From: Brian Norris <briannorris@chromium.org> To: Boris Brezillon <boris.brezillon@free-electrons.com> 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: Fri, 3 Jun 2016 13:03:26 -0700 [thread overview] Message-ID: <20160603200326.GA124478@google.com> (raw) In-Reply-To: <1464942192-25967-4-git-send-email-boris.brezillon@free-electrons.com> 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) ... Seems much saner to me. Now, I note that in patch 10 you're now using pwm_prepare_new_state() to avoid this propagation problem entirely (good idea anyway, IMO), but I just wanted to further note what kind of real problems we can see when we don't round to the closest value. > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com> Reviewed-by: Brian Norris <briannorris@chromium.org> Tested-by: Brian Norris <briannorris@chromium.org> Tested this whole series on rk3399's PWM regulators used for the CPUs, to clarify what my Tested-by means. Thanks for the patches. > --- > drivers/pwm/pwm-rockchip.c | 7 +++---- > 1 file changed, 3 insertions(+), 4 deletions(-) > > diff --git a/drivers/pwm/pwm-rockchip.c b/drivers/pwm/pwm-rockchip.c > index 7d9cc90..68d72ce 100644 > --- a/drivers/pwm/pwm-rockchip.c > +++ b/drivers/pwm/pwm-rockchip.c > @@ -114,12 +114,11 @@ static int rockchip_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm, > * default prescaler value for all practical clock rate values. > */ > div = clk_rate * period_ns; > - do_div(div, pc->data->prescaler * NSEC_PER_SEC); > - period = div; > + period = DIV_ROUND_CLOSEST_ULL(div, > + pc->data->prescaler * NSEC_PER_SEC); > > div = clk_rate * duty_ns; > - do_div(div, pc->data->prescaler * NSEC_PER_SEC); > - duty = div; > + duty = DIV_ROUND_CLOSEST_ULL(div, pc->data->prescaler * NSEC_PER_SEC); > > ret = clk_enable(pc->clk); > if (ret) > -- > 2.7.4 >
WARNING: multiple messages have this Message-ID (diff)
From: briannorris@chromium.org (Brian Norris) To: linux-arm-kernel@lists.infradead.org Subject: [PATCH 03/14] pwm: rockchip: Fix period and duty_cycle approximation Date: Fri, 3 Jun 2016 13:03:26 -0700 [thread overview] Message-ID: <20160603200326.GA124478@google.com> (raw) In-Reply-To: <1464942192-25967-4-git-send-email-boris.brezillon@free-electrons.com> 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) ... Seems much saner to me. Now, I note that in patch 10 you're now using pwm_prepare_new_state() to avoid this propagation problem entirely (good idea anyway, IMO), but I just wanted to further note what kind of real problems we can see when we don't round to the closest value. > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com> Reviewed-by: Brian Norris <briannorris@chromium.org> Tested-by: Brian Norris <briannorris@chromium.org> Tested this whole series on rk3399's PWM regulators used for the CPUs, to clarify what my Tested-by means. Thanks for the patches. > --- > drivers/pwm/pwm-rockchip.c | 7 +++---- > 1 file changed, 3 insertions(+), 4 deletions(-) > > diff --git a/drivers/pwm/pwm-rockchip.c b/drivers/pwm/pwm-rockchip.c > index 7d9cc90..68d72ce 100644 > --- a/drivers/pwm/pwm-rockchip.c > +++ b/drivers/pwm/pwm-rockchip.c > @@ -114,12 +114,11 @@ static int rockchip_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm, > * default prescaler value for all practical clock rate values. > */ > div = clk_rate * period_ns; > - do_div(div, pc->data->prescaler * NSEC_PER_SEC); > - period = div; > + period = DIV_ROUND_CLOSEST_ULL(div, > + pc->data->prescaler * NSEC_PER_SEC); > > div = clk_rate * duty_ns; > - do_div(div, pc->data->prescaler * NSEC_PER_SEC); > - duty = div; > + duty = DIV_ROUND_CLOSEST_ULL(div, pc->data->prescaler * NSEC_PER_SEC); > > ret = clk_enable(pc->clk); > if (ret) > -- > 2.7.4 >
