linux-kernel - Re: [PATCH 3/4] pwm: omap-dmtimer: Do not disable pwm before changing period/duty

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Message-ID: <20200225064833.kmvaplfqqf53s3iy@pengutronix.de>
Date:   Tue, 25 Feb 2020 07:48:33 +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>
Subject: Re: [PATCH 3/4] pwm: omap-dmtimer: Do not disable pwm before
 changing period/duty_cycle

Hello,

On Tue, Feb 25, 2020 at 10:32:42AM +0530, Lokesh Vutla wrote:
> On 24/02/20 2:25 PM, Uwe Kleine-König wrote:
> > Hello,
> > 
> > On Mon, Feb 24, 2020 at 10:51:34AM +0530, Lokesh Vutla wrote:
> >> Only the Timer control register(TCLR) can be updated only when the timer
> >> is stopped. 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.
> >>
> >> Signed-off-by: Lokesh Vutla <lokeshvutla@...com>
> >> ---
> >>  drivers/pwm/pwm-omap-dmtimer.c | 14 --------------
> >>  1 file changed, 14 deletions(-)
> >>
> >> diff --git a/drivers/pwm/pwm-omap-dmtimer.c b/drivers/pwm/pwm-omap-dmtimer.c
> >> index f13be7216847..58c61559e72f 100644
> >> --- a/drivers/pwm/pwm-omap-dmtimer.c
> >> +++ b/drivers/pwm/pwm-omap-dmtimer.c
> >> @@ -102,7 +102,6 @@ static int pwm_omap_dmtimer_config(struct pwm_chip *chip,
> >>  	u32 load_value, match_value;
> >>  	struct clk *fclk;
> >>  	unsigned long clk_rate;
> >> -	bool timer_active;
> >>  
> >>  	dev_dbg(chip->dev, "requested duty cycle: %d ns, period: %d ns\n",
> >>  		duty_ns, period_ns);
> >> @@ -178,25 +177,12 @@ static int pwm_omap_dmtimer_config(struct pwm_chip *chip,
> >>  	load_value = (DM_TIMER_MAX - period_cycles) + 1;
> >>  	match_value = load_value + duty_cycles - 1;
> >>  
> >> -	/*
> >> -	 * We MUST stop the associated dual-mode timer before attempting to
> >> -	 * write its registers, but calls to omap_dm_timer_start/stop must
> >> -	 * be balanced so check if timer is active before calling timer_stop.
> >> -	 */
> >> -	timer_active = pm_runtime_active(&omap->dm_timer_pdev->dev);
> >> -	if (timer_active)
> >> -		omap->pdata->stop(omap->dm_timer);
> >> -
> >>  	omap->pdata->set_load(omap->dm_timer, true, load_value);
> >>  	omap->pdata->set_match(omap->dm_timer, true, match_value);
> > 
> > (Without having looked into the depths of the driver I assume
> > .set_load() sets the period of the PWM and .set_match() the duty cycle.)
> 
> Right.
> 
> > 
> > What happens on a running PWM if you change the period? Consider you
> > change from duty_cycle = 1000, period = 5000 to duty_cycle = 4000,
> > period = 10000. As you set the period first, can it happen the hardware
> > produces a cycle with duty_cycle = 1000, period = 10000?
> 
> No. So, the current cycle is un affected with duty_cycle = 1000 and period =
> 5000. Starting from next cycle new settings gets reflected with duty_cycle =
> 4000 and period = 10000.

Is the reference manual for this hardware publically available?

So the .set_load callback just writes a shadow register and .set_match
latches it into hardware atomically with its own register changes? A
comment in the source code about this would be good. Also if .set_load
doesn't work without .set_match I wonder if it is sane to put their
logic in two different functions.

Best regards
Uwe

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