linux-kernel - Re: [PATCH] drm/msm/a6xx+: Insert a fence wait before SMMU table update

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Message-ID: <CAF6AEGvgS-DD0+qGX_Mud75aES4AQQjmWx2j2gyz7uakTpnp0w@mail.gmail.com>
Date: Tue, 17 Sep 2024 08:30:25 -0700
From: Rob Clark <robdclark@...il.com>
To: Konrad Dybcio <konradybcio@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org, 
	freedreno@...ts.freedesktop.org, Akhil P Oommen <quic_akhilpo@...cinc.com>, 
	Connor Abbott <cwabbott0@...il.com>, Rob Clark <robdclark@...omium.org>, 
	Sean Paul <sean@...rly.run>, Abhinav Kumar <quic_abhinavk@...cinc.com>, 
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, 
	Marijn Suijten <marijn.suijten@...ainline.org>, David Airlie <airlied@...il.com>, 
	Daniel Vetter <daniel@...ll.ch>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm/a6xx+: Insert a fence wait before SMMU table update

On Tue, Sep 17, 2024 at 6:47 AM Konrad Dybcio <konradybcio@...nel.org> wrote:
>
> On 13.09.2024 9:51 PM, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > The CP_SMMU_TABLE_UPDATE _should_ be waiting for idle, but on some
> > devices (x1-85, possibly others), it seems to pass that barrier while
> > there are still things in the event completion FIFO waiting to be
> > written back to memory.
>
> Can we try to force-fault around here on other GPUs and perhaps
> limit this workaround?

not sure what you mean by "force-fault"... we could probably limit
this to certain GPUs, the only reason I didn't is (a) it should be
harmless when it is not needed, and (b) I have no real good way to get
an exhaustive list of where it is needed.  Maybe/hopefully it is only
x1-85, but idk.

It does bring up an interesting question about preemption, though

BR,
-R

> Akhil, do we have any insight on this?
>
> Konrad