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

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Message-ID: <20220930133933.br5kanbh3clvahvr@pengutronix.de>
Date:   Fri, 30 Sep 2022 15:39:33 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Conor Dooley <conor.dooley@...rochip.com>
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 10:45:56AM +0100, Conor Dooley wrote:
> 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.

So set duty_cycle to 0 first?

A problem of the sysfs interface is that you can only set one parameter
after the other. So there you have to find a sequence of valid
pwm_states that only differ in a single parameter between the initial
and the desired state.

That's nothing a "normal" pwm consumer would be affected by. (IMHO we
should have a userspace API that benefits from the properties of
pwm_apply().)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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