drm/i915/bxt: don't allow cached GEM mappings on A stepping

Due to a coherency issue on BXT A steppings we can't guarantee a
coherent view of cached (CPU snooped) GPU mappings, so fail such
requests. User space is supposed to fall back to uncached mappings in
this case.

v2:
- limit the WA to A steppings, on later stepping this HW issue is fixed
v3:
- return error instead of trying to work around the issue in kernel,
  since that could confuse user space (Chris)

Testcast: igt/gem_store_dword_batches_loop/cached-mapping
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3742,6 +3742,15 @@ int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
 		level = I915_CACHE_NONE;
 		break;
 	case I915_CACHING_CACHED:
+		/*
+		 * Due to a HW issue on BXT A stepping, GPU stores via a
+		 * snooped mapping may leave stale data in a corresponding CPU
+		 * cacheline, whereas normally such cachelines would get
+		 * invalidated.
+		 */
+		if (IS_BROXTON(dev) && INTEL_REVID(dev) < BXT_REVID_B0)
+			return -ENODEV;
+
 		level = I915_CACHE_LLC;
 		break;
 	case I915_CACHING_DISPLAY: