linux-kernel - Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

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Date:   Thu, 15 Oct 2020 16:43:00 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>,
        Robin Murphy <robin.murphy@....com>,
        Jonathan Marek <jonathan@...ek.ca>,
        David Airlie <airlied@...ux.ie>,
        freedreno@...ts.freedesktop.org,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>,
        iommu@...ts.linux-foundation.org,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>, Sean Paul <sean@...rly.run>
Subject: Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent
 cache maintenance

On Thu, Oct 15, 2020 at 05:33:34PM +0200, Daniel Vetter wrote:
> On Thu, Oct 15, 2020 at 07:55:32AM +0100, Christoph Hellwig wrote:
> > On Tue, Oct 13, 2020 at 02:42:38PM +0100, Robin Murphy wrote:
> > > I still think this situation would be best handled with a variant of
> > > dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set
> > > automatically when attaching to an unmanaged IOMMU domain.
> > 
> > dma_ops_bypass should mostly do the right thing as-is.  swiotlb bouncing
> > is triggered of two things:
> > 
> >  1) the dma_mask.  This is under control of the driver, and obviously
> >     if it is too small for a legit reason we can't just proceed
> 
> Somewhat related, but is there a way to tell the dma-api to fail instead
> of falling back to swiotlb? In many case for gpu drivers it's much better
> if we fall back to dma_alloc_coherent and manage the copying ourselves
> instead of abstracting this away in the dma-api. Currently that's "solved"
> rather pessimistically by always allocating from dma_alloc_coherent if
> swiotlb could be in the picture (at least for ttm based drivers, i915 just
> falls over).

Is this for the alloc_pages plus manually map logic in various drivers?

They should switch to the new dma_alloc_pages API that I'll send to
Linus for 5.10 soon.