asahi: Add ASAHI_MESA_DEBUG=nowc flag

Add a debug flag to disable write-combining as a performance hack. This may help
diagnose slowness with glReadPixels() heavy workloads like screen capture.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23480>

docs/envvars.rst

@@ -1633,6 +1633,10 @@ Asahi driver environment variables
       possible) or added in the Mesa-wide driconf (if closed source).
    ``dirty``
       In debug builds only: disable dirty tracking optimizations.
+   ``nowc``
+      Disable write-combining (force all allocations to be write-through). This
+      may be useful for diagnosing certain performance issues. Note imported
+      buffers may still be write-combined.
 
 .. envvar:: AGX_MESA_DEBUG
 

src/asahi/lib/agx_device.h

@@ -25,6 +25,7 @@ enum agx_dbg {
    AGX_DBG_STATS = BITFIELD_BIT(9),
    AGX_DBG_RESOURCE = BITFIELD_BIT(10),
    AGX_DBG_BATCH = BITFIELD_BIT(11),
+   AGX_DBG_NOWC = BITFIELD_BIT(12),
 };
 
 /* Dummy partial declarations, pending real UAPI */

src/gallium/drivers/asahi/agx_pipe.c

@@ -69,6 +69,7 @@ static const struct debug_named_value agx_debug_options[] = {
    {"stats",     AGX_DBG_STATS,    "Show command execution statistics"},
    {"resource",  AGX_DBG_RESOURCE, "Log resource operations"},
    {"batch",     AGX_DBG_BATCH,    "Log batches"},
+   {"nowc",      AGX_DBG_NOWC,     "Disable write-combining"},
    DEBUG_NAMED_VALUE_END
 };
 /* clang-format on */
@@ -557,6 +558,11 @@ agx_resource_create_with_modifiers(struct pipe_screen *screen,
       create_flags |= AGX_BO_WRITEBACK;
    }
 
+   /* Allow disabling write-combine to debug performance issues */
+   if (dev->debug & AGX_DBG_NOWC) {
+      create_flags |= AGX_BO_WRITEBACK;
+   }
+
    /* Create buffers that might be shared with the SHAREABLE flag */
    if (bind & (PIPE_BIND_SCANOUT | PIPE_BIND_DISPLAY_TARGET | PIPE_BIND_SHARED))
       create_flags |= AGX_BO_SHAREABLE;