[Mesa-dev] [PATCH] i965: Don't tell the hardware about our UAV access.

Francisco Jerez currojerez at riseup.net
Fri Aug 14 09:27:33 PDT 2015

The hardware documentation relating to the UAV HW-assisted coherency
mechanism and UAV access enable bits is scarce and sometimes
contradictory, and there's quite some guesswork behind this commit, so
let me summarize the background first: HSW and later hardware have
infrastructure to support a stricter form of data coherency between
shader invocations from separate primitives.  The mechanism is
controlled by the "Accesses UAV" bits on 3DSTATE_VS, _HS, _DS, _GS and
_PS (or _PS_EXTRA on BDW+), and the "UAV Coherency Required" bit on
the 3DPRIMITIVE command.

Regardless of whether "UAV Coherency Required" is set, the hardware
fixed-function units will increment a per-stage semaphore for each
request received if "Accesses UAV" is set for the same or any lower
stage.  An implicit DC flush is emitted by the lowermost stage with
"Accesses UAV" set once it's done processing the request, this also
happens regardless of the value of "UAV Coherency Required".  The
completion of the DC flush will cause the same stage and all previous
ones to decrement the semaphore, marking the UAV accesses for the
primitive as coherent with L3.

The "UAV Coherency Required" 3DPRIMITIVE bit will cause a pipeline
stall before any threads are dispatched for the first FF stage with
"Accesses UAV" set until the semaphore is cleared for the same stage.
Effectively this guarantees that UAV memory accesses performed by
previous primitives from any stage will be strictly ordered (and
thanks to the implicit DC flush visible in memory) with UAV accesses
from the following primitives.

None of this is required by the usual image, atomic counter and SSBO
GL APIs which have very relaxed cross-primitive coherency and ordering
requirements, so we don't actually ever set the "UAV Coherency
Required" bit -- Ordering with respect to shader invocations from
previous stages on the same primitive where there is a data dependency
is of course already guaranteed as the spec requires, regardless of
this mechanism being enabled.  We do set the "Accesses UAV" bits
though since my commit ac7664e493655e290783c23a0412b9c70936da50 (which
this patch partially reverts), mainly because of comments like the
following from the BDW PRM:

> 12 Accesses UAV
>    Format: Enable
>    This field must be set when GS has a UAV access.

There are similar comments in the documentation for the other
3DSTATE_*S commands.  The "must" part is misleading and unjustified
AFAIK.  Most of the "Accesses UAV" bits don't seem to have any side
effects other than the implicit DC flushes and the related
book-keeping in anticipation for a subsequent primitive with "UAV
Coherency Required" set, so in most cases they are unnecessary and may
incur a performance penalty.  There is an exception though.  On Gen8+
the PS_EXTRA UAV access bit influences the calculation of the PS
UAV-only and ThreadDispatchEnable signals which on previous
generations were set explicitly by the driver, so we cannot always
avoid enabling it on the PS stage.

The primary motivation for this change is that in fact the hardware
coherency mechanism is buggy and will cause a rather non-deterministic
hang on Gen8 when VS is the only stage with "Accesses UAV" set and the
processing of a request terminates immediately after the implicit DC
flush is sent for a previous primitive with no additional vertices
being emitted for the second primitive, what will cause the hardware
to skip sending a second DC flush and cause the VS to stall
indefinitely waiting for a response from the DC (BDWGFX HSD 1912017).
This hardware bug can be reproduced on current master with the
spec at arb_shader_image_load_store@host-mem-barrier at Indirect/RaW piglit
subtest (if you have the patience to run it a few dozen times).

