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

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Date:   Thu, 29 Nov 2018 20:15:23 -0500
From:   Rob Clark <robdclark@...il.com>
To:     hch@....de
Cc:     Daniel Vetter <daniel@...ll.ch>, David 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 <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 10:57 AM Christoph Hellwig <hch@....de> wrote:
>
> On Thu, Nov 29, 2018 at 03:43:50PM +0100, Daniel Vetter wrote:
> > Yeah we had patches to add manual cache management code to drm, so we
> > don't have to abuse the dma streaming api anymore. Got shouted down.
> > Abusing the dma streaming api also gets shouted down. It's a gpu, any
> > idea of these drivers actually being platform independent is out of
> > the window from the start anyway, so we're ok with tying this to
> > platforms.
>
> Manual or not the iommu API is missing APIs for cache management,
> which makes it kinda surprising it actually ever worked for non-coherent
> devices.
>
> And fortunately while some people spent the last year ot two bickering
> about the situation others actually did work, and we now have a
> generic arch_sync_dma_for_device/arch_sync_dma_for_cpu kernel-internal
> API.  This is only used for DMA API internals so far, and explicitly
> not intended for direct driver use, but it would be perfect as the
> backend for iommu API cache maintainance functions.  It exists on all
> but two architectures on mainline.  Out of those powerpc is in the works,
> only arm32 will need some major help.

oh, hmm, I realized I'd been looking still at the armv7 dma-mapping, I
hadn't noticed arch_sync_* yet.. that does look like a step in the
right direction.

As far as hiding cache ops behind iommu layer, I guess I'd been
thinking more of just letting the drivers that want to bypass dma
layer take things into their own hands.. tbh I think/assume/hope
drm/msm is more the exception than the rule as far as needing to
bypass the dma layer.  Or at least I guess the # of drivers having
problems w/ the dma layer is less than the # of iommu drivers.

(Not sure if that changes my thoughts on this patch, it isn't like
what this patch replaces isn't also a problematic hack around the
inability to bypass the dma layer.  In the short term I just want
*something* that works, I'm totally happy to refactor later when there
are better options.)

BR,
-R