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

[Aaron Lu]

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;
 	}