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From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Michael Walle <michael@walle.cc>
Cc: Thierry Reding <thierry.reding@gmail.com>,
	linux-gpio@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-hwmon@vger.kernel.org,
	linux-pwm@vger.kernel.org, linux-watchdog@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Linus Walleij <linus.walleij@linaro.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Rob Herring <robh+dt@kernel.org>,
	Jean Delvare <jdelvare@suse.com>,
	Guenter Roeck <linux@roeck-us.net>,
	Lee Jones <lee.jones@linaro.org>,
	Wim Van Sebroeck <wim@linux-watchdog.org>,
	Shawn Guo <shawnguo@kernel.org>, Li Yang <leoyang.li@nxp.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jason Cooper <jason@lakedaemon.net>,
	Marc Zyngier <maz@kernel.org>, Mark Brown <broonie@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Subject: Re: [PATCH v5 07/13] pwm: add support for sl28cpld PWM controller
Date: Wed, 15 Jul 2020 18:36:20 +0200
Message-ID: <20200715163620.xhi24mct5b64qpyp@pengutronix.de> (raw)
In-Reply-To: <eedceb44cba9b54e0634f0e8e4f96f70@walle.cc>


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Hello Michael,

On Tue, Jul 14, 2020 at 11:09:28PM +0200, Michael Walle wrote:
> > My wishlist (just as it comes to my mind, so no guarantee of
> > completeness):
> > 
> >  - can do 0% duty cycle for all supported period lengths
> >  - can do 100% duty cycle for all supported period lengths
> >  - supports both polarities
> >  - supports immediate change of configuration and after completion of
> >    the currently running period
> >  - atomic update (i.e. if you go from configuration A to configuration B
> >    the hardware guarantees to only emit periods of type A and then type
> >    B. (Depending on the item above, the last A period might be cut off.)
> 
> We actually discussed this, because the implementation would be easier. But
> if the change takes place immediately you might end up with a longer duty
> cycle. Assume the PWM runs at 80% duty cycle and starts with the on-period.
> If you now change that to 50% you might end up with one successive duty
> cycle of "130%". Eg. the 80% of the old and right after that you switch to
> the new 50% and then you'd have a high output which corresponds to a 130%
> cycle. I don't know if that is acceptable for all applications.

I thought this is a "change takes place immediately" implementation?! So
these problems are actually real here. (And this not happening is exactly
my wish here. Is there a mis-understanding?)

> >  - emits an irq when configuration changes
> 
> Why would you need the interrupt?

To know that the new setting is active. Currently Thierry's ideal PWM
implementation blocks in pwm_apply_state() until the new setting is
active. So some signaling is nice. 

> > > > If you change only cycle but not mode, does the hardware complete the
> > > > currently running period?
> > > 
> > > No it does not.
> > 
> > Please document this as a Limitation.
> 
> I've discussed this internally, for now its a limitation. In the worst
> case you'd do one 100% duty cycle. Maybe we can fix the hardware. I
> acknowledge that this is a severe limitation, esp. if you use the PWM
> for controlling stuff (for now its only LCD backlight.. so thats ok).

That happens if you reduce the duty cycle from A to B and the counter is
already bigger than B but smaller than A, right? The fix would be to
compare for counter >= match instead of counter = match. (Which then
would result in a period with a duty cycle bigger than B but smaller
than A. Also not ideal, but probably better.)

> > > > What about disable()?
> > > 
> > > Mhh well, it would do one 100% cycle.. mhh ;) Lets see if there we can
> > > fix that (in hardware), not much we can do in the driver here. We are
> > > _very_ constraint in size, therefore all that little edge cases fall
> > > off
> > > the table.
> > 
> > You're saying that on disable the hardware emits a constant high level
> > for one cycle? I hope not ...
> 
> Mh, I was mistaken, disabling the PWM will turn it off immediately, but

And does turn off mean, the output gets inactive?
If so you might also disable the hardware if a 0% duty cycle is
configured assuming this saves some energy without modifying the
resulting wave form.

