intel-gem: Disable tiling if we get junk from the MCHBAR read.

One of our systems has been returning 0xffffffff from all MCHBAR reads, which
means we'll need to figure out why, or add an alternate detection method.

diff --git a/linux-core/i915_gem_tiling.c b/linux-core/i915_gem_tiling.c
index d66276d..ddf83ce 100644 (file)
--- a/linux-core/i915_gem_tiling.c
+++ b/linux-core/i915_gem_tiling.c
@@ -155,7 +155,10 @@ i915_gem_detect_bit_6_swizzle(struct drm_device *dev)
                 */
                chdecmisc = readb(mchbar + CHDECMISC);
 
-               if (chdecmisc & CHDECMISC_FLEXMEMORY) {
+               if (chdecmisc == 0xff) {
+                       DRM_ERROR("Couldn't read from MCHBAR.  "
+                                 "Disabling tiling.\n");
+               } else if (chdecmisc & CHDECMISC_FLEXMEMORY) {
                        swizzle_x = I915_BIT_6_SWIZZLE_NONE;
                        swizzle_y = I915_BIT_6_SWIZZLE_NONE;
                } else {
@@ -197,6 +200,12 @@ i915_gem_detect_bit_6_swizzle(struct drm_device *dev)
                        }
                        break;
                }
+               if (dcc == 0xffffffff) {
+                       DRM_ERROR("Couldn't read from MCHBAR.  "
+                                 "Disabling tiling.\n");
+                       swizzle_x = I915_BIT_6_SWIZZLE_UNKNOWN;
+                       swizzle_y = I915_BIT_6_SWIZZLE_UNKNOWN;
+               }
        }
 
        iounmap(mchbar);