linux-kernel - Re: pwm: sun4i: pwm-backlight not working since 5.6-rc1

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Date:   Mon, 16 Mar 2020 08:26:13 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Pascal Roeleven <dev@...calroeleven.nl>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        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,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com
Subject: Re: pwm: sun4i: pwm-backlight not working since 5.6-rc1

Hello,

On Thu, Mar 12, 2020 at 04:06:07PM +0100, Pascal Roeleven wrote:
> On 2020-03-12 14:29, Uwe Kleine-König wrote:
> > On Thu, Mar 12, 2020 at 01:22:13PM +0100, Pascal Roeleven wrote:
> > > Hi all,
> > > 
> > > I am working on adding an old A10 device to mainline and noticed an
> > > issue
> > > when testing on 5.5.8 vs master.
> > > 
> > > Since 5.6-rc1, I can't control the brightness of my LCD backlight
> > > anymore.
> > > The backlight stays on full brightness instead. I am controlling the
> > > brightness value via sysfs for testing.
> > > 
> > > I am not sure if this is a general pwm-sun4i issue or if it is
> > > related to
> > > the backlight. However I narrowed it down to one commit for pwm-sun4i:
> > > 
> > > fa4d81784681a26bcf7d2a43c6ac5cf991ef28f5
> > > 
> > > If I use pwm-sun4i.c from 5b090b430d750961305030232314b6acdb0102aa on
> > > master, the backlight works fine. Unfortunately, due to my lack of
> > > kernel
> > > experience, I can't see how the commit above broke it.
> > 
> > Hmm, I cannot see how fa4d81784681a26bcf7d2a43c6ac5cf991ef28f5 breaks
> > this. Looking at the output of
> > 
> > 	git show -b fa4d81784681a26bcf7d2a43c6ac5cf991ef28f5
> > 
> > (i.e. ignoring whitespace changes) I don't see how the behaviour you're
> > reporting can be explained.
> > 
> > Are you sure that fa4d81784681a26bcf7d2a43c6ac5cf991ef28f5 is the bad
> > commit?
> > 
> > Can you install a tool to inspect register values and check how the
> > affected registers change if you switch kernel versions and/or pwm
> > settings?
> > 
> > (e.g.
> > 	memtool md 0x1c20e00+0xc
> > )
> > 
> > Best regards
> > Uwe
> 
> Thanks for your response.
> 
> Yes I am sure that is the commit. If I am on master, and replace pwm-sun4i.c
> with the one from 5b090b43, everything works. If I then apply fa4d8178, it
> stops working.
> 
> And strangely the output of the registers is exactly the same before and
> after fa4d8178:
> 
> 01c20e00: 00000050 00130014 00000000 (full brightness)
> 01c20e00: 00000050 00130006 00000000 (min brightness)
> 
> Even when I'm on 5b090b43 and cherry-pick fa4d8178 can I reproduce the
> issue.

Very strange. I'm out of sensible ideas. The remaining ones are:

- enable tracing in the kernel and boot with

	trace_event=pwm

  And then check after the problem occurred in
  /sys/kernel/debug/tracing/trace if something sticks out.

- Try modifying the registers using memtool. E.g.

	memtool mw 0x01c20e04 0x00130012

- Do you have equipment to check the actual output of the PWM hardware?
  If so, what do you see?

Best regards
Uwe

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