linux-kernel - Re: [Freedreno] [PATCH 0/3] iommu/drm/msm: Allow non-coherent masters to use system cache

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Message-ID: <20210809145651.GC1458@willie-the-truck>
Date:   Mon, 9 Aug 2021 15:56:51 +0100
From:   Will Deacon <will@...nel.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Georgi Djakov <djakov@...nel.org>,
        "Isaac J. Manjarres" <isaacm@...eaurora.org>,
        David Airlie <airlied@...ux.ie>,
        Akhil P Oommen <akhilpo@...eaurora.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sean Paul <sean@...rly.run>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Kristian H Kristensen <hoegsberg@...gle.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel@...ll.ch>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robin Murphy <robin.murphy@....com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Freedreno] [PATCH 0/3] iommu/drm/msm: Allow non-coherent
 masters to use system cache

On Mon, Aug 02, 2021 at 06:36:04PM -0700, Rob Clark wrote:
> On Mon, Aug 2, 2021 at 8:14 AM Will Deacon <will@...nel.org> wrote:
> >
> > On Mon, Aug 02, 2021 at 08:08:07AM -0700, Rob Clark wrote:
> > > On Mon, Aug 2, 2021 at 3:55 AM Will Deacon <will@...nel.org> wrote:
> > > >
> > > > On Thu, Jul 29, 2021 at 10:08:22AM +0530, Sai Prakash Ranjan wrote:
> > > > > On 2021-07-28 19:30, Georgi Djakov wrote:
> > > > > > On Mon, Jan 11, 2021 at 07:45:02PM +0530, Sai Prakash Ranjan wrote:
> > > > > > > commit ecd7274fb4cd ("iommu: Remove unused IOMMU_SYS_CACHE_ONLY flag")
> > > > > > > removed unused IOMMU_SYS_CACHE_ONLY prot flag and along with it went
> > > > > > > the memory type setting required for the non-coherent masters to use
> > > > > > > system cache. Now that system cache support for GPU is added, we will
> > > > > > > need to set the right PTE attribute for GPU buffers to be sys cached.
> > > > > > > Without this, the system cache lines are not allocated for GPU.
> > > > > > >
> > > > > > > So the patches in this series introduces a new prot flag IOMMU_LLC,
> > > > > > > renames IO_PGTABLE_QUIRK_ARM_OUTER_WBWA to IO_PGTABLE_QUIRK_PTW_LLC
> > > > > > > and makes GPU the user of this protection flag.
> > > > > >
> > > > > > Thank you for the patchset! Are you planning to refresh it, as it does
> > > > > > not apply anymore?
> > > > > >
> > > > >
> > > > > I was waiting on Will's reply [1]. If there are no changes needed, then
> > > > > I can repost the patch.
> > > >
> > > > I still think you need to handle the mismatched alias, no? You're adding
> > > > a new memory type to the SMMU which doesn't exist on the CPU side. That
> > > > can't be right.
> > > >
> > >
> > > Just curious, and maybe this is a dumb question, but what is your
> > > concern about mismatched aliases?  I mean the cache hierarchy on the
> > > GPU device side (anything beyond the LLC) is pretty different and
> > > doesn't really care about the smmu pgtable attributes..
> >
> > If the CPU accesses a shared buffer with different attributes to those which
> > the device is using then you fall into the "mismatched memory attributes"
> > part of the Arm architecture. It's reasonably unforgiving (you should go and
> > read it) and in some cases can apply to speculative accesses as well, but
> > the end result is typically loss of coherency.
> 
> Ok, I might have a few other sections to read first to decipher the
> terminology..
> 
> But my understanding of LLC is that it looks just like system memory
> to the CPU and GPU (I think that would make it "the point of
> coherence" between the GPU and CPU?)  If that is true, shouldn't it be
> invisible from the point of view of different CPU mapping options?

You could certainly build a system where mismatched attributes don't cause
loss of coherence, but as it's not guaranteed by the architecture and the
changes proposed here affect APIs which are exposed across SoCs, then I
don't think it helps much.

Will