Re: [PATCH v2 00/11] pwm: Allow .get_state to fail

Conor Dooley
2022-12-01T11:12:45Z

Hey Uwe!

On Wed, Nov 30, 2022 at 04:21:37PM +0100, Uwe Kleine-König wrote:
> Hello,
>
> I forgot about this series and was remembered when I talked to Conor
> Dooley about how .get_state() should behave in an error case.

In the context of "my" driver, get_state() the proposal was to fail with
-ETIMEDOUT rather than block a caller, potentially, for seconds or
report a potentially "random" state.

Specifically, values writen to the registers that control the PWM duty
cycle are not visible to the cpu until the changes have propagated to
the waveform at the start of a new period.
The timeout would occur if the bit that signifies that the "shadow
registers" contain a value which has not yet propagated. This bit is
per PWM "controller" and not per PWM channel.

Returning from apply() without waiting, possibly for seconds, for the
writes to become visible could cause get_state() to see anything between
the new and old states, inclusive!

If anyone cares at all, the discussion is here:
https://lore.kernel.org/linux-pwm/20221110093512.333881-1-conor.dooley@microchip.com/T/#m800eeabad29067940a5684e54106fd0bb7261944

> In v1 Thierry had the concern:
>
> | That raises the question about what to do in these cases. If we return
> | an error, that could potentially throw off consumers. So perhaps the
> | closest would be to return a disabled PWM?
> | Or perhaps it'd be up to the
> | consumer to provide some fallback configuration for invalidly configured
> | or unconfigured PWMs.
>
> .get_state() is only called in pwm_device_request on a pwm_state that a
> consumer might see. Before my series a consumer might have seen a
> partial modified pwm_state (because .get_state() might have modified
> .period, then stumbled and returned silently). The last patch ensures
> that this partial modification isn't given out to the consumer. Instead
> they now see the same as if .get_state wasn't implemented at all.

Getting the same thing as if get_state() did not exist seems
preferable to me in this context than "lying" and pretending that a PWM
is disabled or potentially inconsistent reports from get_state() that I
mentioned above.

TL;DR, I quite like the ability to return an error and not mislead the
caller.

Thanks for sending a v2 of this so quickly :)
Conor.