The proposed workaround is to insert CS STALLs speculatively between
3DPRIMITIVE commands when "Accesses UAV" is enabled for the VS stage
only.  Because this would affect one of the hottest paths in the
driver and likely decrease performance even further due to the
unnecessary serialization, and because we don't actually need the
implicit DC flushes, it seems better to just disable them.
 src/mesa/drivers/dri/i965/gen7_gs_state.c |  4 +---
 src/mesa/drivers/dri/i965/gen7_vs_state.c |  4 +---
 src/mesa/drivers/dri/i965/gen7_wm_state.c | 12 ++++++++----
 src/mesa/drivers/dri/i965/gen8_gs_state.c |  4 +---
 src/mesa/drivers/dri/i965/gen8_ps_state.c | 32 ++++++++++++++++++++++++++++---
 src/mesa/drivers/dri/i965/gen8_vs_state.c |  4 +---
 6 files changed, 41 insertions(+), 19 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/gen7_gs_state.c b/src/mesa/drivers/dri/i965/gen7_gs_state.c
index 497ecec..8d6d3fe 100644
--- a/src/mesa/drivers/dri/i965/gen7_gs_state.c
+++ b/src/mesa/drivers/dri/i965/gen7_gs_state.c
@@ -59,9 +59,7 @@ upload_gs_state(struct brw_context *brw)
       OUT_BATCH(((ALIGN(stage_state->sampler_count, 4)/4) <<
                  GEN6_GS_SAMPLER_COUNT_SHIFT) |
                 ((brw->gs.prog_data->base.base.binding_table.size_bytes / 4) <<
-                (brw->is_haswell && prog_data->base.nr_image_params ?
-                 HSW_GS_UAV_ACCESS_ENABLE : 0));
       if (brw->gs.prog_data->base.base.total_scratch) {
diff --git a/src/mesa/drivers/dri/i965/gen7_vs_state.c b/src/mesa/drivers/dri/i965/gen7_vs_state.c
index b7e4858..a18dc69 100644
--- a/src/mesa/drivers/dri/i965/gen7_vs_state.c
+++ b/src/mesa/drivers/dri/i965/gen7_vs_state.c
@@ -126,9 +126,7 @@ upload_vs_state(struct brw_context *brw)
 	     ((ALIGN(stage_state->sampler_count, 4)/4) <<
               GEN6_VS_SAMPLER_COUNT_SHIFT) |
              ((brw->vs.prog_data->base.base.binding_table.size_bytes / 4) <<
-             (brw->is_haswell && prog_data->base.nr_image_params ?
-              HSW_VS_UAV_ACCESS_ENABLE : 0));
    if (prog_data->base.total_scratch) {
diff --git a/src/mesa/drivers/dri/i965/gen7_wm_state.c b/src/mesa/drivers/dri/i965/gen7_wm_state.c
index fd6dab5..06d5e65 100644
--- a/src/mesa/drivers/dri/i965/gen7_wm_state.c
+++ b/src/mesa/drivers/dri/i965/gen7_wm_state.c
@@ -113,7 +113,14 @@ upload_wm_state(struct brw_context *brw)
    else if (prog_data->base.nr_image_params)
+   /* The "UAV access enable" bits are unnecessary on HSW because they only
+    * seem to have an effect on the HW-assisted coherency mechanism which we
+    * don't need, and the rasterization-related UAV_ONLY flag and the
+    * DISPATCH_ENABLE bit can be set independently from it.
+    * C.f. gen8_upload_ps_extra().
+    *
+    */
    if (brw->is_haswell &&
        !(brw_color_buffer_write_enabled(brw) || writes_depth) &&
@@ -221,9 +228,6 @@ gen7_upload_ps_state(struct brw_context *brw,
       _mesa_get_min_invocations_per_fragment(ctx, fp, false);
    assert(min_inv_per_frag >= 1);
-   if (brw->is_haswell && prog_data->base.nr_image_params)
    if (prog_data->prog_offset_16 || prog_data->no_8) {
       dw4 |= GEN7_PS_16_DISPATCH_ENABLE;
       if (!prog_data->no_8 && min_inv_per_frag == 1) {
diff --git a/src/mesa/drivers/dri/i965/gen8_gs_state.c b/src/mesa/drivers/dri/i965/gen8_gs_state.c
index 81bd3b2..26a02d3 100644
--- a/src/mesa/drivers/dri/i965/gen8_gs_state.c
+++ b/src/mesa/drivers/dri/i965/gen8_gs_state.c
@@ -52,9 +52,7 @@ gen8_upload_gs_state(struct brw_context *brw)
