linux-kernel - Re: [PATCH] drm/msm: Disable frequency clamping on a630

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Date:   Thu, 9 Sep 2021 21:42:52 +0530
From:   Amit Pundir <amit.pundir@...aro.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Akhil P Oommen <akhilpo@...eaurora.org>,
        Caleb Connolly <caleb.connolly@...aro.org>,
        Rob Clark <robdclark@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jordan Crouse <jordan@...micpenguin.net>,
        Jonathan Marek <jonathan@...ek.ca>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        open list <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH] drm/msm: Disable frequency clamping on a630

On Thu, 9 Sept 2021 at 17:47, Amit Pundir <amit.pundir@...aro.org> wrote:
>
> On Wed, 8 Sept 2021 at 07:50, Bjorn Andersson
> <bjorn.andersson@...aro.org> wrote:
> >
> > On Mon 09 Aug 10:26 PDT 2021, Akhil P Oommen wrote:
> >
> > > On 8/9/2021 9:48 PM, Caleb Connolly wrote:
> > > >
> > > >
> > > > On 09/08/2021 17:12, Rob Clark wrote:
> > > > > On Mon, Aug 9, 2021 at 7:52 AM Akhil P Oommen
> > > > > <akhilpo@...eaurora.org> wrote:
> > [..]
> > > > > > I am a bit confused. We don't define a power domain for gpu in dt,
> > > > > > correct? Then what exactly set_opp do here? Do you think this usleep is
> > > > > > what is helping here somehow to mask the issue?
> > > > The power domains (for cx and gx) are defined in the GMU DT, the OPPs in
> > > > the GPU DT. For the sake of simplicity I'll refer to the lowest
> > > > frequency (257000000) and OPP level (RPMH_REGULATOR_LEVEL_LOW_SVS) as
> > > > the "min" state, and the highest frequency (710000000) and OPP level
> > > > (RPMH_REGULATOR_LEVEL_TURBO_L1) as the "max" state. These are defined in
> > > > sdm845.dtsi under the gpu node.
> > > >
> > > > The new devfreq behaviour unmasks what I think is a driver bug, it
> > > > inadvertently puts much more strain on the GPU regulators than they
> > > > usually get. With the new behaviour the GPU jumps from it's min state to
> > > > the max state and back again extremely rapidly under workloads as small
> > > > as refreshing UI. Where previously the GPU would rarely if ever go above
> > > > 342MHz when interacting with the device, it now jumps between min and
> > > > max many times per second.
> > > >
> > > > If my understanding is correct, the current implementation of the GMU
> > > > set freq is the following:
> > > >   - Get OPP for frequency to set
> > > >   - Push the frequency to the GMU - immediately updating the core clock
> > > >   - Call dev_pm_opp_set_opp() which triggers a notify chain, this winds
> > > > up somewhere in power management code and causes the gx regulator level
> > > > to be updated
> > >
> > > Nope. dev_pm_opp_set_opp() sets the bandwidth for gpu and nothing else. We
> > > were using a different api earlier which got deprecated -
> > > dev_pm_opp_set_bw().
> > >
> >
> > On the Lenovo Yoga C630 this is reproduced by starting alacritty and if
> > I'm lucky I managed to hit a few keys before it crashes, so I spent a
> > few hours looking into this as well...
> >
> > As you say, the dev_pm_opp_set_opp() will only cast a interconnect vote.
> > The opp-level is just there for show and isn't used by anything, at
> > least not on 845.
> >
> > Further more, I'm missing something in my tree, so the interconnect
> > doesn't hit sync_state, and as such we're not actually scaling the
> > buses. So the problem is not that Linux doesn't turn on the buses in
> > time.
> >
> > So I suspect that the "AHB bus error" isn't saying that we turned off
> > the bus, but rather that the GPU becomes unstable or something of that
> > sort.
> >
> >
> > Lastly, I reverted 9bc95570175a ("drm/msm: Devfreq tuning") and ran
> > Aquarium for 20 minutes without a problem. I then switched the gpu
> > devfreq governor to "userspace" and ran the following:
> >
> > while true; do
> >   echo 257000000 > /sys/class/devfreq/5000000.gpu/userspace/set_freq
> >   echo 710000000 > /sys/class/devfreq/5000000.gpu/userspace/set_freq
> > done
> >
> > It took 19 iterations of this loop to crash the GPU.
>
> Ack. With your above script, I can reproduce a crash too on db845c
> (A630) running v5.14. I didn't get any crash log though and device
> just rebooted to USB crash mode.
>
> And same crash on RB5 (A650) too https://hastebin.com/raw/ejutetuwun

fwiw I can't reproduce this crash on RB5 so far with v5.15-rc1 merge
window (HEAD: 477f70cd2a67)

>
> >
> > So the problem doesn't seem to be Rob's change, it's just that prior to
> > it the chance to hitting it is way lower. Question is still what it is
> > that we're triggering.
> >
> > Regards,
> > Bjorn