linux-kernel - Re: [PATCH v3 4/5] pwm: omap-dmtimer: Do not disable pwm before changing period/duty

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Message-ID: <20200312064042.p7himm3odxjyzroi@pengutronix.de>
Date:   Thu, 12 Mar 2020 07:40:42 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Lokesh Vutla <lokeshvutla@...com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Tony Lindgren <tony@...mide.com>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
        Sekhar Nori <nsekhar@...com>, Vignesh R <vigneshr@...com>
Subject: Re: [PATCH v3 4/5] pwm: omap-dmtimer: Do not disable pwm before
 changing period/duty_cycle

On Thu, Mar 12, 2020 at 09:52:09AM +0530, Lokesh Vutla wrote:
> Only the Timer control register(TCLR) cannot be updated when the timer
> is running. Registers like Counter register(TCRR), loader register(TLDR),
> match register(TMAR) can be updated when the counter is running. Since
> TCLR is not updated in pwm_omap_dmtimer_config(), do not stop the
> timer for period/duty_cycle update.

I'm not sure what is sensible here. Stopping the PWM for a short period
is bad, but maybe emitting a wrong period isn't better. You can however
optimise it if only one of period or duty_cycle changes.

@Thierry, what is your position here? I tend to say a short stop is
preferable.

> Tested-by: Tony Lindgren <tony@...mide.com>
> Signed-off-by: Lokesh Vutla <lokeshvutla@...com>
> ---
>  drivers/pwm/pwm-omap-dmtimer.c | 21 +++++++--------------
>  1 file changed, 7 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-omap-dmtimer.c b/drivers/pwm/pwm-omap-dmtimer.c
> index 85b17b49980b..c56e7256e923 100644
> --- a/drivers/pwm/pwm-omap-dmtimer.c
> +++ b/drivers/pwm/pwm-omap-dmtimer.c
> @@ -19,6 +19,13 @@
>   * Limitations:
>   * - When PWM is stopped, timer counter gets stopped immediately. This
>   *   doesn't allow the current PWM period to complete and stops abruptly.
> + * - When PWM is running and changing both duty cycle and period,
> + *   we cannot prevent in software that the output might produce
> + *   a period with mixed settings. Especially when period/duty_cyle
> + *   is updated while the pwm pin is high, current pwm period/duty_cycle
> + *   can get updated as below based on the current timer counter:
> + *   	- period for current cycle =  current_period + new period
> + *   	- duty_cycle for current period = current period + new duty_cycle.

In case we stay with a short stop, adding something like:

 - The PWM has to be stopped for updates of both period and duty_cycle.

Best regards
Uwe

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