                 ((ALIGN(stage_state->sampler_count, 4)/4) <<
                  GEN6_GS_SAMPLER_COUNT_SHIFT) |
                 ((prog_data->base.binding_table.size_bytes / 4) <<
-                (prog_data->base.nr_image_params ?
-                 HSW_GS_UAV_ACCESS_ENABLE : 0));
       if (brw->gs.prog_data->base.base.total_scratch) {
diff --git a/src/mesa/drivers/dri/i965/gen8_ps_state.c b/src/mesa/drivers/dri/i965/gen8_ps_state.c
index ae18f0f..a6c9ab3 100644
--- a/src/mesa/drivers/dri/i965/gen8_ps_state.c
+++ b/src/mesa/drivers/dri/i965/gen8_ps_state.c
@@ -25,6 +25,7 @@
 #include "program/program.h"
 #include "brw_state.h"
 #include "brw_defines.h"
+#include "brw_wm.h"
 #include "intel_batchbuffer.h"
@@ -61,8 +62,33 @@ gen8_upload_ps_extra(struct brw_context *brw,
    if (brw->gen >= 9 && prog_data->pulls_bary)
-   if (_mesa_active_fragment_shader_has_atomic_ops(&brw->ctx) ||
-       prog_data->base.nr_image_params)
+   /* The stricter cross-primitive coherency guarantees that the hardware
+    * gives us with the "Accesses UAV" bit set for at least one shader stage
+    * and the "UAV coherency required" bit set on the 3DPRIMITIVE command are
+    * redundant within the current image, atomic counter and SSBO GL APIs,
+    * which all have very loose ordering and coherency requirements and
+    * generally rely on the application to insert explicit barriers when a
+    * shader invocation is expected to see the memory writes performed by the
+    * invocations of some previous primitive.  Regardless of the value of "UAV
+    * coherency required", the "Accesses UAV" bits will implicitly cause an in
+    * most cases useless DC flush when the lowermost stage with the bit set
+    * finishes execution.
+    *
+    * It would be nice to disable it, but in some cases we can't because on
+    * Gen8+ it also has an influence on rasterization via the PS UAV-only
+    * signal (which could be set independently from the coherency mechanism in
+    * the 3DSTATE_WM command on Gen7), and because in some cases it will
+    * determine whether the hardware skips execution of the fragment shader or
+    * not via the ThreadDispatchEnable signal.  However if we know that
+    * GEN8_PS_BLEND_HAS_WRITEABLE_RT is going to be set and
+    * GEN8_PSX_PIXEL_SHADER_NO_RT_WRITE is not set it shouldn't make any
+    * difference so we may just disable it here.
+    *
+    */
+   if ((_mesa_active_fragment_shader_has_atomic_ops(&brw->ctx) ||
+        prog_data->base.nr_image_params) &&
+       !brw_color_buffer_write_enabled(brw))
       dw1 |= GEN8_PSX_SHADER_HAS_UAV;
@@ -87,7 +113,7 @@ upload_ps_extra(struct brw_context *brw)
 const struct brw_tracked_state gen8_ps_extra = {
    .dirty = {
-      .mesa  = 0,
+      .mesa  = _NEW_BUFFERS | _NEW_COLOR,
       .brw   = BRW_NEW_CONTEXT |
                BRW_NEW_FRAGMENT_PROGRAM |
                BRW_NEW_FS_PROG_DATA |
diff --git a/src/mesa/drivers/dri/i965/gen8_vs_state.c b/src/mesa/drivers/dri/i965/gen8_vs_state.c
index 8b5048b..28f5add 100644
--- a/src/mesa/drivers/dri/i965/gen8_vs_state.c
+++ b/src/mesa/drivers/dri/i965/gen8_vs_state.c
@@ -53,9 +53,7 @@ upload_vs_state(struct brw_context *brw)
              ((ALIGN(stage_state->sampler_count, 4) / 4) <<
                GEN6_VS_SAMPLER_COUNT_SHIFT) |
              ((prog_data->base.binding_table.size_bytes / 4) <<
-             (prog_data->base.nr_image_params ?
-              HSW_VS_UAV_ACCESS_ENABLE : 0));
    if (prog_data->base.total_scratch) {

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