linux-kernel - Re: [PATCH] ARM: multi_v7_defconfig: switch CONFIG_PWM

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Date:   Fri, 30 Nov 2018 00:37:48 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     khilman@...libre.com
Cc:     linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux@...linux.org.uk
Subject: Re: [PATCH] ARM: multi_v7_defconfig: switch CONFIG_PWM_MESON to built-in

Hi Kevin,

On Fri, Nov 30, 2018 at 12:28 AM Kevin Hilman <khilman@...libre.com> wrote:
>
> Martin Blumenstingl <martin.blumenstingl@...glemail.com> writes:
>
> > Hi Kevin,
> >
> > On Thu, Nov 29, 2018 at 1:30 AM Kevin Hilman <khilman@...libre.com> wrote:
> >>
> >> Martin Blumenstingl <martin.blumenstingl@...glemail.com> writes:
> >>
> >> > Some Meson8b boards (Odroid-C1, EC-100) use a PWM regulator which is the
> >> > voltage supply of the CPU cores (this regulator is typically called
> >> > "VCCK").
> >> > Now that we are preparing support for CPU frequency scaling on Meson8,
> >> > Meson8b and Meson8m2 we should build the pwm-meson driver into the
> >> > kernel so we can configure the CPU voltage early in the boot process.
> >>
> >> Can you explain a little more why the configuration of CPU voltage
> >> cannot wait a bit so this could be properly probed?
> >
> > I was under the impression that cpufreq-dt would still initialize even
> > if the regulator is not ready yet. however (after reading the code
> > again) this is clearly NOT the case.
>
> Hmm, a quick look at cpufreq-dt suggests that it should -EPROBE_DEFER if
> the regulator isn't ready yet.  Why doesn't that work?
sorry for not being clear:
I was under the impression that cpufreq-dt wouldn't handle -EPROBE_DEFER.
I'm not sure why I assumed that. As you already said: deferred probing
of the regulator works fine.

> > there's still a benefit with this change: your Odroid-C1 in your
> > KernelCI lab would also be able to change the CPU frequency (so in
> > case something breaks we could spot it there).
> > will you accept this patch after I updated the description to mention
> > that it's for KernelCI test coverage?
>
> Possibly, but I'd still rather see the dependencies worked out correctly
> so that this can be module, and cpufreq-dt would defer until it's ready.
KernelCI shows multiple "pwm-regulator regulator-vcck: Failed to get
PWM: -517" messages on Odroid-C1.
however, what I failed to notice so far is that there's a
"pwm-regulator: supplied by regulator-dummy" at the end of the boot
process. does this mean that the Odroid-C1 in your KernelCI lab can
load kernel modules? in that case this patch can be ignored.

(I will still send a PWM regulator related patch: that "Failed to get
PWM: -517" message is very noisy and we typically suppress that in
other drivers)


Regards
Martin