linux-kernel - Re: [PATCH v3 1/3] clk: qcom: gdsc: Fix the handling of PWRSTS

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Date:   Wed, 21 Sep 2022 11:18:10 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Rajendra Nayak <quic_rjendra@...cinc.com>, agross@...nel.org,
        andersson@...nel.org, konrad.dybcio@...ainline.org,
        mturquette@...libre.com, sboyd@...nel.org, mka@...omium.org,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>
Cc:     linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        johan+linaro@...nel.org, quic_kriskura@...cinc.com,
        dianders@...omium.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH v3 1/3] clk: qcom: gdsc: Fix the handling of PWRSTS_RET
 support

Il 21/09/22 11:05, Rajendra Nayak ha scritto:
> 
> On 9/21/2022 1:21 PM, AngeloGioacchino Del Regno wrote:
>> Il 20/09/22 15:39, Rajendra Nayak ha scritto:
>>>
>>> On 9/20/2022 6:09 PM, AngeloGioacchino Del Regno wrote:
>>>> Il 20/09/22 13:15, Rajendra Nayak ha scritto:
>>>>> GDSCs cannot be transitioned into a Retention state in SW.
>>>>> When either the RETAIN_MEM bit, or both the RETAIN_MEM and
>>>>> RETAIN_PERIPH bits are set, and the GDSC is left ON, the HW
>>>>> takes care of retaining the memory/logic for the domain when
>>>>> the parent domain transitions to power collapse/power off state.
>>>>>
>>>>> On some platforms where the parent domains lowest power state
>>>>> itself is Retention, just leaving the GDSC in ON (without any
>>>>> RETAIN_MEM/RETAIN_PERIPH bits being set) will also transition
>>>>> it to Retention.
>>>>>
>>>>> The existing logic handling the PWRSTS_RET seems to set the
>>>>> RETAIN_MEM/RETAIN_PERIPH bits if the cxcs offsets are specified
>>>>> but then explicitly turns the GDSC OFF as part of _gdsc_disable().
>>>>> Fix that by leaving the GDSC in ON state.
>>>>>
>>>>> Signed-off-by: Rajendra Nayak <quic_rjendra@...cinc.com>
>>>>> Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
>>>>> ---
>>>>> v3:
>>>>> Updated changelog
>>>>>
>>>>> There are a few existing users of PWRSTS_RET and I am not
>>>>> sure if they would be impacted with this change
>>>>>
>>>>> 1. mdss_gdsc in mmcc-msm8974.c, I am expecting that the
>>>>> gdsc is actually transitioning to OFF and might be left
>>>>> ON as part of this change, atleast till we hit system wide
>>>>> low power state.
>>>>> If we really leak more power because of this
>>>>> change, the right thing to do would be to update .pwrsts for
>>>>> mdss_gdsc to PWRSTS_OFF_ON instead of PWRSTS_RET_ON
>>>>> I dont have a msm8974 hardware, so if anyone who has can report
>>>>> any issues I can take a look further on how to fix it.
>>>>
>>>> I think that the safest option is to add a PWRSTS_RET_HW_CTRL flag (or similar),
>>>> used for the specific cases of SC7180 and SC7280 (and possibly others) where the
>>>> GDSC is automatically transitioned to a Retention state by HW control, with no
>>>> required software (kernel driver) intervention.
>>>
>>> Having a PWRSTS_RET_HW_CTRL flag would make sense if there was also a
>>> PWRSTS_RET_SW_CTRL way of achieving Retention state, but FWIK there isn't.
>>> I am sure that's the way it is on 8974 as well, I just don't have hardware to
>>> confirm.
>>>
>>>>
>>>>>
>>>>> 2. gpu_gx_gdsc in gpucc-msm8998.c and
>>>>>     gpu_gx_gdsc in gpucc-sdm660.c
>>>>> Both of these seem to add support for 3 power state
>>>>> OFF, RET and ON, however I dont see any logic in gdsc
>>>>> driver to handle 3 different power states.
>>>>> So I am expecting that these are infact just transitioning
>>>>> between ON and OFF and RET state is never really used.
>>>>> The ideal fix for them would be to just update their resp.
>>>>> .pwrsts to PWRSTS_OFF_ON only.
>>>>
>>>> static int gdsc_init(struct gdsc *sc)
>>>> {
>>>>
>>>>      ...
>>>>
>>>>      if (on || (sc->pwrsts & PWRSTS_RET))
>>>>          gdsc_force_mem_on(sc);
>>>>      else
>>>>          gdsc_clear_mem_on(sc);
>>>>
>>>>      ...
>>>> }
>>>>
>>>> On MSM8998 and SDM630/636/660, we're reaching that point with a GDSC that is
>>>> left OFF from the bootloader, but we want (at least for 630/660) memretain
>>>> without periph-retain: this is required to make the hypervisor happy.
>>>
>>> Ideally setting the memretain bits while the GDSC is OFF should have no affect
>>> at all. Is this for the gpu_gx_gdsc on 630/660? Is this needed only at the init
>>> time (when the bootloader has left it OFF) or is it needed everytime the kernel
>>> turns it OFF too?
>>
>> Even though I don't remember the flow in a clear way (this entire thing was done
>> years ago), I'm sure that for PWRSTS_OFF memretain can be cleared, so, the current
>> flow that we have in gdsc.c does work correctly.
>>
>> Ideally, I agree with you that the memretain bits should have no effect at all
>> while the GDSC is OFF, but that's the situation on these platforms.
> 
> Would you be able to test this patch on these platforms to see if we end up
> with regressions?
> 

Not in a timely manner.

Konrad, Marijn, Jami, can any of you perform a "fast" test?

Thanks.

>>
>>> How did we come up with this trick to keep the hypervisor happy, was it picked
>>> up from some downstream reference code?
>>
>> Yes, it was found in various releases of the downstream kernel for 8998/630/660.
>>
>>>
>>>>
>>>> Regards,
>>>> Angelo
>>>>
>>