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

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Message-ID: <37cc424e-cafd-acb6-d7d2-c6c98abf53de@ysoft.com>
Date:   Fri, 14 Dec 2018 16:40:30 +0000
From:   Vokáč Michal <Michal.Vokac@...ft.com>
To:     Uwe Kleine-König <uwe@...ine-koenig.org>
CC:     Thierry Reding <thierry.reding@...il.com>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [3/3] pwm: imx: Implement get_state() function for hardware
 readout

On 12.12.2018 11:51, Uwe Kleine-König wrote:
> Hello,
> 
> On Mon, Oct 01, 2018 at 04:19:48PM +0200, Michal Vokáč 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.

Michal