linux-kernel - Re: [PATCH 2/2] hwmon: pwm-fan: Set fan speed to 0 on suspend

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Message-ID: <20180921132354.GA16764@roeck-us.net>
Date:   Fri, 21 Sep 2018 06:23:54 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Jean Delvare <jdelvare@...e.com>, Kamil Debski <kamil@...as.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] hwmon: pwm-fan: Set fan speed to 0 on suspend

On Fri, Sep 21, 2018 at 12:10:48PM +0200, Thierry Reding wrote:
> From: Thierry Reding <treding@...dia.com>
> 
> Technically this is not required because disabling the PWM should be
> enough. However, when support for atomic operations was implemented in
> the PWM subsystem, only actual changes to the PWM channel are applied
> during pwm_config(), which means that during after resume from suspend
> the old settings won't be applied.
> 
> One possible solution is for the PWM driver to implement its own PM
> operations such that settings from before suspend get applied on resume.
> This has the disadvantage of completely ignoring any particular ordering
> requirements that PWM user drivers might have, so it is best to leave it
> up to the user drivers to apply the settings that they want at the
> appropriate time.
> 
> Another way to solve this would be to read back the current state of the
> PWM at the time of resume. That way, in case the configuration was lost
> during suspend, applying the old settings in PWM user drivers would
> actually get them applied because they differ from the current settings.
> However, not all PWM drivers support reading the hardware state, and not
> all hardware may support it.
> 
> The best workaround at this point seems to be to let PWM user drivers
> tell the PWM subsystem that the PWM is turned off by, in addition to
> disabling it, also setting the duty cycle to 0. This causes the resume
> operation to apply a configuration that is different from the current
> configuration, resulting in the proper state from before suspend getting
> restored.
> 
> Signed-off-by: Thierry Reding <treding@...dia.com>

Applied.

Thanks,
Guenter