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.
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