Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

From: Daniel Vetter <daniel@ffwll.ch>
To: "Clark, Rob" <robdclark@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>, Dave 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@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 15:43:50 +0100	[thread overview]
Message-ID: <CAKMK7uHOKtX2a12itSA0sDg=b77F+0vnLAdEvc_aNGRJTh85=Q@mail.gmail.com> (raw)
In-Reply-To: <CAF6AEGvsFbXGkC+=4oq1xXnoC2EVX2J0k-GqShefcek1=uw4Lw@mail.gmail.com>

On Thu, Nov 29, 2018 at 3:26 PM Rob Clark <robdclark@gmail.com> wrote:
>
> On Thu, Nov 29, 2018 at 9:14 AM Christoph Hellwig <hch@lst.de> wrote:
> >
> > On Thu, Nov 29, 2018 at 07:33:15PM +0530, Vivek Gautam wrote:
> > > dma_map_sg() expects a DMA domain. However, the drm devices
> > > have been traditionally using unmanaged iommu domain which
> > > is non-dma type. Using dma mapping APIs with that domain is bad.
> > >
> > > Replace dma_map_sg() calls with dma_sync_sg_for_device{|cpu}()
> > > to do the cache maintenance.
> >
> > As I told you before:  hell no.  If you spent the slightest amount of
> > actually trying to understand what you are doing here you'd know this
> > can't work.  Just turn on dma debugging and this will blow up in your
> > face.
>
> you can tone it down..  we weren't the ones who created the dma/iommu
> mess, we are just trying to find a way to work around it
>
> > Either you use the DMA API properly, that is you use it to map and
> > to sync, or you don't use it at all.  Mix and match between iommu
> > APIs and DMA APIs is simply not possible.
>
> I'd *love* nothing more to not use the dma api.. but on arm there is
> no other way to do cache maint.

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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch