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Re: Backlight mystery - anyone shed light?

On 22 Apr 2011, shawn wilson wrote:
> On Tue, Apr 19, 2011 at 12:14 PM, Anthony Campbell <ac@acampbell.org.uk> wrote:
> > My HP laptop tends to boot with a dim screen. I can fix this with
> >
> >   echo 10 > /sys/devices/virtual/backlight/acpi_video0/brightness
> >
> > which I run from /etc/rc,local.
> >
> the right way would be to use sysctl. that said, i always do things
> the way you've done it :)
> > It works well with all kernels up to 2.6.37. With 2.6.38 there is
> > usually no /sys/devices/virtual/backlight... so the above command
> > can't work.
> >
> i'd check the kernel changelog or maybe the debian kernel package
> changelog and see if something was changed for sysfs and see why. i'm
> not sure whether this line of question is appropriate for that forum,
> but if it is, someone on debian-dev should know. (reading through the
> archives, they seem to be hard asses on topicality, so i'd do my
> research first). also, you could try to ask on irc in #kernel
> > What is really odd is that if I first boot with a 2.6.37 kernel and then
> > reboot with a 2.6.38 version, everything works correctly. The file I
> > need to modify is there and there is full brightness.
> >
> iirc, this is stored in ram. i'll bet if you booted in the one kernel,
> and then did a shutdown -h and immediately powered up into the newer
> kernel, you'd notice a fail. that said, i think this is improper
> behavior. you didn't mention what version of debian you're running?
> ie, i don't think this happens onstable (than again, i'm only on .32,
> so...)
> > Is this perhaps something to do with udev?  I've found ubuntu bug
> > reports describing somewhat similar problems but mostly with backlight
> > being too bright rather than too dim.
> >
> i would think it is more in the kernel than in udev. (i think udev
> gets its info from sysfs and proc - that's probably a bad way of
> stating this though. it's fron the kernel at any rate)
> > I would make a bug report if I knew which package was at fault. Can
> > anyone shed any light (literally)? It's an annoyance rather than a major
> > problem but I've been trying to figure it out for at least two weeks
> > wthout success,
> >
> this does sound like a bug. it's at least an issue with some default
> config. if you can, swap out discs and do a fresh install and see what
> happens. i'm pretty sure you'll notice the same thing. however, that
> would be the right thing to do before putting in a bug report (and it
> would be great of you to follow up on this - helps the community and
> all)

Many thanks for these comments. I'll look into the kernel changelog
files though I find them pretty intimidating these days. Time was when I
used to compile my own kernels, but for a long time now I've settled for
a quiet life and just install the standard Debian kernels. But you are
probably right to say that it's a kernel problem. When it goes wrong, it
starts up bright but then goes dim at a certain point in the boot
process. I couldn't see anything in dmesg to explain this but I'll have
another look.


Anthony Campbell - ac@acampbell.org.uk 
Microsoft-free zone - Using Debian GNU/Linux 
http://www.acampbell.org.uk - sample my ebooks at

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