From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paul Cercueil Subject: Re: [PATCH 4/7] pwm: jz4740: Improve algorithm of clock calculation Date: Fri, 09 Aug 2019 19:14:45 +0200 Message-ID: <1565370885.2091.2@crapouillou.net> References: <20190809123031.24219-1-paul@crapouillou.net> <20190809123031.24219-5-paul@crapouillou.net> <20190809170551.u4ybilf5ay2rsvnn@pengutronix.de> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <20190809170551.u4ybilf5ay2rsvnn@pengutronix.de> Sender: linux-kernel-owner@vger.kernel.org To: Uwe =?iso-8859-1?q?Kleine-K=F6nig?= Cc: Thierry Reding , od@zcrc.me, linux-pwm@vger.kernel.org, linux-kernel@vger.kernel.org, Mathieu Malaterre , Artur Rojek List-Id: linux-pwm@vger.kernel.org Le ven. 9 ao=FBt 2019 =E0 19:05, Uwe =3D?iso-8859-1?q?Kleine-K=3DF6nig?=3D=20 a =E9crit : > On Fri, Aug 09, 2019 at 02:30:28PM +0200, Paul Cercueil wrote: >> The previous algorithm hardcoded details about how the TCU clocks=20 >> work. >> The new algorithm will use clk_round_rate to find the perfect clock=20 >> rate >> for the PWM channel. >>=20 >> Signed-off-by: Paul Cercueil >> Tested-by: Mathieu Malaterre >> Tested-by: Artur Rojek >> --- >> drivers/pwm/pwm-jz4740.c | 60=20 >> +++++++++++++++++++++++++++++----------- >> 1 file changed, 44 insertions(+), 16 deletions(-) >>=20 >> diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c >> index 6ec8794d3b99..f20dc2e19240 100644 >> --- a/drivers/pwm/pwm-jz4740.c >> +++ b/drivers/pwm/pwm-jz4740.c >> @@ -110,24 +110,56 @@ static int jz4740_pwm_apply(struct pwm_chip=20 >> *chip, struct pwm_device *pwm, >> struct jz4740_pwm_chip *jz4740 =3D to_jz4740(pwm->chip); >> struct clk *clk =3D pwm_get_chip_data(pwm), >> *parent_clk =3D clk_get_parent(clk); >> - unsigned long rate, period, duty; >> + unsigned long rate, parent_rate, period, duty; >> unsigned long long tmp; >> - unsigned int prescaler =3D 0; >> + int ret; >>=20 >> - rate =3D clk_get_rate(parent_clk); >> - tmp =3D (unsigned long long)rate * state->period; >> - do_div(tmp, 1000000000); >> - period =3D tmp; >> + parent_rate =3D clk_get_rate(parent_clk); >> + >> + jz4740_pwm_disable(chip, pwm); >>=20 >> - while (period > 0xffff && prescaler < 6) { >> - period >>=3D 2; >> - rate >>=3D 2; >> - ++prescaler; >> + /* Reset the clock to the maximum rate, and we'll reduce it if=20 >> needed */ >> + ret =3D clk_set_max_rate(clk, parent_rate); >=20 > What is the purpose of this call? IIUC this limits the allowed range=20 > of > rates for clk. I assume the idea is to prevent other consumers to=20 > change > the rate in a way that makes it unsuitable for this pwm. But this only > makes sense if you had a notifier for clk changes, doesn't it? I'm > confused. Nothing like that. The second call to clk_set_max_rate() might have set a maximum clock rate that's lower than the parent's rate, and we want to undo that. > I think this doesn't match the commit log, you didn't even introduced=20 > a > call to clk_round_rate(). Right, I'll edit the commit message. >> + if (ret) { >> + dev_err(chip->dev, "Unable to set max rate: %d\n", ret); >> + return ret; >> } >>=20 >> - if (prescaler =3D=3D 6) >> - return -EINVAL; >> + ret =3D clk_set_rate(clk, parent_rate); >> + if (ret) { >> + dev_err(chip->dev, "Unable to reset to parent rate (%lu Hz)", >> + parent_rate); >> + return ret; >> + } >> + >> + /* >> + * Limit the clock to a maximum rate that still gives us a period=20 >> value >> + * which fits in 16 bits. >> + */ >> + tmp =3D 0xffffull * NSEC_PER_SEC; >> + do_div(tmp, state->period); >>=20 >> + ret =3D clk_set_max_rate(clk, tmp); >=20 > And now you change the maximal rate again? Basically, we start from the maximum clock rate we can get for that PWM - which is the rate of the parent clk - and from that compute the=20 maximum clock rate that we can support that still gives us < 16-bits hardware values for the period and duty. We then pass that computed maximum clock rate to clk_set_max_rate(),=20 which may or may not update the current PWM clock's rate to match the new=20 limits. Finally we read back the PWM clock's rate and compute the period and=20 duty from that. >> + if (ret) { >> + dev_err(chip->dev, "Unable to set max rate: %d\n", ret); >> + return ret; >> + } >> + >> + /* >> + * Read back the clock rate, as it may have been modified by >> + * clk_set_max_rate() >> + */ >> + rate =3D clk_get_rate(clk); >> + >> + if (rate !=3D parent_rate) >> + dev_dbg(chip->dev, "PWM clock updated to %lu Hz\n", rate); >> + >> + /* Calculate period value */ >> + tmp =3D (unsigned long long)rate * state->period; >> + do_div(tmp, NSEC_PER_SEC); >> + period =3D (unsigned long)tmp; >> + >> + /* Calculate duty value */ >> tmp =3D (unsigned long long)period * state->duty_cycle; >> do_div(tmp, state->period); >> duty =3D period - tmp; >> @@ -135,14 +167,10 @@ static int jz4740_pwm_apply(struct pwm_chip=20 >> *chip, struct pwm_device *pwm, >> if (duty >=3D period) >> duty =3D period - 1; >>=20 >> - jz4740_pwm_disable(chip, pwm); >> - >> /* Set abrupt shutdown */ >> regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm), >> TCU_TCSR_PWM_SD, TCU_TCSR_PWM_SD); >>=20 >> - clk_set_rate(clk, rate); >> - >=20 > It's not obvious to me why removing these two lines belong in the > current patch. They're not removed, they're both moved up in the function. > Best regards > Uwe >=20 > -- > Pengutronix e.K. | Uwe Kleine-K=F6nig =20 > | > Industrial Linux Solutions |=20 > http://www.pengutronix.de/ | =