Re: ACPI/i915: Cannot configure display brightness on Dell Latitude E6440

From: Aaron Lu
Date: Wed Sep 24 2014 - 23:15:43 EST

Hi Hans,

Thanks for following up and explaining the situation to Pali.

On 09/25/2014 02:21 AM, Pali Rohár wrote:
> On Wednesday 24 September 2014 16:34:21 Hans de Goede wrote:
>> Ok, so the dell-laptop interface is just an obsolete wrapper
>> around the i915 opregion code, which shows that the right
>> interface to use is the i915 one, which we do if you don't
>> specify any kernel commandline parameters, case closed.
>> Regards,
>> Hans
> Nope, its not closed.
> Still i915 interface has problem with setting backlight. It
> exports lot of levels which turning display off. Which breaking
> exiting applications for configuring display brightness. This is
> still big regression as black screen is not want people want to
> see.
> Driver dell-laptop has exported only few - not thousands level
> (which is insane) and only usefull levels (not lot of levels
> which turn display off).
> So for this reason using i915 backlight interface is not possible
> and also Dell (for E6440) set kernel param acpi_backlight=vendor
> to use dell_laptop module for controlling brightness.
> On my laptop E6440 is better for using dell-laptop and not acpi
> or i915.

Hi Pali,

Please test this patch:

diff --git a/drivers/gpu/drm/i915/intel_opregion.c b/drivers/gpu/drm/i915/intel_opregion.c
index ca52ad2ae7d1..15534345bd57 100644
--- a/drivers/gpu/drm/i915/intel_opregion.c
+++ b/drivers/gpu/drm/i915/intel_opregion.c
@@ -396,6 +396,24 @@ int intel_opregion_notify_adapter(struct drm_device *dev, pci_power_t state)
return -EINVAL;

+ * Some of the Thinkpads' firmware will issue a backlight change operation
+ * region request unconditionally on AC plug/unplug, this is undesirable and
+ * should be ignored. Then there is a Dell laptop whose vendor backlight
+ * interface also makes use of operation region request to change backlight
+ * level and we have to keep it work. The rule used here is: if the vendor
+ * backlight interface is not in use and the ACPI backlight interface is
+ * broken, we ignore the requests; oterwise, we keep processing them.
+ */
+static bool should_ignore_backlight_request(void)
+ if (acpi_video_backlight_support() &&
+ !acpi_video_verify_backlight_support())
+ return true;
+ return false;
static u32 asle_set_backlight(struct drm_device *dev, u32 bclp)
struct drm_i915_private *dev_priv = dev->dev_private;
@@ -404,11 +422,7 @@ static u32 asle_set_backlight(struct drm_device *dev, u32 bclp)

DRM_DEBUG_DRIVER("bclp = 0x%08x\n", bclp);

- /*
- * If the acpi_video interface is not supposed to be used, don't
- * bother processing backlight level change requests from firmware.
- */
- if (!acpi_video_verify_backlight_support()) {
+ if (should_ignore_backlight_request()) {
DRM_DEBUG_KMS("opregion backlight request ignored\n");
return 0;
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