linux-kernel - Re: [PATCH v7 0/6] clk/qcom: Support gdsc collapse polling using 'reset' interface

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Message-ID: <20221207165457.kwdwwiycbwjpogxl@builder.lan>
Date:   Wed, 7 Dec 2022 10:54:57 -0600
From:   Bjorn Andersson <andersson@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Akhil P Oommen <quic_akhilpo@...cinc.com>,
        freedreno <freedreno@...ts.freedesktop.org>,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        Rob Clark <robdclark@...il.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Douglas Anderson <dianders@...omium.org>,
        krzysztof.kozlowski@...aro.org,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>, Sean Paul <sean@...rly.run>,
        Stephen Boyd <sboyd@...nel.org>, devicetree@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 0/6] clk/qcom: Support gdsc collapse polling using
 'reset' interface

On Wed, Dec 07, 2022 at 05:00:51PM +0100, Ulf Hansson wrote:
> On Thu, 1 Dec 2022 at 23:57, Bjorn Andersson <andersson@...nel.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.

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().

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...

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.

Regards,
Bjorn