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Date: Tue, 3 Jan 2017 23:46:58 +0100
From: Lukasz Majewski <lukma@denx.de>
To: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Stefan Agner <stefan@agner.ch>,
        Thierry Reding <thierry.reding@gmail.com>,
        Sascha Hauer <s.hauer@pengutronix.de>, linux-pwm@vger.kernel.org,
        Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>,
        linux-kernel@vger.kernel.org, Fabio Estevam <fabio.estevam@nxp.com>,
        Fabio Estevam <festevam@gmail.com>,
        Lothar Wassmann <LW@karo-electronics.de>, kernel@pengutronix.de
Subject: Re: [PATCH v3 RESEND 07/11] pwm: imx: Provide atomic PWM support
 for i.MX PWMv2
Message-ID: <20170103234658.2632350e@jawa>
In-Reply-To: <20170103231826.6d11e65e@bbrezillon>
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Hi Boris,

> On Tue, 3 Jan 2017 23:01:11 +0100
> Lukasz Majewski <lukma@denx.de> wrote:
> 
> > Hi Boris, Stefan,
> > 
> > > On Tue, 03 Jan 2017 09:29:40 -0800
> > > Stefan Agner <stefan@agner.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);
> 
> That's what I had in mind.

OK.

> 
> > 
> > >   
> > > >   
> > > > >>    
> > > > >> >
> > > > >> > 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.
> > 
> 
> This looks even more complicated.
> Sorry, but I don't see the problem with modifying patch 2 to enable
> per_clk instead of ipg_clk. Can you explain what's bothering you?

But in patch 2:
"pwm: imx: remove ipg clock"

 we _remove_ the clk_ipg from imx_pwm_config() and imx_pwm_probe(), so
 I'm quite puzzled with your above statement.

> 
> If you really want to do the change after patch 7, fine, but in this
> case, keep the existing logic: enable/disable ipg_clk in
> imx_pwm_apply_v2() until you drop the ipg_clk and replace the ipg_clk
> enable/disable sequence by the equivalent enable/disable per_clk one.
> 

Frankly, I do agree with Stefan here - we should drop patch 2, squash
all changes (including imx7 clock issues) to patch 7 (including verbose
commit message) and remove the non-atomic code in patch 8.


Best regards,

Lukasz Majewski

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