linux-kernel - Re: [PATCH v10 3/4] pwm: add microchip soft ip corePWM driver

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Message-ID: <Yza61MO9hbuFytmM@wendy>
Date:   Fri, 30 Sep 2022 10:45:56 +0100
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
CC:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Daire McNamara <daire.mcnamara@...rochip.com>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-pwm@...r.kernel.org>, <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v10 3/4] pwm: add microchip soft ip corePWM driver

On Fri, Sep 30, 2022 at 11:13:16AM +0200, Uwe Kleine-König wrote:
> On Mon, Sep 19, 2022 at 03:29:19PM +0100, Conor Dooley wrote:
> > Hey Uwe,
> > 
> > On Mon, Sep 19, 2022 at 03:50:08PM +0200, Uwe Kleine-König wrote:
> > > On Mon, Sep 19, 2022 at 01:53:56PM +0100, Conor Dooley wrote:
> > > > Because I was running into conflicts between the reporting here and some
> > > > of the checks that I have added to prevent the PWM being put into an
> > > > invalid state. On boot both negedge and posedge will be zero & this was
> > > > preventing me from setting the period at all.
> > > 
> > > I don't understood that.
> > 
> > On startup, (negedge == posedge) is true as both are zero, but the reset
> > values for prescale and period are actually 0x8. If on reset I try to
> > set a small period, say "echo 1000 > period" apply() returns -EINVAL
> > because of a check in the pwm core in pwm_apply_state() as I am
> > attempting to set the period to lower than the out-of-reset duty cycle.
> 
> You're supposed to keep the period for pwm#1 untouched while configuring
> pwm#0 only if pwm#1 already has a consumer. So if pwm#1 isn't requested,
> you can change the period for pwm#0.

I must have done a bad job of explaining here, as I don't think this is
an answer to my question.

On reset, the prescale and period_steps registers are set to 0x8. If I
attempt to set the period to do "echo 1000 > period", I get -EINVAL back
from pwm_apply_state() (in next-20220928 it's @ L562 in pwm/core.c) as
the duty cycle is computed as twice the period as, on reset, we have
posedge = negedge = 0x0. The check of state->duty_cycle > state->period
fails in pwm_apply_state() as a result.

This failure to assign a value is unrelated to having multiple PWMs, I
think I may have horribly worded my statement when I originally replied
to you with:
> Because I was running into conflicts between the reporting here and some
> of the checks that I have added to prevent the PWM being put into an
> invalid state.

"reporting here" from that quote being the period/duty cycle
calculations in the drivers get_state(). By "the checks" I meant making
sure that a period where posedge = negedge is not set by the driver. I
think I also may have mistakenly assumed the -EINVAL came from my code
and not from the core - but I cannot be sure as it has been a few weeks.

The check in the core looks to be things "working as intended", and it
looks like I am working around it here. Should I just note what the
values are on reset in the "limitations" comment and the top & it is up
to applications that control the PWMs to first "fix" the duty cycle
before changing the period?

Hopefully I've done a better job at explaning this time,
Conor.