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: Tue, 13 Aug 2019 00:25:35 +0200 Message-ID: <1565648735.2007.4@crapouillou.net> References: <20190809123031.24219-1-paul@crapouillou.net> <20190809123031.24219-5-paul@crapouillou.net> <20190809170551.u4ybilf5ay2rsvnn@pengutronix.de> <1565370885.2091.2@crapouillou.net> <20190812061520.lwzk3us4ginwwxov@pengutronix.de> <1565642590.2007.1@crapouillou.net> <20190812214838.e5hyhnlcyykjfvsb@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: <20190812214838.e5hyhnlcyykjfvsb@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 [Re-send my message in plain text, as it was bounced by the lists - sorry about that] Le lun. 12 ao=FBt 2019 =E0 23:48, Uwe =3D?iso-8859-1?q?Kleine-K=3DF6nig?=3D= =20 a =E9crit : > Hello Paul, >=20 > On Mon, Aug 12, 2019 at 10:43:10PM +0200, Paul Cercueil wrote: >> Le lun. 12 ao=FBt 2019 =E0 8:15, Uwe =3D?iso-8859-1?q?Kleine-K=3DF6nig?= =3D >> a =E9crit : >> > On Fri, Aug 09, 2019 at 07:14:45PM +0200, Paul Cercueil wrote: >> > > Le ven. 9 ao=FBt 2019 =E0 19:05, Uwe=20 >> =3D?iso-8859-1?q?Kleine-K=3DF6nig?=3D >> > > a =E9crit : >> > > > On Fri, Aug 09, 2019 at 02:30:28PM +0200, Paul Cercueil=20 >> wrote: >> > > > > [...] >> > > > > + /* Reset the clock to the maximum rate, and we'll=20 >> reduce it if needed */ >> > > > > + ret =3D clk_set_max_rate(clk, parent_rate); >> > > > >> > > > What is the purpose of this call? IIUC this limits the=20 >> allowed range of >> > > > rates for clk. I assume the idea is to prevent other=20 >> consumers to change >> > > > the rate in a way that makes it unsuitable for this pwm. But=20 >> this only >> > > > makes sense if you had a notifier for clk changes, doesn't=20 >> it? I'm >> > > > confused. >> > > >> > > Nothing like that. The second call to clk_set_max_rate() might=20 >> have set >> > > a maximum clock rate that's lower than the parent's rate, and=20 >> we want to >> > > undo that. >> > >> > I still don't get the purpose of this call. Why do you limit the=20 >> clock >> > rate at all? >>=20 >> As it says below, we "limit the clock to a maximum rate that still=20 >> gives >> us a period value which fits in 16 bits". So that the computed=20 >> hardware >> values won't overflow. >=20 > But why not just using clk_set_rate? You want to have the clock=20 > running > at a certain rate, not any rate below that certain rate, don't you? I'll let yourself answer yourself: https://patchwork.ozlabs.org/patch/1018969/ It's enough to run it below a certain rate, yes. The actual rate doesn't actually matter that much. >=20 >> E.g. if at a rate of 12 MHz your computed hardware value for the=20 >> period >> is 0xf000, then at a rate of 24 MHz it won't fit in 16 bits. So the=20 >> clock >> rate must be reduced to the highest possible that will still give=20 >> you a >> < 16-bit value. >>=20 >> We always want the highest possible clock rate that works, for the=20 >> sake of >> precision. >=20 > This is dubious; but ok to keep the driver simple. (Consider a PWM=20 > that > can run at i MHz for i in [1, .. 30]. If a period of 120 ns and a duty > cycle of 40 ns is requested you can get an exact match with 25 MHz,=20 > but > not with 30 MHz.) The clock rate is actually (parent_rate >> (2 * x) ) for x =3D 0, 1, 2, ... So if your parent_rate is 30 MHz the next valid one is 7.5 MHz, and the next one is 1.875 MHz. It'd be very unlikely that you get a better=20 match at a lower clock. >> > > Basically, we start from the maximum clock rate we can get for=20 >> that PWM >> > > - which is the rate of the parent clk - and from that compute=20 >> the maximum >> > > clock rate that we can support that still gives us < 16-bits=20 >> hardware >> > > values for the period and duty. >> > > >> > > We then pass that computed maximum clock rate to=20 >> clk_set_max_rate(), which >> > > may or may not update the current PWM clock's rate to match=20 >> the new limits. >> > > Finally we read back the PWM clock's rate and compute the=20 >> period and duty >> > > from that. >> > >> > If you change the clk rate, is this externally visible on the PWM >> > output? Does this affect other PWM instances? >>=20 >> The clock rate doesn't change the PWM output because the hardware=20 >> values for >> the period and duty are adapted accordingly to reflect the change. >=20 > It doesn't change it in the end. But in the (short) time frame between > the call to change the clock and the update of the PWM registers there > is a glitch, right? The PWM is disabled, so the line is in inactive state, and will be in=20 that state until the PWM is enabled again. No glitch to fear. > You didn't answer to the question about other PWM instances. Does that > mean others are not affected? Sorry. Yes, they are not affected - all PWM channels are independent. > Best regards > Uwe >=20 > PS: It would be great if you could fix your mailer to not damage the > quoted mail. Also it doesn't seem to understand how my name is encoded > in the From line. I fixed up the quotes in my reply. I guess I'll submit a bug report to Geary then. >=20 > -- > Pengutronix e.K. | Uwe Kleine-K=F6nig =20 > | > Industrial Linux Solutions |=20 > http://www.pengutronix.de/ | =