From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jerome Brunet Subject: Re: [PATCH v2 3/3] pwm: meson: improve pwm calculation precision. Date: Fri, 07 Jul 2017 10:57:45 +0200 Message-ID: <1499417865.15362.15.camel@baylibre.com> References: <20170608122416.1993-1-jbrunet@baylibre.com> <20170608122416.1993-4-jbrunet@baylibre.com> <20170706212154.GA28918@mithrandir> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20170706212154.GA28918@mithrandir> Sender: linux-kernel-owner@vger.kernel.org To: Thierry Reding Cc: Kevin Hilman , Neil Armstrong , Carlo Caione , linux-pwm@vger.kernel.org, linux-amlogic@lists.infradead.org, linux-kernel@vger.kernel.org List-Id: linux-pwm@vger.kernel.org On Thu, 2017-07-06 at 23:21 +0200, Thierry Reding wrote: > On Thu, Jun 08, 2017 at 02:24:16PM +0200, Jerome Brunet wrote: > > When using input clocks with high rates, such as clk81 (166MHz), the > > fin_ns = NSEC_PER_SEC / fin_freq can introduce a significant error. > > > > Ex: fin_freq = 166666667, NSEC_PER_SEC = 1000000000 > >     fin_ns = 5,9999999 > > > > which is, of course, rounded down to 5. This introduce an error of ~20% > > on the period requested from the pwm. > > > > This patch use ps instead of ns (and 64bits integer) to perform the > > calculation. This should give a good enough precision. > > > > Fixes: 211ed630753d ("pwm: Add support for Meson PWM Controller") > > Signed-off-by: Jerome Brunet > > --- > >  drivers/pwm/pwm-meson.c | 15 +++++++++------ > >  1 file changed, 9 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c > > index b911a944744a..4cdc66f7f718 100644 > > --- a/drivers/pwm/pwm-meson.c > > +++ b/drivers/pwm/pwm-meson.c > > @@ -163,7 +163,8 @@ static int meson_pwm_calc(struct meson_pwm *meson, > >     unsigned int duty, unsigned int period) > >  { > >   unsigned int pre_div, cnt, duty_cnt; > > - unsigned long fin_freq = -1, fin_ns; > > + unsigned long fin_freq = -1; > > + u64 fin_ps; > >   > >   if (~(meson->inverter_mask >> id) & 0x1) > >   duty = period - duty; > > @@ -179,13 +180,14 @@ static int meson_pwm_calc(struct meson_pwm *meson, > >   } > >   > >   dev_dbg(meson->chip.dev, "fin_freq: %lu Hz\n", fin_freq); > > - fin_ns = NSEC_PER_SEC / fin_freq; > > + fin_ps = ((u64)NSEC_PER_SEC * 1000) / fin_freq; > > This failed to build, so I had to change this division to a do_div(). Oh ! Indeed :( I have mostly tested with arm64 which is fine. I thought I had tested arm(32) build as well but I just noticed that MESON_PWM option is disabled in multi_v7_defconfig. Thanks for catching and fixing this Thierry ! > > Thierry