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From: Rob Clark <robdclark@gmail.com>
To: hch@lst.de
Cc: Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Tomasz Figa <tfiga@chromium.org>,
	Sean Paul <seanpaul@chromium.org>,
	Vivek Gautam <vivek.gautam@codeaurora.org>,
	freedreno <freedreno@lists.freedesktop.org>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*
Date: Thu, 29 Nov 2018 20:15:23 -0500	[thread overview]
Message-ID: <CAF6AEGsS6LnuoL9+_LsKLOWxKF6P0nTrxs7b0j0Kqnqz6tmocw@mail.gmail.com> (raw)
In-Reply-To: <20181129155758.GC26537@lst.de>

On Thu, Nov 29, 2018 at 10:57 AM Christoph Hellwig <hch@lst.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

  parent reply	other threads:[~2018-11-30  1:15 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-29 14:03 [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg* Vivek Gautam
2018-11-29 14:14 ` Christoph Hellwig
2018-11-29 14:25   ` Rob Clark
2018-11-29 14:42     ` Rob Clark
2018-11-29 15:54       ` Christoph Hellwig
2018-11-29 18:48         ` Rob Clark
2018-11-29 19:40           ` Jordan Crouse
2018-11-29 19:57             ` Tomasz Figa
2018-11-29 20:03               ` Robin Murphy
2018-11-30  0:23                 ` Tomasz Figa
2018-12-01  2:05                   ` Tomasz Figa
2018-12-01 11:46                     ` Rob Clark
2018-12-03  0:12                       ` Tomasz Figa
2018-11-29 14:43     ` Daniel Vetter
2018-11-29 15:57       ` Christoph Hellwig
2018-11-29 16:28         ` Daniel Vetter
2018-11-29 16:57           ` Christoph Hellwig
2018-11-29 17:09             ` Daniel Vetter
2018-11-29 17:24               ` Tomasz Figa
2018-11-29 18:35                 ` Christoph Hellwig
2018-11-29 18:57                 ` Rob Clark
2018-11-30  9:40                   ` Daniel Vetter
2018-11-30  9:35                 ` Daniel Vetter
2018-11-30  9:44                   ` Christoph Hellwig
2018-11-29 18:33               ` Christoph Hellwig
2018-11-30  9:46                 ` Daniel Vetter
2018-12-07  1:38                   ` Christoph Hellwig
2018-12-07 14:29                     ` Rob Clark
2018-11-29 17:33             ` Brian Starkey
2018-11-29 18:35               ` Christoph Hellwig
2018-11-30  1:15         ` Rob Clark [this message]
2018-11-30  9:35           ` Christoph Hellwig
2018-11-29 15:53     ` Christoph Hellwig
2018-11-29 18:44       ` Rob Clark

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