[PATCH v3] drm/i915: Fix not finding the VBT when it overlaps with OPREGION_ASLE_EXT

[Jani Nikula <jani.nikula@intel.com>]

From: Hans de Goede <hdegoede@redhat.com>

If there is no OPREGION_ASLE_EXT then a VBT stored in mailbox #4 may
use the ASLE_EXT parts of the opregion. Adjust the vbt_size calculation
for a vbt in mailbox #4 for this.

This fixes the driver not finding the VBT on a jumper ezpad mini3
cherrytrail tablet and on a ACER SW5_017 machine.

Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

---

Hans, I'm resending because for some reason our CI didn't pick this
up. Fixed a s/one/on/ typo in the comment while at it.
---
 drivers/gpu/drm/i915/intel_opregion.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_opregion.c b/drivers/gpu/drm/i915/intel_opregion.c
index f4429f67a4e3..4a862a358c70 100644
--- a/drivers/gpu/drm/i915/intel_opregion.c
+++ b/drivers/gpu/drm/i915/intel_opregion.c
@@ -982,7 +982,18 @@ int intel_opregion_setup(struct drm_i915_private *dev_priv)
 			opregion->vbt_size = vbt_size;
 		} else {
 			vbt = base + OPREGION_VBT_OFFSET;
-			vbt_size = OPREGION_ASLE_EXT_OFFSET - OPREGION_VBT_OFFSET;
+			/*
+			 * The VBT specification says that if the ASLE ext
+			 * mailbox is not used its area is reserved, but
+			 * on some CHT boards the VBT extends into the
+			 * ASLE ext area. Allow this even though it is
+			 * against the spec, so we do not end up rejecting
+			 * the VBT on those boards (and end up not finding the
+			 * LCD panel because of this).
+			 */
+			vbt_size = (mboxes & MBOX_ASLE_EXT) ?
+				OPREGION_ASLE_EXT_OFFSET : OPREGION_SIZE;
+			vbt_size -= OPREGION_VBT_OFFSET;
 			if (intel_bios_is_valid_vbt(vbt, vbt_size)) {
 				DRM_DEBUG_KMS("Found valid VBT in ACPI OpRegion (Mailbox #4)\n");
 				opregion->vbt = vbt;
-- 
2.1.4