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

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Message-ID: <20190501172912.GA19375@lst.de>
Date:   Wed, 1 May 2019 19:29:12 +0200
From:   Christoph Hellwig <hch@....de>
To:     Paul Burton <paul.burton@...s.com>
Cc:     Christoph Hellwig <hch@....de>, 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

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.

> 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.

View attachment "0001-dma-direct-provide-generic-support-for-uncached-kern.patch" of type "text/x-patch" (3567 bytes)