Re: [PATCH 1/4] cpufreq: qcom-nvmem: Enable virtual power domain devices

[Stephan Gerhold <stephan@gerhold.net>]

On Wed, Sep 13, 2023 at 12:56:16PM +0200, Ulf Hansson wrote:
> On Tue, 12 Sept 2023 at 11:40, Stephan Gerhold
> <stephan.gerhold@kernkonzept.com> wrote:
> >
> > The genpd core ignores performance state votes from devices that are
> > runtime suspended as of commit 5937c3ce2122 ("PM: domains: Drop/restore
> > performance state votes for devices at runtime PM").
> 
> I think you are referring to the wrong commit above. Please have a
> look at commit 3c5a272202c2 ("PM: domains: Improve runtime PM
> performance state handling"), instead.
> 
> I also suggest rephrasing the above into saying that the performance
> state vote for a device is cached rather than carried out, if
> pm_runtime_suspended() returns true for it.
> 
> Another relevant information in the commit message would be to add
> that during device-attach (genpd_dev_pm_attach_by_id()), calls
> pm_runtime_enable() the device.
> 

Thanks, I will try to clarify this a bit! I was actually looking at that
commit originally but decided to reference the commit that "started the
change", since the this commit is marked as fix of the one I referenced.
But I think you're right, it would be more clear to reference "PM:
domains: Improve runtime PM performance state handling" directly.

> > However, at the
> > moment nothing ever enables the virtual devices created in
> > qcom-cpufreq-nvmem for the cpufreq power domain scaling, so they are
> > permanently runtime-suspended.
> >
> > Fix this by enabling the devices after attaching them and use
> > dev_pm_syscore_device() to ensure the power domain also stays on when
> > going to suspend. Since it supplies the CPU we can never turn it off
> > from Linux. There are other mechanisms to turn it off when needed,
> > usually in the RPM firmware or the cpuidle path.
> >
> > Without this fix performance states votes are silently ignored, and the
> > CPU/CPR voltage is never adjusted. This has been broken since 5.14 but
> > for some reason no one noticed this on QCS404 so far.
> >
> > Cc: stable@vger.kernel.org
> > Fixes: 1cb8339ca225 ("cpufreq: qcom: Add support for qcs404 on nvmem driver")
> > Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
> > ---
> >  drivers/cpufreq/qcom-cpufreq-nvmem.c | 21 ++++++++++++++++++++-
> >  1 file changed, 20 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpufreq/qcom-cpufreq-nvmem.c b/drivers/cpufreq/qcom-cpufreq-nvmem.c
> > index 84d7033e5efe..17d6ab14c909 100644
> > --- a/drivers/cpufreq/qcom-cpufreq-nvmem.c
> > +++ b/drivers/cpufreq/qcom-cpufreq-nvmem.c
> > @@ -25,6 +25,7 @@
> >  #include <linux/platform_device.h>
> >  #include <linux/pm_domain.h>
> >  #include <linux/pm_opp.h>
> > +#include <linux/pm_runtime.h>
> >  #include <linux/slab.h>
> >  #include <linux/soc/qcom/smem.h>
> >
> > @@ -280,6 +281,7 @@ static int qcom_cpufreq_probe(struct platform_device *pdev)
> >         }
> >
> >         for_each_possible_cpu(cpu) {
> > +               struct device **virt_devs = NULL;
> >                 struct dev_pm_opp_config config = {
> >                         .supported_hw = NULL,
> >                 };
> > @@ -300,7 +302,7 @@ static int qcom_cpufreq_probe(struct platform_device *pdev)
> >
> >                 if (drv->data->genpd_names) {
> >                         config.genpd_names = drv->data->genpd_names;
> > -                       config.virt_devs = NULL;
> > +                       config.virt_devs = &virt_devs;
> >                 }
> >
> >                 if (config.supported_hw || config.genpd_names) {
> > @@ -311,6 +313,23 @@ static int qcom_cpufreq_probe(struct platform_device *pdev)
> >                                 goto free_opp;
> >                         }
> >                 }
> > +
> > +               if (virt_devs) {
> > +                       const char * const *name = config.genpd_names;
> > +                       int i;
> > +
> > +                       for (i = 0; *name; i++, name++) {
> > +                               ret = pm_runtime_resume_and_get(virt_devs[i]);
> > +                               if (ret) {
> > +                                       dev_err(cpu_dev, "failed to resume %s: %d\n",
> > +                                               *name, ret);
> > +                                       goto free_opp;
> > +                               }
> 
> Shouldn't we restore the usage count at ->remove() too?
> 
> > +
> > +                               /* Keep CPU power domain always-on */
> > +                               dev_pm_syscore_device(virt_devs[i], true);
> 
> Is this really correct? cpufreq is suspended/resumed by the PM core
> during system wide suspend/resume. See dpm_suspend|resume(). Isn't
> that sufficient?
> 
> Moreover, it looks like the cpr genpd provider supports genpd's
> ->power_on|off() callbacks. Is there something wrong with this, that I
> am missing?
> 

