drm/i915: Resolving the memory region conflict for Stolen area

There is a conflict seen when requesting the kernel to reserve
the physical space used for the stolen area. This is because
some BIOS are wrapping the stolen area in the root PCI bus, but have
an off-by-one error. As a workaround we retry the reservation with an
offset of 1 instead of 0.

v2: updated commit message & the comment in source file (Daniel)
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_gem_stolen.c

@@ -81,11 +81,24 @@ static unsigned long i915_stolen_to_physical(struct drm_device *dev)
 	 */
 	r = devm_request_mem_region(dev->dev, base, dev_priv->gtt.stolen_size,
 				    "Graphics Stolen Memory");
+	if (r == NULL) {
+		/*
+		 * One more attempt but this time requesting region from
+		 * base + 1, as we have seen that this resolves the region
+		 * conflict with the PCI Bus.
+		 * This is a BIOS w/a: Some BIOS wrap stolen in the root
+		 * PCI bus, but have an off-by-one error. Hence retry the
+		 * reservation starting from 1 instead of 0.
+		 */
+		r = devm_request_mem_region(dev->dev, base + 1,
+					    dev_priv->gtt.stolen_size - 1,
+					    "Graphics Stolen Memory");
 		if (r == NULL) {
 			DRM_ERROR("conflict detected with stolen region: [0x%08x - 0x%08x]\n",
 				  base, base + (uint32_t)dev_priv->gtt.stolen_size);
 			base = 0;
 		}
+	}
 
 	return base;
 }