Re: [PATCH v3 07/11] pwm: imx: Provide atomic PWM support for i.MX PWMv2

[Lukasz Majewski]

Dear Boris, Stefan,

> On Mon, 28 Nov 2016 06:50:31 +0100
> Lukasz Majewski <l.majewski@xxxxxxxxx> wrote:
> 
> > Dear Stefan, Boris,
> > 
> > > On 2016-11-23 00:38, Boris Brezillon wrote:  
> > > > On Tue, 22 Nov 2016 13:55:33 -0800
> > > > Stefan Agner <stefan@xxxxxxxx> wrote:
> > > >   
> > > >> On 2016-11-01 00:10, Lukasz Majewski wrote:  
> > > >> > This commit provides apply() callback implementation for
> > > >> > i.MX's PWMv2.
> > > >> >
> > > >> > Suggested-by: Stefan Agner <stefan@xxxxxxxx>
> > > >> > Suggested-by: Boris Brezillon
> > > >> > <boris.brezillon@xxxxxxxxxxxxxxxxxx> Signed-off-by: Lukasz
> > > >> > Majewski <l.majewski@xxxxxxxxx> Reviewed-by: Boris Brezillon
> > > >> > <boris.brezillon@xxxxxxxxxxxxxxxxxx> ---
> > > >> > Changes for v3:
> > > >> > - Remove ipg clock enable/disable functions
> > > >> >
> > > >> > Changes for v2:
> > > >> > - None
> > > >> > ---
> > > >> >  drivers/pwm/pwm-imx.c | 70
> > > >> > +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file
> > > >> > changed, 70 insertions(+)
> > > >> >
> > > >> > diff --git a/drivers/pwm/pwm-imx.c b/drivers/pwm/pwm-imx.c
> > > >> > index ebe9b0c..cd53c05 100644
> > > >> > --- a/drivers/pwm/pwm-imx.c
> > > >> > +++ b/drivers/pwm/pwm-imx.c
> > > >> > @@ -159,6 +159,75 @@ static void
> > > >> > imx_pwm_wait_fifo_slot(struct pwm_chip *chip, }
> > > >> >  }
> > > >> >
> > > >> > +static int imx_pwm_apply_v2(struct pwm_chip *chip, struct
> > > >> > pwm_device *pwm,
> > > >> > +			    struct pwm_state *state)
> > > >> > +{
> > > >> > +	unsigned long period_cycles, duty_cycles, prescale;
> > > >> > +	struct imx_chip *imx = to_imx_chip(chip);
> > > >> > +	struct pwm_state cstate;
> > > >> > +	unsigned long long c;
> > > >> > +	u32 cr = 0;
> > > >> > +	int ret;
> > > >> > +
> > > >> > +	pwm_get_state(pwm, &cstate);
> > > >> > +  
> > > >>
> > > >> Couldn't we do:
> > > >>
> > > >> if (cstate.enabled) { ...
> > > >>  
> > > >> > +	c = clk_get_rate(imx->clk_per);
> > > >> > +	c *= state->period;
> > > >> > +
> > > >> > +	do_div(c, 1000000000);
> > > >> > +	period_cycles = c;
> > > >> > +
> > > >> > +	prescale = period_cycles / 0x10000 + 1;
> > > >> > +
> > > >> > +	period_cycles /= prescale;
> > > >> > +	c = (unsigned long long)period_cycles *
> > > >> > state->duty_cycle;
> > > >> > +	do_div(c, state->period);
> > > >> > +	duty_cycles = c;
> > > >> > +
> > > >> > +	/*
> > > >> > +	 * according to imx pwm RM, the real period value
> > > >> > should be
> > > >> > +	 * PERIOD value in PWMPR plus 2.
> > > >> > +	 */
> > > >> > +	if (period_cycles > 2)
> > > >> > +		period_cycles -= 2;
> > > >> > +	else
> > > >> > +		period_cycles = 0;
> > > >> > +
> > > >> > +	/* Enable the clock if the PWM is being enabled. */
> > > >> > +	if (state->enabled && !cstate.enabled) {
> > > >> > +		ret = clk_prepare_enable(imx->clk_per);
> > > >> > +		if (ret)
> > > >> > +			return ret;
> > > >> > +	}
> > > >> > +
> > > >> > +	/*
> > > >> > +	 * Wait for a free FIFO slot if the PWM is already
> > > >> > enabled, and flush
> > > >> > +	 * the FIFO if the PWM was disabled and is about to
> > > >> > be enabled.
> > > >> > +	 */
> > > >> > +	if (cstate.enabled)
> > > >> > +		imx_pwm_wait_fifo_slot(chip, pwm);
> > > >> > +	else if (state->enabled)
> > > >> > +		imx_pwm_sw_reset(chip);
> > > >> > +
> > > >> > +	writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
> > > >> > +	writel(period_cycles, imx->mmio_base + MX3_PWMPR);
> > > >> > +
> > > >> > +	cr |= MX3_PWMCR_PRESCALER(prescale) |
> > > >> > +	      MX3_PWMCR_DOZEEN | MX3_PWMCR_WAITEN |
> > > >> > +	      MX3_PWMCR_DBGEN | MX3_PWMCR_CLKSRC_IPG_HIGH;
> > > >> > +
> > > >> > +	if (state->enabled)
> > > >> > +		cr |= MX3_PWMCR_EN;  
> > > >>
> > > >> } else if (state->enabled) {
> > > >> 	imx_pwm_sw_reset(chip);
> > > >> }
