Re: ACPI/i915: Cannot configure display brightness on Dell Latitude E6440
From: Hans de Goede
Date: Wed Sep 24 2014 - 04:20:00 EST
On 09/23/2014 10:44 PM, Pali RohÃr wrote:
> On Tuesday 23 September 2014 22:31:31 you wrote:
>> On 09/23/2014 10:06 PM, Pali RohÃr wrote:
>>> after big changes in acpi video/i915 code I cannot change
>>> display brightness on my Dell Latitude E6440 with kernel
>>> 3.17-rc6. With kernel 3.13 everything worked fine.
>>> More information about this problem:
>>> For configuring brightness on Dell laptops there are 4 ways:
>>> 1) via acpi video driver
>>> 2) via dell-laptop driver
>>> 3) via i915 drm driver
>>> 4) from userspace with special dell SMI call
>>> (e.g with program dellLcdBrightness from libsmbios
>>> Methods 2) and 4) are same, both making special SMI call and
>>> Bios handing this request (just 2 is from kernel and 4 from
>>> Method 1) via acpi video driver working, but is not perfect.
>>> Driver can be used to change brightness (but only some
>>> levels, probably this depends on acpi/DSDT tables), but
>>> cannot be used to retrieve current brightness (when
>>> BIOS/SMI change brightness acpi driver report old incorrect
>>> value). So I prefer dell-laptop driver instead acpi video.
>>> Method 3) working even with 3.17-rc6 kernel but because that
>>> backlight device exported by i915 is marked as raw, desktop
>>> programs prefer to use other devices.
>>> Moreover it looks like that methods 1) 2) and 4) just
>>> forward request to method 3). So in any cased brightness is
>>> changed by i915 drm driver.
>>> I'm not sure (correct me if I'm wrong!) but I think that
>>> intel i915 drm driver accept changes (file
>>> intel_opregion.c) only if acpi function
>>> acpi_video_verify_backlight_support() returns true.
>>> Function acpi_video_verify_backlight_support() returns true
>>> iff: function acpi_video_backlight_support() returns true
>>> AND at least one of these functions returns false:
>>> On my notebook acpi_osi_is_win8() returns true (as is win8
>>> compliant), backlight_device_registered(BACKLIGHT_RAW)
>>> returns true as I'm using intel i915 drm driver with raw
>>> backlight device and acpi_video_use_native_backlight()
>>> returns true/false depending on
>>> "video.use_native_backlight" kernel param. Default is true.
>>> So if I want to have working acpi video driver with display
>>> brightness support I need to boot kernel with param:
>>> "video.use_native_backlight=0". I tested it with kernel
>>> 3.17-rc6 and this param really enabled display brightness
>>> support via acpi video driver -- which is good.
>>> Driver dell-laptop creating backligh device for brightness
>>> control only if acpi_video_backlight_support() returns
>>> false. There is complicated condition for it and when
>>> kernel is booted with "video.use_native_backlight=0" that
>>> function returns true.
>>> So conclusion is: With current code in kernel 3.17-rc6 it is
>>> not possible to control brightness of display with native
>>> driver dell-laptop on Dell Latitude E6440 (and probably on
>>> others too)!!!
>>> And Because laptop is win8 compliant and you create decision
>>> to use native driver (instead acpi one) it is not possible
>>> to control display brightness without tweeks in kernel
>>> As I wrote I would rather to use native dell-laptop driver
>>> for controlling brightness, but it is not possible.
>>> So how to solve this problem?
>>> Quick solution would be to set use_native_backlight false
>>> for some Dell laptops which means, that acpi video will be
>>> used and in this case intel i915 driver will *not* drop
>>> backlight change request.
>>> Another solution could be to disable check in dell_laptop
>>> driver and add use_native_backlight=0 to hooks. But this
>>> create two backlight interfaces (which is not good), but
>>> only way (for now) how to make dell_laptop working again.
>>> Better and maybe only one proper solution would be to teach
>>> intel drm i915 driver to not drop backlight change request
>>> for Dell laptops (or all??). (This allows to work both acpi
>>> video and dell_laptop drivers without any change and with
>>> *any* value in param use_native_backlight). I think that
>>> problematic code is in function asle_set_backlight() in
>>> file intel_opregion.c (but I'm not sure). My idea is that
>>> "drop" event function it is caused by this commit
>>> 0b9f7d93ca6109048a4eb06332b666b6e29df4fe (but I'm not
>>> At least something must be done as I think that I'm not only
>>> one who has Dell laptop and brightness configuration is
>>> really important...
>> I don't understand your problem, the kernel is selecting the
>> i915 backlight driver, making that the only one available to
>> userspace, so the one problem you state with the i915 driver,
>> that it is "raw" is not an issue, as userspace will pick it
>> when it is the only one.
> It is not only one.
Are you sure, if you do not specify any commandline parameters, then
neither dell-laptop nor acpi-video should register a backlight interface.
/* In the event of an ACPI backlight being available, don't
* register the platform controller.
And acpi_video_backlight_support() will (normally) return true on
acpi-backlight capable systems independent of use_native_backlight.
And drivers/acpi/video.c has this:
/* no warning message if acpi_backlight=vendor or a quirk is used */
if (acpi_osi_is_win8() && acpi_video_use_native_backlight() &&
So unlike the check in dell-laptop this one does depend on the
I've just checked 3.17 on my E6430 and there this works as it should,
I only get the intel_backlight in /sys/class/backlight
> Also dell-laptop (or acpi video) backlight
> interface is created (depends on use_native_backlight param). And
> userspace will pick dell-laptop (or acpi video) to use (which
> cannot change brightness).
>> Why would you want to use dell-laptop despite the i915 driver
>> working fine ?
> i915 working only if I compile kernel without acpi video dell-
> laptop support (distributions compile kernel with these drivers)
> and i915 is not good for usage. First it provides more then
> thousand brightness levels and lot of (with low numbers)
> completely turn display off. And if userspace map these thousand
> levels into 5-10 levels (as nobody want to press brightness key
> up/down 1000x) two lowest levels cause display off.
More and more laptops will only have working backlight control at all
when using i915, so userspace will need to learn to better deal with
backlight controls with these ranges. And low pwm levels turning
the backlight of is normal for raw interfaces, if userspace does
not want this, then they should not go so low.
I suggest that you file a bug against your desktop environment
that it is not using the backlight controls in an optimal manner,
from the kernel pov everything is working fine.
> With acpi
> video and dell-laptop driver levels are mapped into small level
> space in perfect way. So this is reason I want to use dell-laptop
> for controlling brightness.
If you want to use dell-laptop, specify acpi_backlight=vendor on
the kernel commandline, that will give you dell_laptop +
intel_backlight as backlight interfaces, and userspace should prefer
dell_laptop. But IMHO it would be better to file a bug with your
desktop environment, and get that fixed to work properly with
intel_backlight (or with raw backlight interfaces in general).
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