4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit 998d75730b40afc218c059d811869abe9676b305 upstream.

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.

Signed-off-by: Hans de Goede <hdego...@redhat.com>
Signed-off-by: Jani Nikula <jani.nik...@intel.com>
Link: 
http://patchwork.freedesktop.org/patch/msgid/1487088758-30050-1-git-send-email-jani.nik...@intel.com
(cherry picked from commit dfb65e71ea2c1d97ac373cc0587dc60b3307581a)
Signed-off-by: Jani Nikula <jani.nik...@intel.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/gpu/drm/i915/intel_opregion.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/i915/intel_opregion.c
+++ b/drivers/gpu/drm/i915/intel_opregion.c
@@ -1031,7 +1031,18 @@ int intel_opregion_setup(struct drm_i915
                        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;


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