> > > >>
> > > >> and get rid of the if (state->enabled) in between? This would
> > > >> safe us useless recalculation when disabling the
> > > >> controller...  
> > > > 
> > > > I get your point, but I'm pretty sure your proposal does not do
> > > > what you want (remember that cstate is the current state, and
> > > > state is the new state to apply).
> > > > 
> > > > Something like that would work better:
> > > > 
> > > > 	if (state->enabled) {  
> > > 
> > > Oops, yes, got that wrong. state->enabled is what I meant.
> > >   
> > > > 		c = clk_get_rate(imx->clk_per);
> > > > 		c *= state->period;
> > > > 
> > > > 		do_div(c, 1000000000);
> > > > 		period_cycles = c;
> > > > 
> > > > 		prescale = period_cycles / 0x10000 + 1;
> > > > 
> > > > 		period_cycles /= prescale;
> > > > 		c = (unsigned long long)period_cycles *
> > > > 		    state->duty_cycle;
> > > > 		do_div(c, state->period);
> > > > 		duty_cycles = c;
> > > > 
> > > > 		/*
> > > > 		 * According to imx pwm RM, the real period
> > > > value
> > > > 		 * should be PERIOD value in PWMPR plus 2.
> > > > 		 */
> > > > 		if (period_cycles > 2)
> > > > 			period_cycles -= 2;
> > > > 		else
> > > > 			period_cycles = 0;
> > > > 
> > > > 		/*
> > > > 		 * Enable the clock if the PWM is not already
> > > > 		 * enabled.
> > > > 		 */
> > > > 		if (!cstate.enabled) {
> > > > 			ret = clk_prepare_enable(imx->clk_per);
> > > > 			if (ret)
> > > > 			return ret;
> > > > 		}
> > > > 
> > > > 		/*
> > > > 		 * Wait for a free FIFO slot if the PWM is
> > > > already
> > > > 		 * enabled, and flush the FIFO if the PWM was
> > > > disabled
> > > > 		 * and is about to be enabled.
> > > > 		 */
> > > > 		if (cstate.enabled)
> > > > 			imx_pwm_wait_fifo_slot(chip, pwm);
> > > > 		else
> > > > 			imx_pwm_sw_reset(chip);
> > > > 
> > > > 		writel(duty_cycles, imx->mmio_base +
> > > > MX3_PWMSAR); writel(period_cycles, imx->mmio_base + MX3_PWMPR);
> > > > 
> > > > 		writel(MX3_PWMCR_PRESCALER(prescale) |
> > > > 		       MX3_PWMCR_DOZEEN | MX3_PWMCR_WAITEN |
> > > > 		       MX3_PWMCR_DBGEN |
> > > > MX3_PWMCR_CLKSRC_IPG_HIGH | MX3_PWMCR_EN,
> > > > 		       imx->mmio_base + MX3_PWMCR);
> > > > 	} else {
> > > > 
> > > > 		writel(0, imx->mmio_base + MX3_PWMCR);
> > > > 
> > > > 		/* Disable the clock if the PWM is currently
> > > > enabled. */ if (cstate.enabled)
> > > > 			clk_disable_unprepare(imx->clk_per);
> > > > 	}
> > > > 
> > > > 
> > > > This being said, I'm a bit concerned by the way this driver
> > > > handles PWM config requests.
> > > > It seems that the new config request is queued, and nothing
> > > > guarantees  
> > > 
> > > Not sure if that is true. The RM says: "A change in the period
> > > value due to a write in PWM_PWMPR results in the counter being
> > > reset to zero and the start of a new count period."
> > > 
> > > And for PWMSAR: "When a new value is written, the duty cycle
> > > changes after the current period is over."
> > > 
> > > So I guess writing the period basically makes sure the next value
> > > from PWMSAR will be active immediately...
> > > 
> > >   
> > > > that it is actually applied when the
> > > > pwm_apply/config/enable/disable() functions return.  
> > > 
> > > 
> > > Given that the driver did it like that since quite some time, I am
> > > assuming it mostly works in practice. 
> > > 
> > > I would rather prefer to do that conversion to atomic PWM API
> > > now, and fix that in a second step...  
> > 
> > I'm also for fixing one problem in a time. The "PWM ->apply()" set
> > of patches now tries to fix all problems in IMX PWM driver.
> > 
> > Could we agree on the scope of this work? I mean what should be
> > included to "->apply()" rework and what will be fixed latter?
> 
> I never asked to fix that in this series ;-), I was just pointing the
> weird behavior of the existing code.
> 
> Let's focus on the atomic support for now.

So Boris, you don't have any comments to the atomic support patches? :-)

Stefan, do you require to change the ipg stuff in the atomic series or
could it be done as a subsequent patch?

If you don't have any more questions, I will prepare next patch
iteration according to Stefan comments.

Best regards,
Åukasz Majewski

> 
> > 
> > Frankly, I think that this patch series comes to the point where it
> > is not manageable anymore.
> > 
> > Please also keep in mind that I do have iMX6q system, Stefan has
> > imx7 and Sasha has HW with PWMv1 working.
> > 
> > Changing the driver in N different places not related to the
> > "->apply()" atomicity support (the ipg clock, FIFO) requires far
> > more work and testing.
> > 
> > 
> > Best regards,
> > Åukasz Majewski
> > 
> > >   
> > > > 
> > > > This approach has several flaws IMO:
> > > > 
> > > > 1/ I'm not sure this is what the PWM users expect. Getting your
> > > > request queued with no guarantees that it is applied can be
> > > > weird in some cases (especially when the user changes the PWM
> > > > config several times in a short period of time).
> > > > 2/ In the disable path, you queue a "stop PWM" request, but
> > > > you're not sure that it's actually dequeued before the per clk
> > > > is disabled. What happens in that case? And more importantly,
> > > > what happens when the PWM is re-enabled to apply a new config?
> > > > AFAICS, there might be a short period of time where the
> > > > re-enabled PWM is actually running with the old config until we
> > > > flush the command queue and queue a new command.
> > > > 3/ The queueing approach complicates the whole logic. You have
> > > > to flush the FIFO in some cases, or wait for an empty slots if
> > > > too many commands are queued.
> > > >    Forcing imx_pwm_apply_v2() to wait for the config request to
> > > > be applied should simplify the whole thing, because you will
> > > > always be guaranteed that the FIFO is empty, and that the
> > > > current configuration is the last requested one.
> > > >   
> > > 
> > > --
> > > Stefan  
> > 
>

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