Re: [PATCH v6 4/7] pwm: pca9685: Support staggered output ON times

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello Clemens,

On Wed, Mar 31, 2021 at 02:26:14PM +0200, Clemens Gruber wrote:
> On Mon, Mar 29, 2021 at 08:02:06PM +0200, Uwe Kleine-König wrote:
> > On Mon, Mar 29, 2021 at 07:16:38PM +0200, Clemens Gruber wrote:
> > > On Mon, Mar 29, 2021 at 07:03:57PM +0200, Uwe Kleine-König wrote:
> > > > On Mon, Mar 29, 2021 at 02:57:04PM +0200, Clemens Gruber wrote:
> > > > > The PCA9685 supports staggered LED output ON times to minimize current
> > > > > surges and reduce EMI.
> > > > > When this new option is enabled, the ON times of each channel are
> > > > > delayed by channel number x counter range / 16, which avoids asserting
> > > > > all enabled outputs at the same counter value while still maintaining
> > > > > the configured duty cycle of each output.
> > > > > 
> > > > > Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
> > > > 
> > > > Is there a reason to not want this staggered output? If it never hurts I
> > > > suggest to always stagger and drop the dt property.
> > > 
> > > There might be applications where you want multiple outputs to assert at
> > > the same time / to be synchronized.
> > > With staggered outputs mode always enabled, this would no longer be
> > > possible as they are spread out according to their channel number.
> > > 
> > > Not sure how often that usecase is required, but just enforcing the
> > > staggered mode by default sounds risky to me.
> > 
> > There is no such guarantee in the PWM framework, so I don't think we
> > need to fear breaking setups. Thierry?
> 
> Still, someone might rely on it? But let's wait for Thierry's opinion.

Someone might rely on the pca9685 driver being as racy as a driver with
legacy bindings usually is. Should this be the reason to drop this whole
series?

> > One reason we might not want staggering is if we have a consumer who
> > cares about config transitions. (This however is moot it the hardware
> > doesn't provide sane transitions even without staggering.)
> > 
> > Did I already ask about races in this driver? I assume there is a
> > free running counter and the ON and OFF registers just define where in
> > the period the transitions happen, right? Given that changing ON and OFF
> > needs two register writes probably all kind of strange things can
> > happen, right? (Example thought: for simplicity's sake I assume ON is
> > always 0. Then if you want to change from OFF = 0xaaa to OFF = 0xccc we
> > might see a period with 0xacc. Depending on how the hardware works we
> > might even see 4 edges in a single period then.)
> 
> Yes, there is a free running counter from 0 to 4095.
> And it is probably true, that there can be short intermediate states
> with our two register writes.
> 
> There is a separate mode "Update on ACK" (MODE2 register, bit 3 "OCH"),
> which is 0 by default (Outputs change on STOP command) but could be set
> to 1 (Outputs change on ACK):
> "Update on ACK requires all 4 PWM channel registers to be loaded before
> outputs will change on the last ACK."

This is about the ACK and STOP in the i2c communication, right? I fail
to understand the relevance of this difference. I guess I have to read
the manual myself.
 
> The chip datasheet also states:
> "Because the loading of the LEDn_ON and LEDn_OFF registers is via the
> I2C-bus, and asynchronous to the internal oscillator, we want to ensure
> that we do not see any visual artifacts of changing the ON and OFF
> values. This is achieved by updating the changes at the end of the LOW
> cycle."

So we're only out of luck if the first register write happens before and
the second after the end of the LOW cycle, aren't we?

Best regards
Uwe

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

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