Re: [PATCH v4 07/11] pwm: imx: Provide atomic PWM support for i.MX PWMv2

[Boris Brezillon]

On Thu, 5 Jan 2017 10:03:47 +0100
Lukasz Majewski <lukma@xxxxxxx> wrote:

> On Thu, 5 Jan 2017 09:50:35 +0100
> Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu,  5 Jan 2017 00:36:50 +0100
> > Lukasz Majewski <lukma@xxxxxxx> 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 v4:
> > > - Avoid recalculation of PWM parameters when disabling PWM signal
> > > - Unconditionally call clk_prepare_enable(imx->clk_per) and
> > >   clk_disable_unprepare(imx->clk_per)  
> > 
> > I don't see that one, but I'm not sure this is actually needed.
> > In the enabled path we enable the clk before accessing the registers,
> > and in the disable path, assuming you change the code according to my
> > suggestion, the clk should already be enabled when you write to
> > MX3_PWMCR.
> >   
> > > 
> > > Changes for v3:
> > > - Remove ipg clock enable/disable functions
> > > 
> > > Changes for v2:
> > > - None
> > > ---
> > >  drivers/pwm/pwm-imx.c | 67
> > > +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed,
> > > 67 insertions(+)
> > > 
> > > diff --git a/drivers/pwm/pwm-imx.c b/drivers/pwm/pwm-imx.c
> > > index 60cdc5c..134dd66 100644
> > > --- a/drivers/pwm/pwm-imx.c
> > > +++ b/drivers/pwm/pwm-imx.c
> > > @@ -249,6 +249,72 @@ static int imx_pwm_config(struct pwm_chip
> > > *chip, return ret;
> > >  }
> > >  
> > > +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;
> > > +	int ret;
> > > +
> > > +	pwm_get_state(pwm, &cstate);
> > > +
> > > +	if (state->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;
> > > +  
> > 
> > Starting here...
> >   
> > > +		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);  
> > 
> > ... till here, should be replaced by:
> > 
> > 
> > 		/*
> > 		 * 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 {
> > 			ret = clk_prepare_enable(imx->clk_per);
> > 			if (ret)
> > 				return ret;  
> 
> In the other mail you mentioned that the clock should be enabled
> unconditionally to fix issue on iMX7 when we want to access registers.

When I said unconditionally, I meant call
clk_prepare_enable(imx->clk_per) just after pwm_get_state() (at the
beginning of the function) and call clk_disable_(imx->clk_per) just
before returning 0 at the end of the function. This way you're
guaranteed that the clk is always enabled when you access registers in
between. Of course, you still need the the clk_prepare_enable() and
clk_disable_unprepare() in the enable and disable path to keep the clk
enabled when the PWM is enabled, and to disable it when the clk is
being disabled.

But, while reviewing your patch I realized this was actually unneeded
(see the explanation in my previous review).

> 
> Now it depends on cstate.enabled flag. 
> 
> So we end up with 
> 
> if (state.enabled && !cstate.enabled)
> 	clk_preapre_enable();

Yep, and that's correct.

> 
> which was the reason of iMX7 failure reported by Stefan to v3.

Stefan, do you confirm that? I don't see how this can possibly fail
since the clk is either already enabled (cstate.enabled) or we enable
it (!cstate.enable) before accessing registers.

What's for sure is that your implementation is introducing possible
unbalanced ref counting on the per clk.

> 
> 
> > 
> > 			imx_pwm_sw_reset(chip);
> > 		}
> > 
> > Otherwise you keep incrementing the prepare/enable counters of the clk
> > when you change the config of an already enabled PWM.
> >   
> > > +
> > > +		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 {  
> > 
> > 	} else if (cstate.enabled) {
> > 
> > Again, this is to avoid unbalanced prepare/enable counters on the per
> > clk if the PWM config is changed several times when the PWM is
> > disabled.
> >   
> > > +		writel(0, imx->mmio_base + MX3_PWMCR);
> > > +
> > > +		clk_disable_unprepare(imx->clk_per);
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > > +
> > >  static int imx_pwm_enable(struct pwm_chip *chip, struct pwm_device
> > > *pwm) {
> > >  	struct imx_chip *imx = to_imx_chip(chip);
> > > @@ -280,6 +346,7 @@ static struct pwm_ops imx_pwm_ops_v1 = {
> > >  };
> > >  
> > >  static struct pwm_ops imx_pwm_ops_v2 = {
> > > +	.apply = imx_pwm_apply_v2,
> > >  	.enable = imx_pwm_enable,
> > >  	.disable = imx_pwm_disable,
> > >  	.config = imx_pwm_config,  
> >   
> 
> 
> 
> 
> Best regards,
> 
> Lukasz Majewski
> 
> --
> 
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