linux-kernel - Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Message-ID: <20181130094604.GV21184@phenom.ffwll.local>
Date:   Fri, 30 Nov 2018 10:46:04 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christoph Hellwig <hch@....de>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        "Clark, Rob" <robdclark@...il.com>, Dave Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Sean Paul <seanpaul@...omium.org>, vivek.gautam@...eaurora.org,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

On Thu, Nov 29, 2018 at 07:33:34PM +0100, Christoph Hellwig wrote:
> On Thu, Nov 29, 2018 at 06:09:05PM +0100, Daniel Vetter wrote:
> > What kind of abuse do you expect? It could very well be that gpu folks
> > call that "standard use case" ... At least on x86 with the i915 driver
> > we pretty much rely on architectural guarantees for how cache flushes
> > work very much. Down to userspace doing the cache flushing for
> > mappings the kernel has set up.
> 
> Mostly the usual bypasses of the DMA API because people know better
> (and with that I don't mean low-level IOMMU API users, but "creative"
> direct mappings).

Ah yes. I think that even gpu folks get behind :-) Best if someone
bothered to explicitly cast to dma_addr_t to shut up the tools, but not
fix the bug ...

> > > As for the buffer sharing: at least for the DMA API side I want to
> > > move the current buffer sharing users away from dma_alloc_coherent
> > > (and coherent dma_alloc_attrs users) and the remapping done in there
> > > required for non-coherent architectures.  Instead I'd like to allocate
> > > plain old pages, and then just dma map them for each device separately,
> > > with DMA_ATTR_SKIP_CPU_SYNC passed for all but the first user to map
> > > or last user to unmap.  On the iommu side it could probably work
> > > similar.
> > 
> > I think this is what's often done. Except then there's also the issue
> > of how to get at the cma allocator if your device needs something
> > contiguous. There's a lot of that still going on in graphics/media.
> 
> Being able to dip into CMA and mayb iommu coalescing if we want to
> get fancy is indeed the only reason for this API.  If we just wanted
> to map pages we could already do that now with just a little bit
> of boilerplate code (and quite a few drivers do - just adding this
> new API will remove tons of code).

Sounds like the future is very bright indeed \o/
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch