linux-kernel - Re: [PATCH 5/7] MIPS: use the generic uncached segment support in dma-direct

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Message-ID: <20190430202947.GA30262@lst.de>
Date:   Tue, 30 Apr 2019 22:29:47 +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 5/7] MIPS: use the generic uncached segment support in
 dma-direct

On Tue, Apr 30, 2019 at 08:10:43PM +0000, Paul Burton wrote:
> This series looks like a nice cleanup to me - the one thing that puzzles
> me is the !PageHighMem check above.
> 
> As far as I can see arch_dma_prep_coherent() should never be called with
> a highmem page, so would it make more sense to either drop this check or
> perhaps wrap it in a WARN_ON()?

dma_alloc_from_contigous can return highmem pages depending on where
the CMA area is located.  But given that these pages don't have a
direct kernel mapping we also shouldn't have to flush the caches
for them.