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

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Message-ID: <20200312132942.2kfspvmoc3mxkdx4@pengutronix.de>
Date:   Thu, 12 Mar 2020 14:29:42 +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

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

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