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

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Date:   Tue, 20 Sep 2022 14:39:21 +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 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.

> 
> 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.

Regards,
Angelo