next prev parent reply other threads:[~2016-06-03 20:03 UTC|newest] Thread overview: 91+ 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 ` Boris Brezillon 2016-06-03 8:22 ` Boris Brezillon 2016-06-03 8:22 ` [PATCH 01/14] pwm: Add new helpers to create/manipulate PWM states Boris Brezillon 2016-06-03 8:22 ` Boris Brezillon 2016-06-03 8:23 ` [PATCH 02/14] regulator: pwm: Drop unneeded pwm_enable() call Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 11:08 ` Applied "regulator: pwm: Drop unneeded pwm_enable() call" to the regulator tree Mark Brown 2016-06-03 11:08 ` Mark Brown 2016-06-03 11:08 ` Mark Brown 2016-06-03 11:08 ` Mark Brown 2016-06-03 8:23 ` [PATCH 03/14] pwm: rockchip: Fix period and duty_cycle approximation Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:03 ` Brian Norris [this message] 2016-06-03 20:03 ` Brian Norris 2016-06-04 6:19 ` Boris Brezillon 2016-06-04 6:19 ` Boris Brezillon 2016-06-07 17:25 ` Brian Norris 2016-06-07 17:25 ` Brian Norris 2016-06-03 8:23 ` [PATCH 04/14] pwm: rockchip: Add support for hardware readout Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:07 ` Brian Norris 2016-06-03 20:07 ` Brian Norris 2016-06-03 20:20 ` Brian Norris 2016-06-03 20:20 ` Brian Norris 2016-06-04 6:24 ` Boris Brezillon 2016-06-04 6:24 ` Boris Brezillon 2016-06-07 17:26 ` Brian Norris 2016-06-07 17:26 ` Brian Norris 2016-06-03 8:23 ` [PATCH 05/14] pwm: rockchip: Avoid glitches on already running PWMs Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:28 ` Brian Norris 2016-06-03 20:28 ` Brian Norris 2016-06-04 6:26 ` Boris Brezillon 2016-06-04 6:26 ` Boris Brezillon 2016-06-03 8:23 ` [PATCH 06/14] pwm: rockchip: Add support for atomic update Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:37 ` Brian Norris 2016-06-03 20:37 ` Brian Norris 2016-06-03 20:37 ` Brian Norris 2016-06-03 8:23 ` [PATCH 07/14] pwm: sti: Add support for hardware readout Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` [PATCH 08/14] pwm: sti: Avoid glitches on already running PWMs Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:38 ` Brian Norris 2016-06-03 20:38 ` Brian Norris 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 8:23 ` Boris Brezillon 2016-06-03 20:41 ` Brian Norris 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 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:50 ` Brian Norris 2016-06-03 20:50 ` Brian Norris 2016-06-03 20:50 ` Brian Norris 2016-06-04 6:28 ` Boris Brezillon 2016-06-04 6:28 ` Boris Brezillon 2016-06-06 6:14 ` Laxman Dewangan 2016-06-06 6:14 ` Laxman Dewangan 2016-06-06 6:14 ` Laxman Dewangan 2016-06-03 8:23 ` [PATCH 11/14] regulator: pwm: properly initialize the ->state field Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:51 ` Brian Norris 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 8:23 ` Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 20:55 ` Brian Norris 2016-06-03 20:55 ` Brian Norris 2016-06-03 8:23 ` [PATCH 13/14] regulator: pwm: Support extra continuous mode cases Boris Brezillon 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 21:03 ` Brian Norris 2016-06-03 21:03 ` Brian Norris 2016-06-04 6:30 ` Boris Brezillon 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 2016-06-03 8:23 ` Boris Brezillon 2016-06-03 21:04 ` Brian Norris 2016-06-03 21:04 ` Brian Norris 2016-06-03 21:04 ` Brian Norris 2016-06-06 14:09 ` Rob Herring 2016-06-06 14:09 ` Rob Herring
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