From: Sean Young <sean@mess.org>
To: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.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 08:44:17 +0000 [thread overview]
Message-ID: <20201204084417.GA2154@gofer.mess.org> (raw)
In-Reply-To: <4683237c-7b40-11ab-b3c0-f94a5dd39b4d@gmx.de>
Hi,
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 || value > U32_MAX)
return -EINVAL;
> > 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.
Thanks
Sean
next prev parent reply other threads:[~2020-12-04 8:45 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 [this message]
2020-12-04 8:58 ` Sean Young
2020-12-04 11:13 ` Uwe Kleine-König
2020-12-04 11:38 ` Sean Young
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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