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Date:   Tue, 30 Aug 2022 11:43:57 -0700
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Rajendra Nayak <quic_rjendra@quicinc.com>
Cc:     Bjorn Andersson <andersson@kernel.org>,
        Andy Gross <agross@kernel.org>,
        Konrad Dybcio <konrad.dybcio@somainline.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Johan Hovold <johan+linaro@kernel.org>,
        linux-clk@vger.kernel.org,
        Krishna Kurapati <quic_kriskura@quicinc.com>,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Douglas Anderson <dianders@chromium.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>
Subject: Re: [PATCH v2 1/2] clk: qcom: gcc-sc7180: Keep the USB GDSC always on
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References: <20220825182152.v2.1.I45235b7c40997bc2abf813e4722b4dcdd6aecf6b@changeid>
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On Mon, Aug 29, 2022 at 01:42:02PM +0530, Rajendra Nayak wrote:
> 
> On 8/26/2022 8:10 AM, Bjorn Andersson wrote:
> > On Thu, Aug 25, 2022 at 06:21:58PM -0700, Matthias Kaehlcke wrote:
> > > When the GDSC is disabled during system suspend USB is broken on
> > > sc7180 when the system resumes. Mark the GDSC as always on to
> > > make sure USB still works after system suspend.
> > > 
> > > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > 
> > Rajendra, where you able to find some time to look into take the GDSC
> > into retention state? Do you suggest that I merge these two patches for
> > now?
> > 
> 
> Hi Bjorn, based on my experiments to support retention on sc7280 these are
> some of my findings
> 
> On Platforms which support CX retention (for example sc7180/sc7280) instead of
> CX PowerCollapse (PC), We can leave the GDSC turned ON. When CX transitions to RET state
> the GDSC goes into retention too (some controller state is retained) and USB wakeups work.
> 
> On platforms which support CX PC, just leaving the GDSC
> turned ON will not help since the GDSC will also transition to OFF state
> when we enter CX PC, hence wake-ups from USB won't work.
> For such platforms we need to make sure gdsc_force_mem_on() is called
> and cxcs (* @cxcs: offsets of branch registers to toggle mem/periph bits in)
> are populated correctly, while leaving the GDSC turned ON.
> This will make sure usb gdsc transitions from being powered by CX to MX
> when CX hits PC and we still get USB wakeups to work.
> So in short we could do the same thing that this patch does on those
> platforms too with additionally populating the right cxcs entries and it
> should just work fine.
> 
> Now the problem that I see with this approach is not with getting USB wakeups
> to work in suspend, but with supporting performance state voting when
> USB is active.
> The last conclusion we had on that [1] was to model usb_gdsc as a subdomain of CX,
> so if we do that and we model usb_gdsc as something that supports ALWAYS_ON,
> we would _never_ drop the CX vote and prevent CX from going down (either to ret
> or pc)
> 
> The only way I think we can solve both the USB wakeups and performance state
> needs (with usb_gdsc as a subdomain of CX) is if we can model a RET state for gdsc
> which sets the mem/periph bits while leaving the GDSC ON (Today the RET state sets
> the mem/periph bits but turns the GDSC OFF)
> 
> That would mean a change in gdsc.c like this
> ---
> 
> diff --git a/drivers/clk/qcom/gdsc.c b/drivers/clk/qcom/gdsc.c
> index d3244006c661..0fe017ba901b 100644
> --- a/drivers/clk/qcom/gdsc.c
> +++ b/drivers/clk/qcom/gdsc.c
> @@ -368,6 +368,10 @@ static int _gdsc_disable(struct gdsc *sc)
>         if (sc->pwrsts & PWRSTS_OFF)
>                 gdsc_clear_mem_on(sc);
> 
> +       /* If the GDSC supports RET, do not explicitly power it off */
> +       if (sc->pwrsts & PWRSTS_RET)
> +               return 0;
> +
>         ret = gdsc_toggle_logic(sc, GDSC_OFF);
>         if (ret)
>                 return ret;
> 
> 
> So with that change, we would then not need the ALWAYS_ON flag set for usb gdsc,
> instead we would update the .pwrsts to PWRSTS_RET_ON instead of PWRSTS_OFF_ON,
> and that should make both usb wake-ups to work and we can still have the usb_gdsc as
> a subdomain of CX for performance state voting.

Krishna and I confirmed that this works from the USB side for sc7180 and sc7280.