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 08:58:39 +0000 [thread overview]
Message-ID: <20201204085839.GA2937@gofer.mess.org> (raw)
In-Reply-To: <20201204084417.GA2154@gofer.mess.org>
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:
> > 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.
Let me rephrase that.
Changing the division to rounding down will exactly revert the fix I made
in commit 11fc4edc483bea8bf0efa0cc726886d2342f6fa6.
So in the case described in that commit, the requested frequency was 455kHz,
but rounding down resulted in a frequency of 476kHz.
That's totally broken and a bad idea.
Sean
next prev parent reply other threads:[~2020-12-04 8:59 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 [this message]
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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