[SOLVED] Control PWM frequency of LED-backlight in Thinkpad laptops

Hi.
I'm seeking for a person who is eager to make a contribution to opensource
Recently I've got Lenovo X220 laptop and noticed that it makes me sick. Eventually I discovered a screen flickering that is caused by low PWM frequency (backlight brightness is controlled with PWM).
This issue is so ubiquitous, that some folk created simple utility for Windows to adjust PWM frequency by interacting with Intel driver.
http://pastebin.com/6iirMbfc
It's only 50 lines of code. So I hope that accomplishing the same under linux won't be very hard.
As you might have guessed, I don't have any experience nor knowledge in C. Thus I'm asking here for help.
So... anyone eager to create similar utility for linux?
Hails and beer for the author implied
P.S. Of course, I would be very glad to hear, that my question is non-sense because our belowed linux already provides handy ways to control PWM frequency (though, I was not able to find anything like this).
UPD: that damn screen flickering made me not only sick, but also dull.
I've just found this in archwiki
https://wiki.archlinux.org/index.php/Ba … at_care.29
Seems like it does the same, that mentioned utility does. Though, some background knowledge is required. Appreciate any help with this.
Solution
Thank you all, folks.
Solution is an wiki already.
https://wiki.archlinux.org/index.php/Ba … 15_only.29
Last edited by eDio (2013-04-21 16:22:23)

I've found some patch to fix default PWM frequency in i915
http://web.archiveorange.com/archive/v/ … BgDr5oIynd
From the code of this patch I've got an address of a register that controls PWM frequency 0xc8254. I was not able to determine anything else. That patch seems to be applicable to an old kernel so I had no any context (new kernel in the Linus's github repo doesn't look very similar). Also, my "native" programming language is java so I'm a little bit scared by C syntax .
To manipulate Intel GPU registers one can use intel_reg_read and intel_reg_write programs from the community/intel-gpu-tools.
By default, register contained value
0 x 12 28 12 28
Experimentally I was able to determine, that register controls the period of PWM modulation (1/frequency).
The most comfortable value for me is *0x03000000*.
I'm going to put this info to the wiki on the weekend. I'm sure it'll be useful for others.
C+Linux programming adepts are welcome to write an application for user-friendly PWM modulation frequency adjustment
P.S. forgot to mention that Linux rocks! I can't imagine investigations like this in other OSes.
Last edited by eDio (2013-03-21 22:16:18)

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Solved!
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Here you go, enjoy! These were made with a Spyder3 and dispcalGUI. I've also used ColorEyes Display Pro and it gives very similar results, but the consistency in the dark gray tones is not as good.
Some important info about these profiles: the first link is a very precise LUT-based profile combined with an inverted matrix-based profile. I'd recommend most of you don't use this profile unless you understand color management. If you use this profile with a program that does not support XYZ-LUT profiles, then it will show the colors as inverted (ex. Windows Photo Viewer). The is done intentionally because it lets you know it's not using the more-accurate LUT-based profile and is instead using the less-accurate matrix based profile (which is the color-inverted profile). However, using this profile with a truly color-managed app like Photoshop or Lightroom will yield very, very nice results. White point 6500K/D65, gamma 2.2.
https://www.dropbox.com/s/hmdbau8c32y5yt9/dispcalGUI%20ThinkPad%20Display%201920%202012-04-30%20D650...
This second profile will work acceptably with most programs. It's not quite as extremely accurate as the first, but still very good. The gamma curve for this profile is L* instead of 2.2, which I prefer for most normal use, because it lessens the "black crush" at the bottom of the gamma curve and makes shadow details more visible. White point, again, is 6500K/D65.
https://www.dropbox.com/s/ruyk59sqdu7orbe/dispcalGUI%20ThinkPad%20Display%201920%202012-04-30%20D650...
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Yes, that's what I think too, thanks.
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I can assure you that Lenovo support is excellent. It's handled by the same original IBM division in Atlanta Georgia. They are there for you 24/7 every day. I would not hesitate to give them a call, as it's also beneficial for them to hear from you. If it's not reported, they won't be aware of it. Many things like this do not show up in the very extensive testing that Lenovo does before releasing a new model. Therefore we are helping them by reporting these issues. Please let us know the outcome of your call....

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Hi @Sarteret
Welcome to the HP Support Forums!
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Message Edited by bjamesw on 12-29-2008 04:11 PM

bjamesw wrote:
I'm astonished. Has all of this been covered already in posts I haven't found? This would seem to be a pretty important topic to anyone interested in thinkpads and it's generated utter silence?
It's the holiday season - forum traffic is down all over, not just here.
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Jane
2015 X1 Carbon, ThinkPad Slate, T410s, X301, X200 Tablet, T60p, HP TouchPad, iPad Air 2, iPhone 5S, IdeaTab A2107A, Yoga Tablet, Yoga 3 Pro
I am not a Lenovo Employee.
I AM one of those crazy ThinkPad zealots!
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[SOLVED] Control PWM frequency of LED-backlight in Thinkpad laptops

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