Date:   Tue, 3 Jan 2017 23:01:11 +0100
From:   Lukasz Majewski <lukma@...x.de>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     Stefan Agner <stefan@...er.ch>,
        Thierry Reding <thierry.reding@...il.com>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-pwm@...r.kernel.org,
        Bhuvanchandra DV <bhuvanchandra.dv@...adex.com>,
        linux-kernel@...r.kernel.org,
        Fabio Estevam <fabio.estevam@....com>,
        Fabio Estevam <festevam@...il.com>,
        Lothar Wassmann <LW@...o-electronics.de>, kernel@...gutronix.de
Subject: Re: [PATCH v3 RESEND 07/11] pwm: imx: Provide atomic PWM support
 for i.MX PWMv2

Hi Boris, Stefan,

> On Tue, 03 Jan 2017 09:29:40 -0800
> Stefan Agner <stefan@...er.ch> wrote:
> 
> > On 2017-01-03 04:46, Boris Brezillon wrote:
> > <snip>
> > >> > Well, regarding the imx_pwm_apply_v2() suggested by Stefan, I
> > >> > think we both agreed that most of the code was unneeded when
> > >> > all we want to do is disable the PWM.  
> > >>
> > >> So for the PATCH 7/11 we fix the issue with recalculating clocks
> > >> when we want to disable PWM.
> > >>
> > >> 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;
> > >>
> > >> 		/*
> > >> 		 * 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);
> > >> 	}
> > >>
> > >>  
> > > 
> > > Yep.
> > >   
> > 
> > This looks like a good transformation of the current Patch 7, but
> > once you merge my patch, it will look slightly different...
> 
> Yes. I think we should just unconditionally enable/disable the per_clk
> at function entry/exit. The prepare_enable() call is almost free
> when the clk is already enabled, so it's not like we're adding a huge
> overhead by doing that.

So in the above snippet we should replace:

if (!cstate.enabled) {
	ret = clk_prepare_enable(imx->clk_per);
	if (ret)
		return ret;
}

with
	ret = clk_prepare_enable(imx->clk_per);
	if (ret)
		return ret;

And 

if (cstate.enabled)
	clk_disable_unprepare(imx->clk_per);

with 
	clk_disable_unprepare(imx->clk_per);

> 
> > 
> > >>  
> > >> >
> > >> > My concern was more about the way PWM changes are applied
> > >> > (->apply() returns before the change is actually applied), but
> > >> > I agreed that it could be fixed later on (if other people
> > >> > think it's really needed), since the existing code already
> > >> > handles it this way.  
> > >>
> > >> This is the issue with FIFO setting - but for now we do not deal
> > >> with it.  
> > > 
> > > Exactly.
> > >   
> > >>  
> > >> >  
> > >> > > No clear decision what to change until today when Stefan
> > >> > > prepared separate (concise) patch (now I see what is the
> > >> > > problem). 
> > >> >
> > >> > The patch proposed by Stefan is addressing a different
> > >> > problem: the periph clock has to be enabled before accessing
> > >> > registers.  
> > >>
> > >> So for this reason Stefan's patch [1] always enable the clock no
> > >> matter if PWM clock is generated or not.  
> > > 
> > > Yes.
> > >   
> > >>  
> > >> >  
> > >> > >  
> > >> > > >
> > >> > > > Same goes for the regression introduced in patch 2: I
> > >> > > > think it's better to keep things bisectable on all
> > >> > > > platforms (even if it appeared to work by chance on imx7,
> > >> > > > it did work before this change).  
> > >> > >
> > >> > > Could you be more specific about your idea to solve this
> > >> > > problem?  
> > >> >
> > >> > Stefan already provided a patch, I just think it should be
> > >> > fixed before patch 2 to avoid breaking bisectibility.  
> > >>
> > >> My idea is as follows:
> > >>
> > >> I will drop patch v2 (prepared by Sasha) and then squash
> > >> Stefan's patch [1] to patch 7/11. The "old" ipg enable code will
> > >> be removed with other not needed code during conversion.  
> > > 
> > > How about keeping patch 2 but enabling/disabling the periph clk
> > > in imx_pwm_config() instead of completely dropping the
> > > enable/disable clk sequence.
> > > 
> > > In patch 7 you just add the logic we talked about earlier:
> > > unconditionally enable the periph clk when entering the
> > > imx_pwm_apply_v2() function and disable it before leaving the
> > > function.
> > > 
> > > This way you can preserve bisectibility and still get rid of the
> > > ipg clk.
> > > 
> > > Stefan, what's your opinion?  
> > 
> > We will get rid of the ipg clocks anyway in patch 8 (which removes
> > those functions completely).
> > 
> > So I think Lukasz approach should be fine, just drop patch 2 and
> > squash my patch into patch 7.
> 
> Well, the end result will be same (ipg_clk will be gone after patch
> 8), but then it's hard to track why this clock suddenly disappeared.
> I still think it's worth adding an extra commit explaining that
> enabling the per_clk before accessing IP registers is needed on some
> platforms (imx7), and that IPG clk is actually not required until we
> start using it as a source for the PWM signal generation.
> 
> Maybe I'm the only one to think so. In this case, feel free to drop
> patch 2.

If you feel really bad about this issue, then we can drop patch 2 and:

reorganize patch 7/11 to 
 - keep code, which adds imx_pwm_apply_v2() function code (just moves it
as is) 
 - remove .apply = imx_pwm_apply_v2 entry from pwm_ops structure.


On top of it add patch to enable/disable unconditionally the
imx->clk_per clock to avoid problems on imx7 (and state them in commit
message).

Then we add separate patch with 
.apply = imx_pwm_apply_v2 to pwm_ops structure to enable "new" atomic
approach.

And at last we apply patch 8/11, which removes the code for old (non
atomic) behaviour.

All the issues are documented in this way on the cost of having
"dead" (I mean not used) imx_pwm_apply_v2() for two commits.



> 
> Regards,
> 
> Boris




Best regards,

Lukasz Majewski

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