necessary then as-is this code will clear the allocated memory
> > using uncached writes which will be pretty slow. It would be much more
> > efficient to perform the memset before arch_dma_prep_coherent() & befor=
e
> > converting ret to an uncached address.
>=20
> Yes, we could do that.

Great; using cached writes would match the existing MIPS behavior.

Thanks,
    Paul