> one 100% duty cycle may happen if you change from a higher to a lower
> duty cycle setting. See above.
> 
> > I never programmed a CPLD to emulate a hardware PWM, but I wonder if
> > these are really edge cases that increase the size of the binary?!
> 
> At the moment there is only one 8bit register which stores the value
> which is used for matching. If you want to change that setting after
> a whole cycle, you'd use another 8bit register to cache the new value.
> So this would at least needs 8 additional flip-flops. This doesn't
> sound much, but we are already near 100% usage of the CPLD. So its
> hard to convince people why this is really necessary.

OK. (Maybe there is enough space to allow implementing 100% for mode 0?)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-06 17:53 [PATCH v5 00/13] Add support for Kontron sl28cpld Michael Walle
2020-07-06 17:53 ` [PATCH v5 01/13] regmap-irq: use fwnode instead of device node in add_irq_chip() Michael Walle
2020-07-08 10:22   ` Mark Brown
2020-07-06 17:53 ` [PATCH v5 02/13] mfd: add simple regmap based I2C driver Michael Walle
2020-07-17  9:04   ` Lee Jones
2020-07-19 22:25     ` Michael Walle
2020-07-17  9:06   ` Lee Jones
2020-07-19 22:31     ` Michael Walle
2020-07-20  7:43       ` Lee Jones
2020-07-06 17:53 ` [PATCH v5 03/13] dt-bindings: mfd: Add bindings for sl28cpld Michael Walle
2020-07-13 16:04   ` Rob Herring
2020-07-06 17:53 ` [PATCH v5 04/13] mfd: simple-mfd-i2c: add sl28cpld support Michael Walle
2020-07-06 17:53 ` [PATCH v5 05/13] irqchip: add sl28cpld interrupt controller support Michael Walle
2020-07-08  9:37   ` Marc Zyngier
2020-07-06 17:53 ` [PATCH v5 06/13] watchdog: add support for sl28cpld watchdog Michael Walle
2020-07-06 17:53 ` [PATCH v5 07/13] pwm: add support for sl28cpld PWM controller Michael Walle
2020-07-09  8:50   ` Uwe Kleine-König
2020-07-11 17:28     ` Michael Walle
2020-07-13  8:47       ` Uwe Kleine-König
2020-07-14 11:31         ` Michael Walle
2020-07-14 16:08           ` Uwe Kleine-König
2020-07-14 21:09             ` Michael Walle
2020-07-15 16:36               ` Uwe Kleine-König [this message]
2020-07-15 17:45                 ` Michael Walle
2020-07-15 18:18                   ` Uwe Kleine-König
2020-07-15 20:41                     ` Michael Walle
2020-07-16  6:10                       ` Uwe Kleine-König
2020-07-06 17:53 ` [PATCH v5 08/13] gpio: add support for the sl28cpld GPIO controller Michael Walle
2020-07-08  7:35   ` Linus Walleij
2020-07-06 17:53 ` [PATCH v5 09/13] hwmon: add support for the sl28cpld hardware monitoring controller Michael Walle
2020-07-06 17:53 ` [PATCH v5 10/13] arm64: dts: freescale: sl28: enable sl28cpld Michael Walle
2020-07-06 17:53 ` [PATCH v5 11/13] arm64: dts: freescale: sl28: map GPIOs to input events Michael Walle
2020-07-06 17:53 ` [PATCH v5 12/13] arm64: dts: freescale: sl28: enable LED support Michael Walle
2020-07-17  8:36   ` Pavel Machek
2020-07-17 21:54     ` Michael Walle
2020-07-06 17:53 ` [PATCH v5 13/13] arm64: dts: freescale: sl28: enable fan support Michael Walle
2020-07-08  7:39   ` Linus Walleij
2020-07-08  7:56     ` Michael Walle

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