linux-kernel - Re: [RFCv1 5/5] arm64/ARM: configs: Change CONFIG_PWM

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Date:   Mon, 07 Oct 2019 15:57:59 -0700
From:   Kevin Hilman <khilman@...libre.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Anand Moon <linux.amoon@...il.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv1 5/5] arm64/ARM: configs: Change CONFIG_PWM_MESON from m to y

Martin Blumenstingl <martin.blumenstingl@...glemail.com> writes:

> On Mon, Oct 7, 2019 at 3:17 PM Anand Moon <linux.amoon@...il.com> wrote:
> [...]
>> diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
>> index c9a867ac32d4..72f6a7dca0d6 100644
>> --- a/arch/arm64/configs/defconfig
>> +++ b/arch/arm64/configs/defconfig
>> @@ -774,7 +774,7 @@ CONFIG_MPL3115=m
>>  CONFIG_PWM=y
>>  CONFIG_PWM_BCM2835=m
>>  CONFIG_PWM_CROS_EC=m
>> -CONFIG_PWM_MESON=m
>> +CONFIG_PWM_MESON=y
>
> some time ago I submitted a similar patch for the 32-bit SoCs
> it turned that that pwm-meson can be built as module because the
> kernel will run without CPU DVFS as long as the clock and regulator
> drivers are returning -EPROBE_DEFER (-517)

On 64-bit SoCs, the kernel boots with PWM as a module also, but DVFS
only works sometimes, and making it built-in fixes the problem.
Actually, it doesn't fix, it just hides the problem, which is likely a
race or timeout happening during deferred probing.

> did you check whether there's some other problem like some unused
> clock which is being disabled at that moment?
> I've been hunting weird problems in the past where it turned out that
> changing kernel config bits changed the boot timing - that masked the
> original problem

Right, I would definitely prefer to not make this built-in without a lot
more information to *why* this is needed.  In figuring that out, we'll
probably find the race/timeout that's the root cause.

Kevin