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

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Date:   Tue, 30 Apr 2019 21:11:07 +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 5/7] MIPS: use the generic uncached segment support in
 dma-direct

Hi Christoph,

On Tue, Apr 30, 2019 at 10:29:47PM +0200, Christoph Hellwig wrote:
> 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.

Right but dma_direct_alloc_pages() already checks for the PageHighMem
case & returns before ever calling arch_dma_prep_coherent(), no?

Thanks,
    Paul