UNSOLVED
[Solved] Screen brightness granularity lost in Ubuntu 15.04
Hello,
Has anyone else that has installed Ubuntu 15.04 fresh on the XPS 13 9343 noticed that the granularity of the brightness control is worse than compared to the stock Ubuntu provided by Dell? What I'm finding is that for any value on the slider from about 20%-100% there's very l little difference in brightness; it's slight differences in "very bright". Below 20% it rapidly jumps to completely off.
I've found some posts that suggest that it's down to the Linux kernel switching from a logarithmic scale to a linear one:
- https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1341108
- https://marc.info/?l=linux-acpi&m=140600387808371&w=2
Right now I'm running 3.19.0-22-generic, the most up to date kernel with Ubuntu 15.04. I've tried to use kernel parameters suggested such as acpi_backlight=vendor or acpi_backlight=0 but these seem to only disable the brightness controls completely.
Can anyone else confirm this experience and if so has anyone got a fix? If not, next steps for me will be to try earlier kernels to prove the regression (since I could have imagined it) and then try later ones to see if it's already been addressed for future releases.
Regards,
Bob
I'm happy to admit I've got it wrong. I went back to factory and I found the steps to be pretty much the same.
Both Dell OEM Ubuntu 14.04 and Ubuntu stock 15.04 are both using video.use_native_backlight=1. The brightness steps are slightly different. For example in 15.04 the brightness steps 0, 46, 92, 138 ... whereas in Dell 14.04 I think I observed 0, 45, 90, 135... Something like that. But the ultimate minimum and maximum values are 0-937 and the steps appear to be numerically linear.
The only difference I can really observe is that the 14.04 at zero doesn't completely go off at zero; it leaves a negligible brightness that is actually pretty workable in a dark room, whereas 15.04 at zero is completely off and the first step up from zero is brighter than the 14.04 is at zero (still pretty dim then).
What would be the cause of this? I don't know. They both seem to have the same drivers listed by lshw and modinfo, and they both feature the same /sys/class/backlight/intel_backlight 'files'. Perhaps somewhere in the kernel the interpretation of the firmware values has been adjusted slightly?
Why am I going into so much detail? Only because I've let myself become too obsessed about a non-problem and want anyone else so inclined to avoid wasting their lives like I did. =) (I took video recordings of both to see how the steps compare and I can really see no obvious difference, but I'll save you all the boredom.)
Pros of the Dell 14.04 behaviour: You can get a really dim but visible display which could be useful if you want to preserve battery yet actively waiting for something to happen.
Pros of the stock 15.04 behaviour: You can completely turn off the display if you're having something crunch unattended.
So there, hope that helps anyone with similar doubts to me. I consider this 'not an issue'.
Regards,
Bob
DELL-Jared D
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Posted July 13th, 2015 10:00
The factory image adds a kernel boot parameter for the backlight, which is added by the workaround-native-backlight-trusty package. It adds the video.use_native_backlight=1 parameter.