linux-kernel - Re: [PATCH 4/7] dma-direct: provide generic support for uncached kernel segments

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Date:   Wed, 1 May 2019 17:40:34 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Christoph Hellwig <hch@....de>
CC:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Ley Foon Tan <lftan@...era.com>,
        Michal Simek <monstr@...str.eu>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>
Subject: Re: [PATCH 4/7] dma-direct: provide generic support for uncached
 kernel segments

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.
> > 
> > On MIPS we currently don't clear the allocated memory with memset. Is
> > doing that really necessary?
> 
> 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() & before
> > converting ret to an uncached address.
> 
> Yes, we could do that.

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

Thanks,
    Paul