From: Sean Young <sean@mess.org>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Lino Sanfilippo <LinoSanfilippo@gmx.de>,
thierry.reding@gmail.com, lee.jones@linaro.org,
nsaenzjulienne@suse.de, f.fainelli@gmail.com, rjui@broadcom.com,
sbranden@broadcom.com, bcm-kernel-feedback-list@broadcom.com,
linux-pwm@vger.kernel.org, linux-rpi-kernel@lists.infradead.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic configuration
Date: Fri, 4 Dec 2020 11:38:46 +0000 [thread overview]
Message-ID: <20201204113846.GA6547@gofer.mess.org> (raw)
In-Reply-To: <20201204111326.qjux6k2472dmukot@pengutronix.de>
Hi,
On Fri, Dec 04, 2020 at 12:13:26PM +0100, Uwe Kleine-König wrote:
> On Fri, Dec 04, 2020 at 08:44:17AM +0000, Sean Young wrote:
> > On Fri, Dec 04, 2020 at 12:42:15AM +0100, Lino Sanfilippo wrote:
> > > > You're storing an unsigned long long (i.e. 64 bits) in an u32. If
> > > > you are sure that this won't discard relevant bits, please explain
> > > > this in a comment for the cursory reader.
> > >
> > > What about an extra check then to make sure that the period has not been truncated,
> > > e.g:
> > >
> > > value = DIV_ROUND_CLOSEST_ULL(state->period, scaler);
> > >
> > > /* dont accept a period that is too small or has been truncated */
> > > if ((value < PERIOD_MIN) ||
> > > (value != DIV_ROUND_CLOSEST_ULL(state->period, scaler)))
> > > return -EINVAL;
> >
> > Rather than doing another 64 bit division which is expensive (esp on 32 bit
> > kernels), you could assign to u64 and check:
> >
> > if (value < PERIOD_MIN || value > U32_MAX)
> > return -EINVAL;
>
> Given that value is a u32, value > U32_MAX will never trigger.
I meant that value is declared u64 as well ("assign to u64").
> Maybe checking period before doing the division is more sensible.
That could introduce rounding errors, exactly why PERIOD_MIN was introduced.
> > > > Also note that round_closed is probably wrong, as .apply() is
> > > > supposed to round down the period to the next achievable period. (But
> > > > fixing this has to do done in a separate patch.)
> > >
> > > According to commit 11fc4edc4 rounding to the closest integer has been introduced
> > > to improve precision in case that the pwm controller is used by the pwm-ir-tx driver.
> > > I dont know how strong the requirement is to round down the period in apply(), but I
> > > can imagine that this may be a good reason to deviate from this rule.
> > > (CCing Sean Young who introduced DIV_ROUND_CLOSEST)
> >
> > There was a problem where the carrier is incorrect for some IR hardware
> > which uses a carrier of 455kHz. With periods that small, rounding errors
> > do really matter and rounding down might cause problems.
> >
> > A policy of rounding down the carrier is not the right thing to do
> > for pwm-ir-tx, and such a change will probably break pwm-ir-tx in some
> > edge cases.
>
> IMO it's not an option to say: pwm-driver A is used for IR, so A's
> .apply uses round-nearest and pwm-driver B is used for $somethingelse,
> so B's .apply uses round-down.
I'm not saying that one driver should have one it one way and another driver
another way.
> To be a sensible API pwm_apply_state
> should have a fixed behaviour. I consider round-down the sensible
> choice (because it is easier to implmement the other options with this)
It's not sensible when it's wrong about half the time.
Why is is easier to implement?
> and for consumers like the IR stuff we need to provide some more
> functions to allow it selecting a better suited state. Something like:
>
> pwm_round_state_nearest(pwm, { .period = 2198, .. }, &state)
>
> which queries the hardwares capabilities and then assigns state.period =
> 2200 instead of 2100.
This is very elaborate and surely not "easier to implement". Why not just
do the right thing in the first place and round-closest?
> Where can I find the affected (consumer) driver?
So there is the pwm-ir-tx driver. The infrared led is directly connected
to the pwm output pin, so that's all there is.
Thanks,
Sean
next prev parent reply other threads:[~2020-12-04 11:39 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <202011281128.54eLfMWr-lkp@intel.com>
2020-11-28 12:02 ` [PATCH v2] pwm: bcm2835: Support apply function for atomic configuration Lino Sanfilippo
2020-11-29 18:10 ` Uwe Kleine-König
2020-12-03 23:42 ` Lino Sanfilippo
2020-12-04 8:44 ` Sean Young
2020-12-04 8:58 ` Sean Young
2020-12-04 11:13 ` Uwe Kleine-König
2020-12-04 11:38 ` Sean Young [this message]
2020-12-04 23:28 ` Uwe Kleine-König
2020-12-05 17:34 ` Sean Young
2020-12-05 19:25 ` Uwe Kleine-König
2020-12-06 14:19 ` Sean Young
2020-12-07 8:16 ` Uwe Kleine-König
2020-12-07 9:43 ` Sean Young
2020-12-07 13:52 ` Uwe Kleine-König
2020-12-07 15:29 ` Thierry Reding
2020-12-07 21:46 ` Uwe Kleine-König
2020-12-07 18:18 ` Sean Young
2020-12-08 0:00 ` Lino Sanfilippo
2020-12-08 9:07 ` Uwe Kleine-König
2020-12-04 23:16 ` Lino Sanfilippo
2020-12-04 11:21 ` Uwe Kleine-König
2020-12-04 11:40 ` Sean Young
2020-12-04 21:55 ` Uwe Kleine-König
2020-12-04 22:44 ` Sean Young
2020-12-04 23:25 ` Lino Sanfilippo
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