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

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Date:   Tue, 17 Mar 2020 18:52:50 +0100
From:   Pascal Roeleven <dev@...calroeleven.nl>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
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

On 2020-03-17 18:32, Uwe Kleine-König wrote:
> Hello Pascal,
> 
> On Mon, Mar 16, 2020 at 08:26:13AM +0100, Uwe Kleine-König wrote:
>> 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?
> 
> I assume the sun4i-series you sent earlier today resolves the problems
> you reported here?
> 
> Best regards
> Uwe

Hi Uwe,

Yes it does, but as Emil mentioned it's probably not complete. It's just 
an RFC for now to make sure it doesn't cause a regression. Turns out the 
Allwinner PWM controller is even more pickier than I thought.

Again, thank you for your help.