xdrm/i915: Use two 32bit reads for select 64bit REG_READ ioctls

commit 648a9bc5308d952f2c80772301b339f73026f013 upstream.

Since the hardware sometimes mysteriously totally flummoxes the 64bit
read of a 64bit register when read using a single instruction, split the
read into two instructions. Since the read here is of automatically
incrementing timestamp counters, we also have to be very careful in
order to make sure that it does not increment between the two
instructions.

However, since userspace tried to workaround this issue and so enshrined
this ABI for a broken hardware read and in the process neglected that
the read only fails in some environments, we have to introduce a new
uABI flag for userspace to request the 2x32 bit accurate read of the
timestamp.

v2: Fix alignment check and include details of the workaround for
userspace.

Reported-by: Karol Herbst <freedesktop@karolherbst.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91317
Testcase: igt/gem_reg_read
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Tested-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index ff57f07c3249..86a07d6ece9c 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1035,6 +1035,14 @@ struct drm_i915_reg_read {
 	__u64 offset;
 	__u64 val; /* Return value */
 };
+/* Known registers:
+ *
+ * Render engine timestamp - 0x2358 + 64bit - gen7+
+ * - Note this register returns an invalid value if using the default
+ *   single instruction 8byte read, in order to workaround that use
+ *   offset (0x2538 | 1) instead.
+ *
+ */
 
 struct drm_i915_reset_stats {
 	__u32 ctx_id;