Re: [3/3] pwm: imx: Implement get_state() function for hardware readout

From: VokÃÄ Michal
Date: Fri Dec 14 2018 - 11:40:36 EST

On 12.12.2018 11:51, Uwe Kleine-KÃnig wrote:
> Hello,
> On Mon, Oct 01, 2018 at 04:19:48PM +0200, Michal VokÃÄ wrote:
>> Implement the get_state() function and set the initial state to reflect
>> real state of the hardware. This allows to keep the PWM running if it was
>> enabled in bootloader. It is very similar to the GPIO behavior. GPIO pin
>> set as output in bootloader keep the same setting in Linux unless it is
>> reconfigured.
>> If we find the PWM block enabled we need to prepare and enable its source
>> clock otherwise the clock will be disabled late in the boot as unused.
>> That will leave the PWM in enabled state but with disabled clock. That has
>> a side effect that the PWM output is left at its current level at which
>> the clock was disabled. It is totally non-deterministic and it may be LOW
>> or HIGH.
> Does this problem still exist if the pwm-imx driver is a module?

Yes. The source clock for PWM is stopped by the clock core shortly
before init process is started and it does not matter if the pwm-imx
driver is built as a module or built into the kernel.

When the module is loaded, the .get_state function is executed and
reads the PWM registers. If the EN bit is set the clock is started
and PWM continues from where it was stopped.