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

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Date: Thu, 23 May 2024 15:54:00 +1000
From: John Watts <contact@...kia.org>
To: きくちゃんさん <kikuchan98@...il.com>
Cc: privatesub2@...il.com, aou@...s.berkeley.edu, bigunclemax@...il.com,
	conor+dt@...nel.org, devicetree@...r.kernel.org,
	fusibrandon13@...il.com, jernej.skrabec@...il.com,
	krzk+dt@...nel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
	mkl@...gutronix.de, p.zabel@...gutronix.de, palmer@...belt.com,
	paul.walmsley@...ive.com, robh@...nel.org, samuel@...lland.org,
	ukleinek@...nel.org, wens@...e.org
Subject: Re: [PATCH v9 0/3] Add support for Allwinner PWM on D1/T113s/R329
 SoCs

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.