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

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>
Cc: stable@vger.kernel.org
Tested-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
index a6d8a3ee7750adecb552415c8ffab06d599f46ff..260389acfb7752d01ce0ebd59597f06a671b2ea1 100644 (file)
--- a/drivers/gpu/drm/i915/intel_uncore.c
+++ b/drivers/gpu/drm/i915/intel_uncore.c
@@ -1274,10 +1274,12 @@ int i915_reg_read_ioctl(struct drm_device *dev,
        struct drm_i915_private *dev_priv = dev->dev_private;
        struct drm_i915_reg_read *reg = data;
        struct register_whitelist const *entry = whitelist;
+       unsigned size;
+       u64 offset;
        int i, ret = 0;
 
        for (i = 0; i < ARRAY_SIZE(whitelist); i++, entry++) {
-               if (entry->offset == reg->offset &&
+               if (entry->offset == (reg->offset & -entry->size) &&
                    (1 << INTEL_INFO(dev)->gen & entry->gen_bitmask))
                        break;
        }
@@ -1285,23 +1287,33 @@ int i915_reg_read_ioctl(struct drm_device *dev,
        if (i == ARRAY_SIZE(whitelist))
                return -EINVAL;
 
+       /* We use the low bits to encode extra flags as the register should
+        * be naturally aligned (and those that are not so aligned merely
+        * limit the available flags for that register).
+        */
+       offset = entry->offset;
+       size = entry->size;
+       size |= reg->offset ^ offset;
+
        intel_runtime_pm_get(dev_priv);
 
-       switch (entry->size) {
+       switch (size) {
+       case 8 | 1:
+               reg->val = I915_READ64_2x32(offset, offset+4);
+               break;
        case 8:
-               reg->val = I915_READ64(reg->offset);
+               reg->val = I915_READ64(offset);
                break;
        case 4:
-               reg->val = I915_READ(reg->offset);
+               reg->val = I915_READ(offset);
                break;
        case 2:
-               reg->val = I915_READ16(reg->offset);
+               reg->val = I915_READ16(offset);
                break;
        case 1:
-               reg->val = I915_READ8(reg->offset);
+               reg->val = I915_READ8(offset);
                break;
        default:
-               MISSING_CASE(entry->size);
                ret = -EINVAL;
                goto out;
        }

diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 6e1a2ed116cb1410958f61631a7dc30839652738..db809b722985d3a1a03480cc1e3d36a656af2d2d 100644 (file)
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1070,6 +1070,14 @@ 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;