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Date:   Thu, 8 Dec 2022 15:06:22 -0600
From:   Bjorn Andersson <andersson@kernel.org>
To:     Ulf Hansson <ulf.hansson@linaro.org>
Cc:     Akhil P Oommen <quic_akhilpo@quicinc.com>,
        freedreno <freedreno@lists.freedesktop.org>,
        dri-devel@lists.freedesktop.org, linux-arm-msm@vger.kernel.org,
        Rob Clark <robdclark@gmail.com>,
        Stephen Boyd <swboyd@chromium.org>,
        Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Douglas Anderson <dianders@chromium.org>,
        krzysztof.kozlowski@linaro.org,
        Abhinav Kumar <quic_abhinavk@quicinc.com>,
        Andy Gross <agross@kernel.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@linux.ie>,
        Konrad Dybcio <konrad.dybcio@somainline.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Rob Herring <robh+dt@kernel.org>, Sean Paul <sean@poorly.run>,
        Stephen Boyd <sboyd@kernel.org>, devicetree@vger.kernel.org,
        linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 0/6] clk/qcom: Support gdsc collapse polling using
 'reset' interface
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On Thu, Dec 08, 2022 at 02:40:55PM +0100, Ulf Hansson wrote:
> On Wed, 7 Dec 2022 at 17:55, Bjorn Andersson <andersson@kernel.org> wrote:
> >
> > On Wed, Dec 07, 2022 at 05:00:51PM +0100, Ulf Hansson wrote:
> > > On Thu, 1 Dec 2022 at 23:57, Bjorn Andersson <andersson@kernel.org> wrote:
> > > >
> > > > On Wed, Oct 05, 2022 at 02:36:58PM +0530, Akhil P Oommen wrote:
> > > > >
> > > >
> > > > @Ulf, Akhil has a power-domain for a piece of hardware which may be
> > > > voted active by multiple different subsystems (co-processors/execution
> > > > contexts) in the system.
> > > >
> > > > As such, during the powering down sequence we don't wait for the
> > > > power-domain to turn off. But in the event of an error, the recovery
> > > > mechanism relies on waiting for the hardware to settle in a powered off
> > > > state.
> > > >
> > > > The proposal here is to use the reset framework to wait for this state
> > > > to be reached, before continuing with the recovery mechanism in the
> > > > client driver.
> > >
> > > I tried to review the series (see my other replies), but I am not sure
> > > I fully understand the consumer part.
> > >
> > > More exactly, when and who is going to pull the reset and at what point?
> > >
> > > >
> > > > Given our other discussions on quirky behavior, do you have any
> > > > input/suggestions on this?
> > > >
> > > > > Some clients like adreno gpu driver would like to ensure that its gdsc
> > > > > is collapsed at hardware during a gpu reset sequence. This is because it
> > > > > has a votable gdsc which could be ON due to a vote from another subsystem
> > > > > like tz, hyp etc or due to an internal hardware signal. To allow
> > > > > this, gpucc driver can expose an interface to the client driver using
> > > > > reset framework. Using this the client driver can trigger a polling within
> > > > > the gdsc driver.
> > > >
> > > > @Akhil, this description is fairly generic. As we've reached the state
> > > > where the hardware has settled and we return to the client, what
> > > > prevents it from being powered up again?
> > > >
> > > > Or is it simply a question of it hitting the powered-off state, not
> > > > necessarily staying there?
> > >
> > > Okay, so it's indeed the GPU driver that is going to assert/de-assert
> > > the reset at some point. Right?
> > >
> > > That seems like a reasonable approach to me, even if it's a bit
> > > unclear under what conditions that could happen.
> > >
> >
> > Generally the disable-path of the power-domain does not check that the
> > power-domain is actually turned off, because the status might indicate
> > that the hardware is voting for the power-domain to be on.
> 
> Is there a good reason why the HW needs to vote too, when the GPU
> driver is already in control?
> 
> Or perhaps that depends on the running use case?
> 
> >
> > As part of the recovery of the GPU after some fatal fault, the GPU
> > driver does something which will cause the hardware votes for the
> > power-domain to be let go, and then the driver does pm_runtime_put().
> 
> Okay. That "something", sounds like a device specific setting for the
> corresponding gdsc, right?
> 
> So somehow the GPU driver needs to manage that setting, right?
> 
> >
> > But in this case the GPU driver wants to ensure that the power-domain is
> > actually powered down, before it does pm_runtime_get() again. To ensure
> > that the hardware lost its state...
> 
> I see.
> 
> >
> > The proposal here is to use a reset to reach into the power-domain
> > provider and wait for the hardware to be turned off, before the GPU
> > driver attempts turning the power-domain on again.
> >
> >
> > In other words, there is no reset. This is a hack to make a normally
> > asynchronous pd.power_off() to be synchronous in this particular case.
> 
> Alright, assuming I understood your clarifications above correctly
> (thanks!), I think I have got a much better picture now.
> 
> Rather than abusing the reset interface, I think we should manage this
> through the genpd's power on/off notifiers (GENPD_NOTIFY_OFF). The GPU
> driver should register its corresponding device for them
> (dev_pm_genpd_add_notifier()).
> 
> The trick however, is to make the behaviour of the power-domain for
> the gdsc (the genpd->power_off() callback) conditional on whether the
> HW is configured to vote or not. If the HW can vote, it should not
> poll for the state - and vice versa when the HW can't vote.
> 

Per Akhil's description I misunderstood who the other voters are; but
either way it's not the same "HW configured" mechanism as the one we're
already discussing.


But if we based on similar means could control if the power_off() ops
should be blocking, waiting for the status indication to show that the
hardware is indeed powered down, I think this would meet the needs.

And GENPD_NOTIFY_OFF seems to provide the notification that it was
successful (i.e. happened within the timeout etc).

> Would this work?
> 

If we can control the behavior of the genpd, I think it would.

Thanks,
Bjorn