I think this question is a quite fundamental one. To explain this
properly I will need to delve a bit into the implementation details of
the two different GENPD providers that are applicable here:

Fundamentally, we are describing the main power supply for the CPU here.
Consider a simple regulator with adjustable voltage. From the Linux
point of view this regulator should be marked as "regulator-always-on".
If we would turn off this regulator, the CPU would be immediately dead
without proper shutdown done by firmware or hardware.

Representing the regulator as power domain does not change much, except
that we now have abstract "performance states" instead of actual voltages.
However, for power domains there is currently no generic mechanism like
"regulator-always-on" in the DT, only drivers can specify
GENPD_FLAG_ALWAYS_ON.

The special situation on MSM8909 is that there are two possible setups
for the CPU power supply depending on the PMIC that is used (see
"[PATCH 4/4] cpufreq: qcom-nvmem: Add MSM8909"): CPR or RPMPD. Both are
GENPD providers so in theory we can just have either

  cpu@0 { power-domains = <&cpr>; }; // or
  cpu@0 { power-domains = <&rpmpd MSM8909_VDDCX_AO>; };

in the DT, without handling this specifically on the cpufreq side.

The two GENPD providers behave quite differently though:

 - CPR: CPR is not really a power domain itself. It's more like a monitor
   on a power supply line coming from some other regulator. CPR provides
   suggestions how to adjust the voltage for best power/stability.

   The GENPD .power_off() disables the CPR state machine and forwards
   this to the regulator with regulator_disable(). On QCS404 the
   regulator is marked regulator-always-on, so it will never be disabled
   from Linux. The SAW/SPM hardware component on Qualcomm SoCs will
   usually disable the regulator during deep cpuidle states.

 - RPMPD: This is the generic driver for all the SoC power domains
   managed by the RPM firmware. It's not CPU-specific. However, as
   special feature each power domain is exposed twice in Linux, e.g.
   "MSM8909_VDDCX" and "MSM8909_VDDCX_AO". The _AO ("active-only")
   variant tells the RPM firmware that the performance/enable vote only
   applies when the CPU is active (not in deep cpuidle state).

   The GENPD .power_off() drops all performance state votes and also
   releases the "enable" vote for the power domain.

Now, imagine what happens during system wide suspend/resume:

 - CPR: The CPR state machine gets disabled. The voltage stays as-is.
     - With "regulator-always-on": The CPU keeps running until WFI.
     - Without: I would expect the CPU is dead immediately(?)

 - RPMPD: The performance/enable vote is dropped. The power domain might
   go to minimal voltage or even turn off completely. However, the CPU
   actually needs to keep running at the same frequency until WFI!
   Worst case, the CPU is dead immediately when the power domain votes
   get dropped.

In case of RPMPD, the votes must remain even during system wide suspend.
The special _AO variant of the power domain tells the firmware to
release the votes once the CPU has been shut down cleanly. It will also
restore them once the CPU wakes up (long before the resume handlers run).

My conclusion was that in both cases we want to keep the "power domain"
enabled, since the CPU must keep running for a short while even after
the system suspend handlers have been called.

Does this help with understanding the problem? It's a bit complicated. :D

Thanks!
Stephan

PS: This is essentially just another manifestation of a discussion we
had a few times already over the years about where to enable power
domains used by cpufreq, e.g. [1, 2, 3, 4]. Apparently I already
mentioned back in 2021 already that QCS404 is broken [5] (I forgot
about that :')).

[1]: https://lore.kernel.org/linux-pm/YLi5N06Qs+gYHgYg@gerhold.net/
[2]: https://lore.kernel.org/linux-pm/20200826093328.88268-1-stephan@gerhold.net/
[3]: https://lore.kernel.org/linux-pm/20200730080146.25185-1-stephan@gerhold.net/
[4]: https://lore.kernel.org/linux-arm-msm/20200426123140.GA190483@gerhold.net/
[5]: https://lore.kernel.org/linux-pm/YLoTl7MfMfq2g10h@gerhold.net/