Re: [PATCH 1/2] iommu/arm-smmu-qcom: Skip the TTBR1 quirk for db820c.

From: Bjorn Andersson <bjorn.andersson@linaro.org>
To: Will Deacon <will@kernel.org>
Cc: Rob Clark <robdclark@gmail.com>, Eric Anholt <eric@anholt.net>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	freedreno <freedreno@lists.freedesktop.org>,
	Sean Paul <sean@poorly.run>,
	Jordan Crouse <jcrouse@codeaurora.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Rob Herring <robh+dt@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	"moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
	<linux-arm-kernel@lists.infradead.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] iommu/arm-smmu-qcom: Skip the TTBR1 quirk for db820c.
Date: Tue, 25 May 2021 12:14:45 -0500	[thread overview]
Message-ID: <YK0whQrWpehkxTrL@builder.lan> (raw)
In-Reply-To: <20210330153050.GB6567@willie-the-truck>

On Tue 30 Mar 10:31 CDT 2021, Will Deacon wrote:

> On Tue, Mar 30, 2021 at 08:03:36AM -0700, Rob Clark wrote:
> > On Tue, Mar 30, 2021 at 2:34 AM Will Deacon <will@kernel.org> wrote:
> > >
> > > On Mon, Mar 29, 2021 at 09:02:50PM -0700, Rob Clark wrote:
> > > > On Mon, Mar 29, 2021 at 7:47 AM Will Deacon <will@kernel.org> wrote:
> > > > >
> > > > > On Fri, Mar 26, 2021 at 04:13:02PM -0700, Eric Anholt wrote:
> > > > > > db820c wants to use the qcom smmu path to get HUPCF set (which keeps
> > > > > > the GPU from wedging and then sometimes wedging the kernel after a
> > > > > > page fault), but it doesn't have separate pagetables support yet in
> > > > > > drm/msm so we can't go all the way to the TTBR1 path.
> > > > >
> > > > > What do you mean by "doesn't have separate pagetables support yet"? The
> > > > > compatible string doesn't feel like the right way to determine this.
> > > >
> > > > the compatible string identifies what it is, not what the sw
> > > > limitations are, so in that regard it seems right to me..
> > >
> > > Well it depends on what "doesn't have separate pagetables support yet"
> > > means. I can't tell if it's a hardware issue, a firmware issue or a driver
> > > issue.
> > 
> > Just a driver issue (and the fact that currently we don't have
> > physical access to a device... debugging a5xx per-process-pgtables by
> > pushing untested things to the CI farm is kind of a difficult way to
> > work)
> 
> But then in that case, this is using the compatible string to identify a
> driver issue, no?
> 

No the compatible addition identifies the hardware, the implementation
then uses this information to know that it needs to behave "differently"
on this platform.

When/if someone decides to add the necessary support in the driver they
can remove the driver quirk, but it doesn't invalidate the specific
compatible.

Regards,
Bjorn