linux-kernel - Re: [PATCH] dma-direct: Export dma_direct_alloc() and dma_direct

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Message-ID: <20190206065749.GA23392@lst.de>
Date:   Wed, 6 Feb 2019 07:57:50 +0100
From:   Christoph Hellwig <hch@....de>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: Export dma_direct_alloc() and
 dma_direct_free()

On Tue, Feb 05, 2019 at 11:29:12PM +0100, Thierry Reding wrote:
> > > 	https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/host1x/cdma.c#n106
> > 
> > Yes, I think we I can come up with a nicer helper for that.
> 
> One thing I could also try is to remove direct IOMMU API usage at least
> from the host1x driver. I think this might work nowadays.
> 
> For Tegra DRM we still need direct IOMMU API usage because we need to be
> able to map into an IOMMU domain without knowing the struct device *
> that we're mapping for (it could be needed by any of up to four display
> controllers). For host1x we always only have one struct device *, so the
> DMA mapping API should be good enough.

If you can convert it to plain DMA API usage, please do.

I did look into an IOMMU API memory allocator, and while we can do it
easily for coherent devices, we need some arch hooks for non-coherent
device support.  With a pending series from me we have those for arm64,
but 32-bit arm support will require a lot more work first.