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

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Date:   Wed, 21 Sep 2022 09:51:04 +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
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 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.

> 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
>>