Received-SPF: pass (google.com: domain of linux-kernel+bounces-39311-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Thu, 25 Jan 2024 22:28:20 +0100
From: Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= <u.kleine-koenig@pengutronix.de>
To: Stefan Wahren <wahrenst@gmx.net>
Cc: Francesco Dolcini <francesco@dolcini.it>, pratikmanvar09@gmail.com, 
	linux-pwm@vger.kernel.org, shawnguo@kernel.org, s.hauer@pengutronix.de, 
	pratik.manvar@ifm.com, linux-kernel@vger.kernel.org, thierry.reding@gmail.com, 
	xiaoning.wang@nxp.com, linux-imx@nxp.com, kernel@pengutronix.de, 
	oe-kbuild-all@lists.linux.dev, festevam@gmail.com, linux-arm-kernel@lists.infradead.org, 
	jun.li@nxp.com
Subject: Re: [PATCH v3] pwm: imx27: workaround of the pwm output bug
Message-ID: <w34gpyikqc6oz5zjoan3ws4ckw5dlzjytddfw4df26dyflx3uk@itw6b34z5luv>
References: <20240103103421.GA3758@francesco-nb>
 <20240103110200.1018-1-pratikmanvar09@gmail.com>
 <20240103122005.GA3347@francesco-nb>
 <71426744-7223-448c-bd5b-ff786c5f8e65@gmx.net>
Precedence: bulk
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
	protocol="application/pgp-signature"; boundary="aqzalxdjxdebfp4u"
Content-Disposition: inline
In-Reply-To: <71426744-7223-448c-bd5b-ff786c5f8e65@gmx.net>


--aqzalxdjxdebfp4u
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

Hello Stefan,

On Wed, Jan 24, 2024 at 12:48:21PM +0100, Stefan Wahren wrote:
> Am 03.01.24 um 13:20 schrieb Francesco Dolcini:
> > Hello Pratik,
> >=20
> > On Wed, Jan 03, 2024 at 04:32:00PM +0530, pratikmanvar09@gmail.com wrot=
e:
> > > From: Clark Wang <xiaoning.wang@nxp.com>
> > >=20
> > > This fixes the pwm output bug when decrease the duty cycle.
> > > This is a limited workaround for the PWM IP issue TKT0577206.
> > >=20
> > > Root cause:
> > > When the SAR FIFO is empty, the new write value will be directly appl=
ied
> > > to SAR even the current period is not over.
> > > If the new SAR value is less than the old one, and the counter is
> > > greater than the new SAR value, the current period will not filp the
> > > level. This will result in a pulse with a duty cycle of 100%.
> > >=20
> > > Workaround:
> > > Add an old value SAR write before updating the new duty cycle to SAR.
> > > This will keep the new value is always in a not empty fifo, and can be
> > > wait to update after a period finished.
> > >=20
> > > Limitation:
> > > This workaround can only solve this issue when the PWM period is long=
er
> > > than 2us(or <500KHz).
> > >=20
> > > Reviewed-by: Jun Li <jun.li@nxp.com>
> > > Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
> > > Link: https://github.com/nxp-imx/linux-imx/commit/16181cc4eee61d87cba=
ba0e5a479990507816317
> > > Tested-by: Pratik Manvar <pratik.manvar@ifm.com>
> > > Signed-off-by: Pratik Manvar <pratik.manvar@ifm.com>
> > A very similar patch was already send in 2021 [1], did it had review
> > comments not addressed? Please have a look.
> >=20
> > In general please refrain from sending a new patch version every other
> > day, while every Linux kernel subsystem has different rules and a
> > difference pace of development, in this specific case sending a v3 just
> > adding your signed-off-by without allowing a little bit of time to wait
> > for more feedback is just not sane.
> >=20
> > [1] https://lore.kernel.org/all/?q=3Ddfn%3Adrivers%2Fpwm%2Fpwm-imx27.c+=
AND+b%3A%22Clark+Wang%22
> thank you, this is very helpful. Unfortunately i don't have the
> knowledge and resources to continue this work.
>=20
> @Uwe It seems that you were able to reproduce this issue. Is it possible
> to trigger this via sysfs and some kind of script?

When I read through the linked thread I was surprised to read that I
could reproduce it. My best guess is that I did that using a long period
and an LED connected to the PWM's output to make the effect easily visible.
Something like: Configure for

	.duty_cycle =3D 1s, .period =3D 5s

and change that to

	.duty_cycle =3D 0s, .period =3D 5s

(e.g. using sysfs) when the output got high.

Best regards
Uwe

--=20
Pengutronix e.K.                           | Uwe Kleine-K=F6nig            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

--aqzalxdjxdebfp4u
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----

iQEzBAABCgAdFiEEP4GsaTp6HlmJrf7Tj4D7WH0S/k4FAmWy0nMACgkQj4D7WH0S
/k568QgAl9kR2hZ1RP9qsAlpFKv1pbgAqj8Cm2Q2SnZ6jmVmw6pEQHAiJ+kjKr7u
XtaA2sUd05WCawm4997HKGx7KRoJAdjzyiLB6MIsvWGjP2IV8rVNMXejmXPRZ92h
VyRGn1U6OeExVQnm+MkVCpDzQEZlHuMVzBUojTvzxip69IylIZ23QmUOWFJfYVFT
LcA/fNE0MjoYqYdqw9Me87u8fuN5wHyGTIGGKjRdKWP0YV/6NeJKM1fRuYMqXXEw
7+FFg7NwQ7mh5PyloJ1Phjh0ZsTiwbBIJHD35Sdogj6tIrGoNv9DQpG7cHUo8jSI
/GzzhiotZlzTOU7JfVKxg2nEnwYPJA==
=Tkun
-----END PGP SIGNATURE-----

--aqzalxdjxdebfp4u--