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From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Yoshihiro Shimoda" <yoshihiro.shimoda.uh@renesas.com>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Linux PWM List" <linux-pwm@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"Michal Vokáč" <Michal.Vokac@ysoft.com>
Subject: Re: [PATCH 2/5] pwm: rcar: Add support "atomic" API
Date: Fri, 7 Dec 2018 11:45:48 +0100	[thread overview]
Message-ID: <20181207104548.cuwlavf62f2f4hdk@pengutronix.de> (raw)
In-Reply-To: <CAMuHMdUfrQ0u3eJ2Dmgmbb2Dt_DRmFNXXDzS=W0MmsoemU61ag@mail.gmail.com>

Hello,

On Fri, Dec 07, 2018 at 10:57:48AM +0100, Geert Uytterhoeven wrote:
> > Is the documentation for this hardware publically available?
> 
> Please check Section 59 of the "User's Manual: Hardware" at
> https://www.renesas.com/eu/en/products/microcontrollers-microprocessors/rz/rzg/rzg1m.html

Thanks.

So the ugly things here are:

 a) if the pwm is stopped (PWMCR.ENO = 0) the output is set to high after
    completion of the currently running period.
 b) setting a duty_cycle of 0 is forbidden

both are bad. A workaround for both is implementation of something
similar to the switch to a gpio suggested by Michal for imx. But this
cannot be done reliably because the current period's end isn't
observable.

Alternatively a confirmation from the Renesas engineers that PWMCNT.PHO
can be set to 0 with the intended effect despite being forbidden in the
reference manual would be great. Did someone with access to such
hardware test what happens if the PHO field is set to 0? Maybe the
forbidden value is just a wrong copy&paste from the CYCO field?

I think it would be a good idea to add the link to the documentation
into a comment at the top of the driver.

@Thierry: Given that nobody seems to have an overview about the features
and ugly implementation details of all the PWMs, what about documenting
them in the driver files in a greppable way. For the rcar driver
something like:

 - duty-counter-bits: 10
 - period-counter-bits: 10
 - hardware-polarity-support: false
 - uglyness:
   - OUTPUT-ACTIVE-ON-DISABLE
   - NO-ZERO-DUTY-CYCLE

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K�nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

WARNING: multiple messages have this Message-ID (diff)
From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Yoshihiro Shimoda" <yoshihiro.shimoda.uh@renesas.com>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Linux PWM List" <linux-pwm@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"Michal Vokáč" <Michal.Vokac@ysoft.com>
Subject: Re: [PATCH 2/5] pwm: rcar: Add support "atomic" API
Date: Fri, 7 Dec 2018 11:45:48 +0100	[thread overview]
Message-ID: <20181207104548.cuwlavf62f2f4hdk@pengutronix.de> (raw)
In-Reply-To: <CAMuHMdUfrQ0u3eJ2Dmgmbb2Dt_DRmFNXXDzS=W0MmsoemU61ag@mail.gmail.com>

Hello,

On Fri, Dec 07, 2018 at 10:57:48AM +0100, Geert Uytterhoeven wrote:
> > Is the documentation for this hardware publically available?
> 
> Please check Section 59 of the "User's Manual: Hardware" at
> https://www.renesas.com/eu/en/products/microcontrollers-microprocessors/rz/rzg/rzg1m.html

Thanks.

So the ugly things here are:

 a) if the pwm is stopped (PWMCR.ENO = 0) the output is set to high after
    completion of the currently running period.
 b) setting a duty_cycle of 0 is forbidden

both are bad. A workaround for both is implementation of something
similar to the switch to a gpio suggested by Michal for imx. But this
cannot be done reliably because the current period's end isn't
observable.

Alternatively a confirmation from the Renesas engineers that PWMCNT.PHO
can be set to 0 with the intended effect despite being forbidden in the
reference manual would be great. Did someone with access to such
hardware test what happens if the PHO field is set to 0? Maybe the
forbidden value is just a wrong copy&paste from the CYCO field?

I think it would be a good idea to add the link to the documentation
into a comment at the top of the driver.

@Thierry: Given that nobody seems to have an overview about the features
and ugly implementation details of all the PWMs, what about documenting
them in the driver files in a greppable way. For the rcar driver
something like:

 - duty-counter-bits: 10
 - period-counter-bits: 10
 - hardware-polarity-support: false
 - uglyness:
   - OUTPUT-ACTIVE-ON-DISABLE
   - NO-ZERO-DUTY-CYCLE

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

  reply	other threads:[~2018-12-07 10:45 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-07  8:29 [PATCH 0/5] pwm: rcar: Add support "atomic" API and workaround Yoshihiro Shimoda
2018-12-07  8:29 ` [PATCH 1/5] pwm: rcar: add rcar_pwm_calc_counter() to calculate PWMCNT value only Yoshihiro Shimoda
2018-12-07  9:00   ` Uwe Kleine-König
2018-12-07  9:00     ` Uwe Kleine-König
2018-12-07  8:29 ` [PATCH 2/5] pwm: rcar: Add support "atomic" API Yoshihiro Shimoda
2018-12-07  9:07   ` Uwe Kleine-König
2018-12-07  9:07     ` Uwe Kleine-König
2018-12-07  9:57     ` Geert Uytterhoeven
2018-12-07 10:45       ` Uwe Kleine-König [this message]
2018-12-07 10:45         ` Uwe Kleine-König
2018-12-10  4:58         ` Yoshihiro Shimoda
2018-12-07  8:29 ` [PATCH 3/5] pwm: rcar: Use "atomic" API on rcar_pwm_resume() Yoshihiro Shimoda
2018-12-07  8:29 ` [PATCH 4/5] pwm: rcar: remove legacy APIs Yoshihiro Shimoda
2018-12-07  8:29 ` [PATCH 5/5] pwm: rcar: add workaround to output "pseudo" low level Yoshihiro Shimoda
2018-12-07  9:13   ` Uwe Kleine-König
2018-12-07  9:13     ` Uwe Kleine-König
2018-12-10  4:49     ` Yoshihiro Shimoda
2018-12-10  8:11       ` Uwe Kleine-König
2018-12-10  8:11         ` Uwe Kleine-König
2018-12-12  3:19         ` Yoshihiro Shimoda
2018-12-12  3:19           ` Yoshihiro Shimoda
2018-12-12  7:37           ` Uwe Kleine-König
2018-12-12  7:37             ` Uwe Kleine-König
2018-12-12 10:49             ` Yoshihiro Shimoda
2018-12-12 10:49               ` Yoshihiro Shimoda
2018-12-13  9:47     ` Yoshihiro Shimoda
2018-12-13  9:47       ` Yoshihiro Shimoda
2018-12-13  9:52       ` Uwe Kleine-König
2018-12-13  9:52         ` Uwe Kleine-König
2018-12-13 10:53         ` Yoshihiro Shimoda
2018-12-13 10:53           ` Yoshihiro Shimoda
2018-12-07 21:49 ` pwm: rcar: improve calculation of divider Uwe Kleine-König
2018-12-07 21:49   ` Uwe Kleine-König
2018-12-09 20:55   ` Laurent Pinchart
2018-12-10  5:09     ` Yoshihiro Shimoda
2018-12-10  8:04       ` Uwe Kleine-König
2018-12-10  8:04         ` Uwe Kleine-König
2018-12-12  3:13         ` Yoshihiro Shimoda
2018-12-12  3:13           ` Yoshihiro Shimoda
2018-12-09 22:48 ` [PATCH 0/5] pwm: rcar: Add support "atomic" API and workaround Laurent Pinchart

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