Re: [PATCH] pwm: lpss: Fix get_state runtime-pm reference handling

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, May 12, 2020 at 01:00:44PM +0200, Hans de Goede wrote:
> Before commit cfc4c189bc70 ("pwm: Read initial hardware state at request
> time"), a driver's get_state callback would get called once per PWM from
> pwmchip_add().
> 
> pwm-lpss' runtime-pm code was relying on this, getting a runtime-pm ref for
> PWMs which are enabled at probe time from within its get_state callback,
> before enabling runtime-pm.
> 
> The change to calling get_state at request time causes a number of
> problems:
> 
> 1. PWMs enabled at probe time may get runtime suspended before they are
> requested, causing e.g. a LCD backlight controlled by the PWM to turn off.
> 
> 2. When the request happens when the PWM has been runtime suspended, the
> ctrl register will read all 1 / 0xffffffff, causing get_state to store
> bogus values in the pwm_state.
> 
> 3. get_state was using an async pm_runtime_get() call, because it assumed
> that runtime-pm has not been enabled yet. If shortly after the request an
> apply call is made, then the pwm_lpss_is_updating() check may trigger
> because the resume triggered by the pm_runtime_get() call is not complete
> yet, so the ctrl register still reads all 1 / 0xffffffff.
> 
> This commit fixes these issues by moving the initial pm_runtime_get() call
> for PWMs which are enabled at probe time to the pwm_lpss_probe() function;
> and by making get_state take a runtime-pm ref before reading the ctrl reg.
> 
> BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=1828927
> Fixes: cfc4c189bc70 ("pwm: Read initial hardware state at request time")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
>  drivers/pwm/pwm-lpss.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)

Applied, thanks.

Thierry

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