From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Brian Norris <briannorris@chromium.org>
Cc: Thierry Reding <thierry.reding@gmail.com>,
linux-pwm@vger.kernel.org, Mark Brown <broonie@kernel.org>,
Liam Girdwood <lgirdwood@gmail.com>,
Heiko Stuebner <heiko@sntech.de>,
linux-rockchip@lists.infradead.org,
Rob Herring <robh+dt@kernel.org>, Pawel Moll <pawel.moll@arm.com>,
Mark Rutland <mark.rutland@arm.com>,
Ian Campbell <ijc+devicetree@hellion.org.uk>,
Kumar Gala <galak@codeaurora.org>,
devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, Milo Kim <milo.kim@ti.com>,
Doug Anderson <dianders@google.com>,
Caesar Wang <wxt@rock-chips.com>,
Stephen Barber <smbarber@chromium.org>,
Ajit Pal Singh <ajitpal.singh@st.com>,
Srinivas Kandagatla <srinivas.kandagatla@gmail.com>,
Maxime Coquelin <maxime.coquelin@st.com>,
Patrice Chotard <patrice.chotard@st.com>,
kernel@stlinux.com
Subject: Re: [PATCH 03/14] pwm: rockchip: Fix period and duty_cycle approximation
Date: Sat, 4 Jun 2016 08:19:55 +0200 [thread overview]
Message-ID: <20160604081955.47393878@bbrezillon> (raw)
In-Reply-To: <20160603200326.GA124478@google.com>
On Fri, 3 Jun 2016 13:03:26 -0700
Brian Norris <briannorris@chromium.org> wrote:
> On Fri, Jun 03, 2016 at 10:23:01AM +0200, Boris Brezillon wrote:
> > The current implementation always round down the duty and period
> > values, while it would be better to round them to the closest integer.
>
> Agreed. As I noted to you elsewhere, not having this change can cause
> problems where doing a series of pwm_get_state() / modify /
> pwm_apply_state() will propagate rounding errors, which will change the
> period unexpectedly. e.g., I have an expected period of 3.333 us and a
> clk rate of 112.666667 MHz -- the clock frequency doesn't divide evenly,
> so the period (stashed in nanoseconds) shrinks when we convert to the
> register value and back, as follows:
>
> pwm_apply_state(): register = period * 112666667 / 1000000000;
> pwm_get_state(): period = register * 1000000000 / 112666667;
>
> or in other words:
>
> period = period * 112666667 / 1000000000 * 1000000000 / 112666667;
>
> which yields a sequence like:
>
> 3333 -> 3328
> 3328 -> 3319
> 3319 -> 3310
> 3310 -> 3301
> 3301 -> 3292
> 3292 -> ... (etc) ...
>
> With this patch, we'd see instead:
>
> period = div_round_closest(period * 112666667, 1000000000) * 1000000000 / 112666667;
>
> which yields a stable sequence:
>
> 3333 -> 3337
> 3337 -> 3337
> 3337 -> ... (etc) ...
Woh! Thanks for the detailed explanation. Do you want me to put that in
a comment explaining why we're using DIV_ROUND_CLOSEST_ULL()?
--
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
next prev parent reply other threads:[~2016-06-04 6:19 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-03 8:22 [PATCH 00/14] regulator: pwm: various improvements Boris Brezillon
2016-06-03 8:22 ` [PATCH 01/14] pwm: Add new helpers to create/manipulate PWM states Boris Brezillon
2016-06-03 8:23 ` [PATCH 04/14] pwm: rockchip: Add support for hardware readout Boris Brezillon
2016-06-03 20:07 ` Brian Norris
2016-06-03 20:20 ` Brian Norris
2016-06-04 6:24 ` Boris Brezillon
2016-06-07 17:26 ` Brian Norris
[not found] ` <1464942192-25967-1-git-send-email-boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
2016-06-03 8:23 ` [PATCH 02/14] regulator: pwm: Drop unneeded pwm_enable() call Boris Brezillon
2016-06-03 11:08 ` Applied "regulator: pwm: Drop unneeded pwm_enable() call" to the regulator tree Mark Brown
2016-06-03 8:23 ` [PATCH 03/14] pwm: rockchip: Fix period and duty_cycle approximation Boris Brezillon
2016-06-03 20:03 ` Brian Norris
2016-06-04 6:19 ` Boris Brezillon [this message]
2016-06-07 17:25 ` Brian Norris
2016-06-03 8:23 ` [PATCH 05/14] pwm: rockchip: Avoid glitches on already running PWMs Boris Brezillon
2016-06-03 20:28 ` Brian Norris
2016-06-04 6:26 ` Boris Brezillon
2016-06-03 8:23 ` [PATCH 07/14] pwm: sti: Add support for hardware readout Boris Brezillon
2016-06-03 8:23 ` [PATCH 11/14] regulator: pwm: properly initialize the ->state field Boris Brezillon
2016-06-03 20:51 ` Brian Norris
2016-06-03 8:23 ` [PATCH 12/14] regulator: pwm: Retrieve correct voltage Boris Brezillon
2016-06-03 20:55 ` Brian Norris
2016-06-03 8:23 ` [PATCH 06/14] pwm: rockchip: Add support for atomic update Boris Brezillon
[not found] ` <1464942192-25967-7-git-send-email-boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
2016-06-03 20:37 ` Brian Norris
2016-06-03 8:23 ` [PATCH 08/14] pwm: sti: Avoid glitches on already running PWMs Boris Brezillon
2016-06-03 20:38 ` Brian Norris
2016-06-03 8:23 ` [PATCH 09/14] regulator: pwm: Adjust PWM config at probe time Boris Brezillon
2016-06-03 20:41 ` Brian Norris
2016-06-03 8:23 ` [PATCH 10/14] regulator: pwm: Switch to the atomic PWM API Boris Brezillon
[not found] ` <1464942192-25967-11-git-send-email-boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
2016-06-03 20:50 ` Brian Norris
2016-06-04 6:28 ` Boris Brezillon
2016-06-06 6:14 ` Laxman Dewangan
2016-06-03 8:23 ` [PATCH 13/14] regulator: pwm: Support extra continuous mode cases Boris Brezillon
2016-06-03 21:03 ` Brian Norris
2016-06-04 6:30 ` Boris Brezillon
2016-06-03 8:23 ` [PATCH 14/14] regulator: pwm: Document pwm-dutycycle-unit and pwm-dutycycle-range Boris Brezillon
[not found] ` <1464942192-25967-15-git-send-email-boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
2016-06-03 21:04 ` Brian Norris
2016-06-06 14:09 ` Rob Herring
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