From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Clemens Gruber <clemens.gruber@pqgruber.com>
Cc: linux-pwm@vger.kernel.org,
Thierry Reding <thierry.reding@gmail.com>,
Sven Van Asbroeck <TheSven73@gmail.com>,
devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 1/8] pwm: pca9685: Switch to atomic API
Date: Wed, 14 Apr 2021 21:21:31 +0200 [thread overview]
Message-ID: <20210414192131.2o4c2eia6jnjatp2@pengutronix.de> (raw)
In-Reply-To: <YHbbaiwK9Tasb7NF@workstation.tuxnet>
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On Wed, Apr 14, 2021 at 02:09:14PM +0200, Clemens Gruber wrote:
> Hi Uwe,
>
> On Tue, Apr 13, 2021 at 09:38:18PM +0200, Uwe Kleine-König wrote:
> > Hello Clemens,
> >
> > On Tue, Apr 13, 2021 at 02:11:38PM +0200, Clemens Gruber wrote:
> > > On Mon, Apr 12, 2021 at 10:10:19PM +0200, Uwe Kleine-König wrote:
> > > > On Mon, Apr 12, 2021 at 06:39:28PM +0200, Clemens Gruber wrote:
> > > > > With your suggested round-down, the example with frequency of 200 Hz
> > > > > would no longer result in 30 but 29 and that contradicts the datasheet.
> > > >
> > > > Well, with PRESCALE = 30 we get a frequency of 196.88 Hz and with
> > > > PRESCALE = 29 we get a frequency of 203.45 Hz. So no matter if you pick
> > > > 29 or 30, you don't get 200 Hz. And which of the two possible values is
> > > > the better one depends on the consumer, no matter what rounding
> > > > algorithm the data sheet suggests. Also note that the math here contains
> > > > surprises you don't expect at first. For example, what PRESCALE value
> > > > would you pick to get 284 Hz? [If my mail was a video, I'd suggest to
> > > > press Space now to pause and let you think first :-)] The data sheet's
> > > > formula suggests:
> > > >
> > > > round(25 MHz / (4096 * 284)) - 1 = 20
> > > >
> > > > The resulting frequency when picking PRESCALE = 20 is 290.644 Hz (so an
> > > > error of 6.644 Hz). If instead you pick PRESCALE = 21 you get 277.433 Hz
> > > > (error = 6.567 Hz), so 21 is the better choice.
> > > >
> > > > Exercise for the reader:
> > > > What is the correct formula to really determine the PRESCALE value that
> > > > yields the best approximation (i.e. minimizing
> > > > abs(real_freq - target_freq)) for a given target_freq?
> >
> > I wonder if you tried this.
>
> We could calculate both round-up and round-down and decide which one is
> closer to "real freq" (even though that is not the actual frequency but
> just our backwards-calculated frequency).
Yeah, the backwards-calculated frequency is the best assumption we
have.
> But I can't give you a formula with minimized abs(real_freq-target_freq)
> Is it a different round point than 0.5 and maybe relative to f ?
>
> Please enlighten us :-)
Sorry, I cannot. I spend ~20 min today after lunch with pencil and
paper, but without success. I was aware that it isn't trivial and this
is the main reason I established round-down as default for new drivers
instead of round-nearest.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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next prev parent reply other threads:[~2021-04-14 19:21 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-12 13:27 [PATCH v8 1/8] pwm: pca9685: Switch to atomic API Clemens Gruber
2021-04-12 13:27 ` [PATCH v8 2/8] pwm: pca9685: Support hardware readout Clemens Gruber
2021-04-12 16:21 ` Uwe Kleine-König
2021-04-12 13:27 ` [PATCH v8 3/8] pwm: pca9685: Improve runtime PM behavior Clemens Gruber
2021-04-12 13:27 ` [PATCH v8 4/8] dt-bindings: pwm: Support new PWM_USAGE_POWER flag Clemens Gruber
2021-04-12 16:27 ` Uwe Kleine-König
2021-04-12 16:46 ` Clemens Gruber
2021-04-13 11:38 ` Uwe Kleine-König
2021-04-13 11:41 ` Thierry Reding
2021-04-13 11:51 ` Thierry Reding
2021-04-13 17:56 ` Uwe Kleine-König
2021-04-15 16:27 ` Thierry Reding
2021-04-16 9:32 ` Uwe Kleine-König
2021-04-16 10:45 ` Thierry Reding
2021-04-18 13:30 ` Uwe Kleine-König
2021-04-16 13:55 ` Thierry Reding
2021-04-16 15:39 ` Rob Herring
2021-04-16 15:54 ` Clemens Gruber
2021-04-17 15:44 ` Uwe Kleine-König
2021-04-12 13:27 ` [PATCH v8 5/8] pwm: core: " Clemens Gruber
2021-04-12 13:27 ` [PATCH v8 6/8] pwm: pca9685: " Clemens Gruber
2021-04-12 16:30 ` Uwe Kleine-König
2021-04-12 17:11 ` Clemens Gruber
2021-04-13 10:34 ` Uwe Kleine-König
2021-04-12 13:27 ` [PATCH v8 7/8] pwm: pca9685: Restrict period change for enabled PWMs Clemens Gruber
2021-04-17 15:46 ` Uwe Kleine-König
2021-04-12 13:27 ` [PATCH v8 8/8] pwm: pca9685: Add error messages for failed regmap calls Clemens Gruber
2021-04-17 15:47 ` Uwe Kleine-König
2021-04-12 16:18 ` [PATCH v8 1/8] pwm: pca9685: Switch to atomic API Uwe Kleine-König
2021-04-12 16:39 ` Clemens Gruber
2021-04-12 20:10 ` Uwe Kleine-König
2021-04-13 12:11 ` Clemens Gruber
2021-04-13 12:17 ` Clemens Gruber
2021-04-13 12:37 ` Thierry Reding
2021-04-13 13:06 ` Clemens Gruber
2021-04-13 19:38 ` Uwe Kleine-König
2021-04-14 12:09 ` Clemens Gruber
2021-04-14 19:21 ` Uwe Kleine-König [this message]
2021-04-14 19:45 ` Clemens Gruber
2021-04-15 6:48 ` Uwe Kleine-König
2021-04-17 15:37 ` Uwe Kleine-König
2021-04-17 16:40 ` Clemens Gruber
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