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commit 4e9bdc8ceeffa48962ae018cf52cc8ada72dc98b
Author: Carlo Caione <carlo@endlessm.com>
Date:   Fri Aug 21 08:58:14 2015 +0200

    drm/i915/opregion: work around buggy firmware that provides 15+ output devices
    
    - DIDL/DDL2/DDL3/... are the DIDL fields in the IGDM OpRegion (Supported
      Display Devices ID List)
    
    - This field indicates which display devices are supported by the platform, and
      therefore enumerable by the graphics driver. A maximum of 15 devices are
      assumed supportable on a given platform and enumerable by the graphics driver
    
    - The graphics driver detects or determines devices during its initialization
      and prior to the first monitor enumeration call it receives from the OS
    
    - This is currently done for the i915 driver in intel_didl_outputs() evaluating
      the _ADR of each output device and setting the DIDL value according to the
      Device ID returned by _ADR (see 3143751ff)
    
    - The firmware enumerates (in this order) devices DD0->DDF (defined as
      'External Digital Monitor') and then an additional device LCDD. This LCDD
      device is what is associated with the function keys via the EC according to
      ACPI (the method called for the brightness keys is \_SB.PCI0.GFX0.LCDD._DCS).
      The device ID for this LCDD device is 0x400 ('Internal/Integrated Digital Flat
      Panel')
    
    - Being this LCDD device the 16th device, it is not used to populate the DIDL
      fields (you can read 'More than 15 outputs detected via ACPI' in the journal)
      for this reason
    
    - The first 8 values in the DIDL fiels are used to initialize the CADL fields
      in the OpRegion (see d627b62ff) using the intel_setup_cadls() function in the
      i915 driver
    
    - The CADL/CAL3/CAL3/... fields indicate which display devices (monitors) are
      currently active. A maximum of 8  monitors are assumed active on a given
      platform. The IDs should be the same as the enumerated monitor or connector
      IDs. The graphics driver determines active monitors during mode set times and
      during boot.
    
    - In \_SB.PCI0.GFX0.CDDS (called by \_SB.PCI0.GFX0.LCDD._DCS) we are using
      these CADL values to determine if we can control the brightness of our
      current active monitor. This is done comparing the value of (SANV.DIDX &
      0x0F0F) to all the CADL values saved in the OpRegion
    
    - Please note that the content of SANV.DIDX is nothing else that the address of
      the LCDD device (+ some flag) since the LCDD device is the only device for
      which we can control the brightness
    
    - The problem is that none of the 8 values in the CADL region is actually the
      ID of the LCDD device (and then DIDX) since intel_setup_cadls() only copied
      the first 8 DIDL values that are the IDs of the first 8 devices read in
      intel_didl_outputs() and the LCDD device is not among the first 8 (it's the
      15th device returned pinging the _ADR methods)
    
    - What we do in the patch is to use the _BCM method to identify the laptop
      panel so we can copy its ID in the last CADL field so that
      \_SB.PCI0.GFX0.CDDS (and \_SB.PCI0.GFX0.LCDD._DCS) never fails.
    
    - This patch is an adaptation of https://patchwork.freedesktop.org/patch/20080/
    
    Signed-off-by: Carlo Caione <carlo@endlessm.com>

diff --git a/drivers/gpu/drm/i915/intel_opregion.c b/drivers/gpu/drm/i915/intel_opregion.c
index cb1c657..ae28d04 100644
--- a/drivers/gpu/drm/i915/intel_opregion.c
+++ b/drivers/gpu/drm/i915/intel_opregion.c
@@ -661,6 +661,7 @@ static void intel_didl_outputs(struct drm_device *dev)
 	acpi_status status;
 	u32 temp, max_outputs;
 	int i = 0;
+	bool done;
 
 	handle = ACPI_HANDLE(&dev->pdev->dev);
 	if (!handle || acpi_bus_get_device(handle, &acpi_dev))
@@ -692,11 +693,18 @@ static void intel_didl_outputs(struct drm_device *dev)
 	max_outputs = ARRAY_SIZE(opregion->acpi->didl) +
 		ARRAY_SIZE(opregion->acpi->did2);
 
+	done = false;
 	list_for_each_entry(acpi_cdev, &acpi_video_bus->children, node) {
 		if (i >= max_outputs) {
-			DRM_DEBUG_KMS("More than %u outputs detected via ACPI\n",
-				      max_outputs);
-			return;
+			DRM_DEBUG_KMS("More than %u outputs detected via ACPI, %s\n",
+				      max_outputs, acpi_device_bid(acpi_cdev));
+			if (acpi_has_method(acpi_cdev->handle, "_BCM")) {
+				DRM_DEBUG_KMS("%s has _BCM, replacing 8th entry\n", acpi_device_bid(acpi_cdev));
+				i = 7;
+				done = true;
+			} else {
+				continue;
+			}
 		}
 		status = acpi_evaluate_integer(acpi_cdev->handle, "_ADR",
 					       NULL, &device_id);
@@ -705,6 +713,9 @@ static void intel_didl_outputs(struct drm_device *dev)
 				goto blind_set;
 			set_did(opregion, i++, (u32)(device_id & 0x0f0f));
 		}
+
+		if (done)
+			return;
 	}
 
 end: