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Date: Tue, 3 Jan 2017 20:35:51 +0100
From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Stefan Agner <stefan@agner.ch>
Cc: Lukasz Majewski <lukma@denx.de>,
        Thierry Reding <thierry.reding@gmail.com>,
        Sascha Hauer <s.hauer@pengutronix.de>, linux-pwm@vger.kernel.org,
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        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: <20170103203551.737e3a45@bbrezillon>
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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.

> 
> >>  
> >> >
> >> > 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.

Regards,

Boris