We don't have safe 64-bit mmio writes as they are really split into
2x32-bit writes. This tearing is dangerous as the hardware *will*
operate on the intermediate value, requiring great care when assigning.
(See, for example, i965_write_fence_reg.) As such we don't currently use
them and strongly advise not to us them. Go one step further and remove
the 64-bit write vfuncs.
v2: Add some more details to the comment about why WRITE64 is absent,
and why you need to think twice before using READ64.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/i915_drv.h | 13 +++++++++----
drivers/gpu/drm/i915/intel_uncore.c | 9 ---------
2 files changed, 9 insertions(+), 13 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 757b1d1b9366..ecfd8e9726b5 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -579,8 +579,6 @@ struct intel_uncore_funcs {
uint16_t val, bool trace);
void (*mmio_writel)(struct drm_i915_private *dev_priv, i915_reg_t r,
uint32_t val, bool trace);
- void (*mmio_writeq)(struct drm_i915_private *dev_priv, i915_reg_t r,
- uint64_t val, bool trace);
};
struct intel_uncore {
@@ -3758,9 +3756,16 @@ int intel_freq_opcode(struct drm_i915_private *dev_priv, int val);
* will be implemented using 2 32-bit writes in an arbitrary order with
* an arbitrary delay between them. This can cause the hardware to
* act upon the intermediate value, possibly leading to corruption and
- * machine death. You have been warned.
+ * machine death. For this reason we do not support I915_WRITE64, or
+ * dev_priv->uncore.funcs.mmio_writeq.
+ *
+ * When reading a 64-bit value as two 32-bit values, the delay may cause
+ * the two reads to mismatch, e.g. a timestamp overflowing. Also note that
+ * occasionally a 64-bit register does not actualy support a full readq
+ * and must be read using two 32-bit reads.
+ *
+ * You have been warned.
*/
-#define I915_WRITE64(reg, val) dev_priv->uncore.funcs.mmio_writeq(dev_priv, (reg), (val), true)
#define I915_READ64(reg) dev_priv->uncore.funcs.mmio_readq(dev_priv, (reg), true)
#define I915_READ64_2x32(lower_reg, upper_reg) ({ \
diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
index e9f68cd56e32..a9b6c936aadd 100644
--- a/drivers/gpu/drm/i915/intel_uncore.c
+++ b/drivers/gpu/drm/i915/intel_uncore.c
@@ -1018,11 +1018,9 @@ gen5_write##x(struct drm_i915_private *dev_priv, i915_reg_t reg, u##x val, bool
__gen5_write(8)
__gen5_write(16)
__gen5_write(32)
-__gen5_write(64)
__gen2_write(8)
__gen2_write(16)
__gen2_write(32)
-__gen2_write(64)
#undef __gen5_write
#undef __gen2_write
@@ -1112,23 +1110,18 @@ gen9_write##x(struct drm_i915_private *dev_priv, i915_reg_t reg, u##x val, \
__gen9_write(8)
__gen9_write(16)
__gen9_write(32)
-__gen9_write(64)
__chv_write(8)
__chv_write(16)
__chv_write(32)
-__chv_write(64)
__gen8_write(8)
__gen8_write(16)
__gen8_write(32)
-__gen8_write(64)
__hsw_write(8)
__hsw_write(16)
__hsw_write(32)
-__hsw_write(64)
__gen6_write(8)
__gen6_write(16)
__gen6_write(32)
-__gen6_write(64)
#undef __gen9_write
#undef __chv_write
@@ -1158,7 +1151,6 @@ static void vgpu_write##x(struct drm_i915_private *dev_priv, \
__vgpu_write(8)
__vgpu_write(16)
__vgpu_write(32)
-__vgpu_write(64)
#undef __vgpu_write
#undef VGPU_WRITE_FOOTER
@@ -1169,7 +1161,6 @@ do { \
dev_priv->uncore.funcs.mmio_writeb = x##_write8; \
dev_priv->uncore.funcs.mmio_writew = x##_write16; \
dev_priv->uncore.funcs.mmio_writel = x##_write32; \
- dev_priv->uncore.funcs.mmio_writeq = x##_write64; \
} while (0)
#define ASSIGN_READ_MMIO_VFUNCS(x) \
--
2.9.3
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