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>

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 1ac57ec..41263cd 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/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: