debugging backlight issues
 

Hello all,

As a member of bugsquad and a regular triager, I've been working on some
power management bugs the past few days. Naturally a few of these have
been backlight brightness issues but I haven't been able to find
helpful, up-to-date info on debugging backlight stuff. So I'm planning
on making a /DebuggingBacklight wiki page with some basic information,
and I have a few questions for you guys and girls if any of you have a sec.

1) Any backlight information found by the kernel will be in
/sys/class/backlight correct? Is all the info from /proc/acpi available
somewhere in /sys now?

2) I have a laptop where the brightness hotkeys work, but there's
nothing in /sys/backlight and /proc/acpi/video/*/backlight reads "<not
supported>". Do some laptops have the keys directly connected to the
hardware or something?

3) Is there any way to "blacklist" ACPI control of the backlight if it
doesn't work? This would be for laptops where acpi=off fixes the
brightness keys but the user naturally doesn't want to use this all the
time.

If there are any other tips you can point me to, particularly regarding
the ACPI information under /proc/acpi and /sys, that would be great.

Thanks in advance,
David

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debugging backlight issues
 

On 06/15/2010 12:36 AM, David Tombs wrote:
> Hello all,
>
> As a member of bugsquad and a regular triager, I've been working on some
> power management bugs the past few days. Naturally a few of these have
> been backlight brightness issues but I haven't been able to find
> helpful, up-to-date info on debugging backlight stuff. So I'm planning
> on making a /DebuggingBacklight wiki page with some basic information,
> and I have a few questions for you guys and girls if any of you have a sec.
>
> 1) Any backlight information found by the kernel will be in
> /sys/class/backlight correct? Is all the info from /proc/acpi available
> somewhere in /sys now?

Yes and no. /sys is a bit more abstracted. In proc you will see discrete
percentage values and the current value in one file. The sysfs interface uses
index levels and has a separate variable for the maximum value.

> 2) I have a laptop where the brightness hotkeys work, but there's
> nothing in /sys/backlight and /proc/acpi/video/*/backlight reads "<not
> supported>". Do some laptops have the keys directly connected to the
> hardware or something?

Yes, this is the case sometimes.

> 3) Is there any way to "blacklist" ACPI control of the backlight if it
> doesn't work? This would be for laptops where acpi=off fixes the
> brightness keys but the user naturally doesn't want to use this all the
> time.
>
acpi_backlight=vendor could help. The opposite acpi_backlight=video had a bug
and will only work in a future Lucid (which includes upstream stable 2.6.32.16)
version or Maverick.

> If there are any other tips you can point me to, particularly regarding
> the ACPI information under /proc/acpi and /sys, that would be great.
>

Both vendor specific and the acpi video driver should place the same interface
files into /sys/class/backlight/*/...

max_brightness would contain the highest number and echoing numbers into
brightness should change the brightness. If supported correctly. In hairy cases
asking the users for a dump (sudo acpidump -o acpidump-<laptop-model>.txt). This
can then extracted into various acpi tables (acpixtract) and disassembled with
iasl. Though reading the result is tedious.

> Thanks in advance,
> David
>

-Stefan

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debugging backlight issues
 

Hi David-

On Mon, 2010-06-14 at 18:36 -0400, David Tombs wrote:

> 2) I have a laptop where the brightness hotkeys work, but there's
> nothing in /sys/backlight and /proc/acpi/video/*/backlight reads "<not
> supported>". Do some laptops have the keys directly connected to the
> hardware or something?

I imagine that's possible... On that laptop, do you see the GUI
"brightness notification popup"? If you do then the software must be
involved somehow; if you don't then maybe the keys are indeed directly
wired to the hardware.


> 3) Is there any way to "blacklist" ACPI control of the backlight if it
> doesn't work? This would be for laptops where acpi=off fixes the
> brightness keys but the user naturally doesn't want to use this all the
> time.

I think the param "acpi.brightness_switch_enabled=0" does what you're
looking for.

Also note: I've written an experimental patch which lets Intel
i915-driver GPU's take over backlight brightness control from ACPI (for
situations where the ACPI method is broken). It is currently pending
review upstream but you're welcome to try it if you wish. See:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/568611
(comment #20 points to my latest test kernel)

-Kamal
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debugging backlight issues
 

Hi Kamal,

Thanks for the extra info!

On 06/16/2010 03:43 PM, Kamal Mostafa wrote:

>> 2) I have a laptop where the brightness hotkeys work, but there's
>> nothing in /sys/backlight and /proc/acpi/video/*/backlight reads "<not
>> supported>". Do some laptops have the keys directly connected to the
>> hardware or something?
>
> I imagine that's possible... On that laptop, do you see the GUI
> "brightness notification popup"? If you do then the software must be
> involved somehow; if you don't then maybe the keys are indeed directly
> wired to the hardware.

Nope, no notification popup. Hardwired, I guess.

>> 3) Is there any way to "blacklist" ACPI control of the backlight if it
>> doesn't work? This would be for laptops where acpi=off fixes the
>> brightness keys but the user naturally doesn't want to use this all the
>> time.
>
> I think the param "acpi.brightness_switch_enabled=0" does what you're
> looking for.

Do you know of any documentation for this? I couldn't find the parameter
in kernel-parameters.txt [1] and the patch I found Googling [2] seems to
imply that it just disables automatic brightness changes or something,
but won't let the hw take over completely like acpi=off does.

Also, is this actually supported in Ubuntu? I couldn't find a parameter
for it anywhere in sysfs.

Thanks,
David

[1] http://www.mjmwired.net/kernel/Documentation/kernel-parameters.txt
[2] http://www.mail-archive.com/linux-acpi@vger.kernel.org/msg11894.html

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debugging backlight issues
 

On 06/15/2010 09:03 AM, Stefan Bader wrote:
> On 06/15/2010 12:36 AM, David Tombs wrote:
>> Hello all,
>>
>> As a member of bugsquad and a regular triager, I've been working on some
>> power management bugs the past few days. Naturally a few of these have
>> been backlight brightness issues but I haven't been able to find
>> helpful, up-to-date info on debugging backlight stuff. So I'm planning
>> on making a /DebuggingBacklight wiki page with some basic information,
>> and I have a few questions for you guys and girls if any of you have a sec.
>>
>> 1) Any backlight information found by the kernel will be in
>> /sys/class/backlight correct? Is all the info from /proc/acpi available
>> somewhere in /sys now?
>
> Yes and no. /sys is a bit more abstracted. In proc you will see discrete
> percentage values and the current value in one file. The sysfs interface uses
> index levels and has a separate variable for the maximum value.
>
>> 2) I have a laptop where the brightness hotkeys work, but there's
>> nothing in /sys/backlight and /proc/acpi/video/*/backlight reads "<not
>> supported>". Do some laptops have the keys directly connected to the
>> hardware or something?
>
> Yes, this is the case sometimes.
>
>> 3) Is there any way to "blacklist" ACPI control of the backlight if it
>> doesn't work? This would be for laptops where acpi=off fixes the
>> brightness keys but the user naturally doesn't want to use this all the
>> time.
>>
> acpi_backlight=vendor could help. The opposite acpi_backlight=video had a bug
> and will only work in a future Lucid (which includes upstream stable 2.6.32.16)
> version or Maverick.
>
>> If there are any other tips you can point me to, particularly regarding
>> the ACPI information under /proc/acpi and /sys, that would be great.
>>
>
> Both vendor specific and the acpi video driver should place the same interface
> files into /sys/class/backlight/*/...
>
> max_brightness would contain the highest number and echoing numbers into
> brightness should change the brightness. If supported correctly. In hairy cases
> asking the users for a dump (sudo acpidump -o acpidump-<laptop-model>.txt). This
> can then extracted into various acpi tables (acpixtract) and disassembled with
> iasl. Though reading the result is tedious.
>
>> Thanks in advance,
>> David
>>
>
> -Stefan
>

I know I already replied to this, but I just finished reading through
the page and I have to thank you again, Stefan! The information is
great, I think the best docs out there on the 'net for backlight stuff.
This will come in very handy during triaging.

David

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debugging backlight issues
 

On Thu, 2010-06-17 at 11:05 -0400, David Tombs wrote:

> > param "acpi.brightness_switch_enabled=0"
>
> Do you know of any documentation for this?

No, I just observed it in the source -- It is present in Ubuntu kernel
source (Lucid onward, at least) but you're right, it does not in fact
disable acpi control of the backlight after all.

As Stefan recommended, acpi_backlight=vendor might be interesting to
you. I find that it does disable acpi's control of the backlight on one
of my laptops but has no effect on another laptop. (The laptop where it
has no effect is likely already using a vendor-specific backlight
driver.)

-Kamal
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debugging backlight issues
 

On 06/17/2010 02:50 PM, Kamal Mostafa wrote:
> On Thu, 2010-06-17 at 11:05 -0400, David Tombs wrote:
>
>>> param "acpi.brightness_switch_enabled=0"
>>
>> Do you know of any documentation for this?
>
> No, I just observed it in the source -- It is present in Ubuntu kernel
> source (Lucid onward, at least) but you're right, it does not in fact
> disable acpi control of the backlight after all.
>
> As Stefan recommended, acpi_backlight=vendor might be interesting to
> you. I find that it does disable acpi's control of the backlight on one
> of my laptops but has no effect on another laptop. (The laptop where it
> has no effect is likely already using a vendor-specific backlight
> driver.)
>
> -Kamal

Thanks Kamal,

I'm now trying to debug an issue where the backlight keys work but not
properly (jumping steps, etc.). I think it's an issue where single
keypresses are getting routed to multiple places and therefore the
brightness is adjusted multiple times.

So, I ask now, what are the different routes in which brightness will
get adjusted? I'm thinking there are three:

1) Pure hw, like we discussed before.

2) Pure kernel ACPI, which brightness_switch_enabled=0 would turn off.

3) Userspace power manager like gnome-power-manager, which can be turned
off by killing the process.

Is this correct?

Thanks in advance,
David

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debugging backlight issues
 

On 06/23/2010 02:38 AM, David Tombs wrote:
> On 06/17/2010 02:50 PM, Kamal Mostafa wrote:
>> On Thu, 2010-06-17 at 11:05 -0400, David Tombs wrote:
>>
>>>> param "acpi.brightness_switch_enabled=0"
>>>
>>> Do you know of any documentation for this?
>>
>> No, I just observed it in the source -- It is present in Ubuntu kernel
>> source (Lucid onward, at least) but you're right, it does not in fact
>> disable acpi control of the backlight after all.
>>
>> As Stefan recommended, acpi_backlight=vendor might be interesting to
>> you. I find that it does disable acpi's control of the backlight on one
>> of my laptops but has no effect on another laptop. (The laptop where it
>> has no effect is likely already using a vendor-specific backlight
>> driver.)
>>
>> -Kamal
>
> Thanks Kamal,
>
> I'm now trying to debug an issue where the backlight keys work but not
> properly (jumping steps, etc.). I think it's an issue where single
> keypresses are getting routed to multiple places and therefore the
> brightness is adjusted multiple times.
>
> So, I ask now, what are the different routes in which brightness will
> get adjusted? I'm thinking there are three:
>
> 1) Pure hw, like we discussed before.
>
> 2) Pure kernel ACPI, which brightness_switch_enabled=0 would turn off.
>
> 3) Userspace power manager like gnome-power-manager, which can be turned
> off by killing the process.
>
> Is this correct?
>
> Thanks in advance,
> David
>

There also might be hw controlled plus acpi events in some odd cases. Or there
is a acpi driver which translates acpi events into input events and accidentally
still a script that is triggered by acpi events, too.

Stefan

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