linux-kernel - Re: [PATCH 0/1] pwm: meson: fix scheduling while atomic issue

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Message-ID: <20190331184730.y767dxhblffxj3om@pengutronix.de>
Date:   Sun, 31 Mar 2019 20:47:30 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     linux-pwm@...r.kernel.org,
        Neil Armstrong <narmstrong@...libre.com>,
        linux-kernel@...r.kernel.org, thierry.reding@...il.com,
        kernel@...gutronix.de, linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, jbrunet@...libre.com
Subject: Re: [PATCH 0/1] pwm: meson: fix scheduling while atomic issue

On Sat, Mar 30, 2019 at 08:29:35PM +0100, Martin Blumenstingl wrote:
> Hello Uwe,
> 
> On Mon, Mar 25, 2019 at 9:07 PM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> [...]
> > > >  - Does stopping the PWM (i.e. clearing MISC_{A,B}_EN in the MISC_AB
> > > >    register) freeze the output, or is the currently running period
> > > >    completed first? (The latter is the right behaviour.)
> > > I don't know, I would have to measure this with a logic analyzer.
> >
> > In practise you can do this with a multimeter, too. Just do something
> > like:
> >
> >         pwm_apply_state({ .enabled = true, .period = 5s, .duty_cycle = 5s, .polarity = PWM_POLARITY_NORMAL });
> >         pwm_apply_state({ .enabled = false, .period = 5s, .duty_cycle = 5s, .polarity = PWM_POLARITY_NORMAL });
> >
> > (assuming the PWM supports periods that long). The expectation is that
> > the last command takes nearly 5 s to complete and while it waits the
> > output is high and on return it's low. If that isn't the case, there is
> > a bug somewhere.
> the longest supported period (using the 24MHz crystal as input, which
> is the slowest input clock and thus gives the longest possible
> duration) is 349514407ns (that's approx. 0.35 seconds). my multimeter
> isn't fast enough to measure this so I'm using my logic analyzer with
> puleseview instead: [0]
> 
> I added the following code to meson_pwm_request:
>   struct pwm_state enable = {
>         .enabled = true,
>         .period = 349514407U,
>         .duty_cycle = 349514407U,
>         .polarity = PWM_POLARITY_NORMAL };
>   struct pwm_state disable = {
>         .enabled = false,
>         .period = 349514407U,
>         .duty_cycle = 349514407U,
>         .polarity = PWM_POLARITY_NORMAL };
>   pwm_apply_state(pwm, &enable);
>   pwm_apply_state(pwm, &disable);
> 
> this returns immediately. my logic analyzer doesn't see signal change
> (I'm sampling at 1MHz).
> 
> can you please confirm that my test code and measurement procedure is correct?
> if it is then my observation is that disabling the PWM does so
> immediately, without waiting for the current period to complete

Ack, with the above two pwm_apply_state the output must be high when the
first pwm_apply_state returns. Then it must stay high for n *
349514407 ns (for an natural n >= 1) and then go low.

Best regards
Uwe

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