linux-kernel - Re: [Freedreno] [PATCH v2 5/7] arm64: dts: qcom: sc7280: Update gpu register list

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Date:   Fri, 22 Jul 2022 08:28:03 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Akhil P Oommen <quic_akhilpo@...cinc.com>
Cc:     Rajendra Nayak <quic_rjendra@...cinc.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Taniya Das <quic_tdas@...cinc.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Jonathan Marek <jonathan@...ek.ca>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Andy Gross <agross@...nel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Herring <robh+dt@...nel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jordan Crouse <jordan@...micpenguin.net>,
        freedreno <freedreno@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Freedreno] [PATCH v2 5/7] arm64: dts: qcom: sc7280: Update gpu
 register list

On Thu, Jul 21, 2022 at 9:04 AM Akhil P Oommen <quic_akhilpo@...cinc.com> wrote:
>
> On 7/20/2022 11:34 AM, Akhil P Oommen wrote:
> > On 7/19/2022 3:26 PM, Rajendra Nayak wrote:
> >>
> >>
> >> On 7/19/2022 12:49 PM, Stephen Boyd wrote:
> >>> Quoting Akhil P Oommen (2022-07-18 23:37:16)
> >>>> On 7/19/2022 11:19 AM, Stephen Boyd wrote:
> >>>>> Quoting Akhil P Oommen (2022-07-18 21:07:05)
> >>>>>> On 7/14/2022 11:10 AM, Akhil P Oommen wrote:
> >>>>>>> IIUC, qcom gdsc driver doesn't ensure hardware is collapsed
> >>>>>>> since they
> >>>>>>> are vote-able switches. Ideally, we should ensure that the hw has
> >>>>>>> collapsed for gpu recovery because there could be transient
> >>>>>>> votes from
> >>>>>>> other subsystems like hypervisor using their vote register.
> >>>>>>>
> >>>>>>> I am not sure how complex the plumbing to gpucc driver would be
> >>>>>>> to allow
> >>>>>>> gpu driver to check hw status. OTOH, with this patch, gpu driver
> >>>>>>> does a
> >>>>>>> read operation on a gpucc register which is in always-on domain.
> >>>>>>> That
> >>>>>>> means we don't need to vote any resource to access this register.
> >>>
> >>> Reading between the lines here, you're saying that you have to read the
> >>> gdsc register to make sure that the gdsc is in some state? Can you
> >>> clarify exactly what you're doing? And how do you know that something
> >>> else in the kernel can't cause the register to change after it is read?
> >>> It certainly seems like we can't be certain because there is voting
> >>> involved.
> > From gpu driver, cx_gdscr.bit[31] (power off status) register can be
> > polled to ensure that it *collapsed at least once*. We don't need to
> > care if something turns ON gdsc after that.
> >
> >>
> >> yes, this looks like the best case effort to get the gpu to recover, but
> >> the kernel driver really has no control to make sure this condition can
> >> always be met (because it depends on other entities like hyp,
> >> trustzone etc right?)
> >> Why not just put a worst case polling delay?
> >
> > I didn't get you entirely. Where do you mean to keep the polling delay?
> >>
> >>>
> >>>>>>>
> >>>>>>> Stephen/Rajendra/Taniya, any suggestion?
> >>>>> Why can't you assert a gpu reset signal with the reset APIs? This
> >>>>> series
> >>>>> seems to jump through a bunch of hoops to get the gdsc and power
> >>>>> domain
> >>>>> to "reset" when I don't know why any of that is necessary. Can't we
> >>>>> simply assert a reset to the hardware after recovery completes so the
> >>>>> device is back into a good known POR (power on reset) state?
> >>>> That is because there is no register interface to reset GPU CX domain.
> >>>> The recommended sequence from HW design folks is to collapse both
> >>>> cx and
> >>>> gx gdsc to properly reset gpu/gmu.
> >>>>
> >>>
> >>> Ok. One knee jerk reaction is to treat the gdsc as a reset then and
> >>> possibly mux that request along with any power domain on/off so that if
> >>> the reset is requested and the power domain is off nothing happens.
> >>> Otherwise if the power domain is on then it manually sequences and
> >>> controls the two gdscs so that the GPU is reset and then restores the
> >>> enable state of the power domain.
> > It would be fatal to asynchronously pull the plug on CX gdsc
> > forcefully because there might be another gpu/smmu driver thread
> > accessing registers in cx domain.
> >
> > -Akhil.
> >
> But, we can move the cx collapse polling to gpucc and expose it to gpu
> driver using 'reset' framework. I am not very familiar with clk driver,
> but I did a rough prototype here (untested):
> https://zerobin.net/?d34b5f958be3b9b8#NKGzdPy9fgcuOqXZ/XqjI7b8JWcivqe+oSTf4yWHSOU=
>
> If this approach is acceptable, I will send it out as a separate series.
>

I'm not super familiar w/ reset framework, but this approach seems
like it would avoid needing to play games with working around runpm as
well.  So that seems like a cleaner approach.

BR,
-R