linux-kernel - Re: [RFC PATCH 0/4] pwm: sun4i: Properly turn pwm off and fix stuck output state

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Date:   Tue, 17 Mar 2020 20:35:51 +0100
From:   Emil Lenngren <emil.lenngren@...il.com>
To:     Pascal Roeleven <dev@...calroeleven.nl>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        linux-pwm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>, linux-sunxi@...glegroups.com
Subject: Re: [RFC PATCH 0/4] pwm: sun4i: Properly turn pwm off and fix stuck
 output state

Hi,

Den tis 17 mars 2020 19:16Pascal Roeleven <dev@...calroeleven.nl> skrev:
>
> On 2020-03-17 17:45, Emil Lenngren wrote:
> > Hi all,
> >
> > Den tis 17 mars 2020 kl 17:00 skrev Pascal Roeleven
> > <dev@...calroeleven.nl>:
> >>
> >> Hi all,
> >>
> >> For the last few days I've been debugging a lot to get pwm working
> >> again since
> >> recent changes in 5.6-rc1 broke it for me.
> >>
> >> Testing shows the pwm controller crashes (or the output gets stuck)
> >> when the
> >> period register is written when the channel is disabled while the
> >> clock gate is
> >> still on. Usually after multiple writes, but one write can also lead
> >> to
> >> unpredictable behaviour. Patch 3 and 4 fix this.
> >>
> >> Patch 2 contains a fix which wouldn't completely turn off the pwm if
> >> the
> >> output is disabled. The clock gate needs to stay on for at least one
> >> more
> >> period to ensure the output is properly disabled. This issue has been
> >> around
> >> for a long time but has probably stayed unnoticed because if the
> >> duty_cycle is
> >> also changed to 0, you can't tell the difference.
> >>
> >> Patch 1 removes some leftovers which aren't needed anymore.
> >>
> >> Obviously these patches work for my device, but I'd like to hear your
> >> opinion
> >> if any of these changes make sense. After days, this one is a bit
> >> blurry for me.
> >>
> >> Thanks to Uwe for some help with debugging.
> >>
> >> Pascal.
> >>
> >> Pascal Roeleven (4):
> >>   pwm: sun4i: Remove redundant needs_delay
> >>   pwm: sun4i: Disable pwm before turning off clock gate
> >>   pwm: sun4i: Move delay to function
> >>   pwm: sun4i: Delay after writing the period
> >>
> >>  drivers/pwm/pwm-sun4i.c | 53
> >> ++++++++++++++++++++---------------------
> >>  1 file changed, 26 insertions(+), 27 deletions(-)
> >>
> >> --
> >> 2.20.1
> >>
> >
> > I also worked on sun4i-pwm some time ago, fixing a bunch of issues.
> > One was that disabling the pwm sometimes didn't turn off the signal,
> > because the gate and enable bit were modified in the same clock cycle.
> > Another was that the current code used an unnecessary sleep of a whole
> > period length (or more?) in case of an update to the period, which
> > could be very time-consuming if it's a very long interval, like 2
> > seconds.
> >
> > Note that the behaviour is not unpredictable, if you know how it works
> > ;)
> > I fiddled around a long time with devmem2, an oscilloscope and the
> > prescaler set to max to figure out how works internally.
> >
> > Please try my version I just posted at https://pastebin.com/GWrhWzPJ.
> > It is based on this version from May 28, 2019:
> > https://github.com/torvalds/linux/blob/f50a7f3d9225dd374455f28138f79ae3074a7a3d/drivers/pwm/pwm-sun4i.c.
> > Sorry for not posting it inline, but GMail would break the formatting.
> > It contains quite many comments about how it works internally. I also
> > wrote a section at http://linux-sunxi.org/PWM_Controller, but it might
> > be a bit old (two years), so please rather look at the code and the
> > comments.
> >
> > /Emil
>
> Hi Emil,
>
> Thank you very much, this is helpful. Ah it was your note on the wiki.
> That is indeed where I took the idea of keeping the gate on and
> disabling the panel from. As a scope is still on my wishlist, the rest
> was just trial-and-error. Judging from your code, there are more edge
> cases which might occur. I will test your code and try to integrate it.
> If it's okay with you, I can post it on your behalf?

Sure! I was thinking of sending a patch last year but never got the time :/
The devices I have tested on are Allwinner GR8 (A13) and V3s.

>
> If you ask me, it's really unfortunate Allwinner didn't provide a timing
> diagram for such a picky controller.

/Emil