Re: [PATCH v9 0/3] Add support for Allwinner PWM on D1/T113s/R329 SoCs

[John Watts]

On Thu, May 23, 2024 at 02:30:32PM +0900, きくちゃんさん wrote:
> Hello John,

Hello,

> 
> Your method is quite impressive and challenging.
> 
> I think PWM is not only for LCD backlighting, but also for signal generation.
> For instance, imagine an application software that sends a stream to
> one PWM channel to synthesize sound by changing its period (plays a
> square wave).
> By your method, if the other PWM channel is used for LCD backlighting,
> it may flicker repeatedly on DIV_M changes.
> (Or simply both channels for streo sound synthesize, you may hear lots
> of pop noise)
> 
> It means the setting of one channel can affect the other, which users
> may not anticipate.

Does Linux guarantee a flicker-free experience with setting up PWM
channels, or that it doesn't affect other channels?
How do other drivers handle this situation?

Maybe out-sourcing the clocking to the device tree and letting people set the
clock and common divisor in the DT would be a good idea, with the HOSC and a
common divisor of 1 by default, then error if the value can't be found.

This would work for periods from 41ns up to 687ms by default. Using APB would
give a better resolution as by default it hangs around 5ns which is nice and
divisible by 10.

> 
> Best regards,
> kikuchan.

John.