gallium/radeon: add a debug flag not to use write combining (v2)

v2: just clear the flag before the allocation

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

src/gallium/drivers/radeon/r600_buffer_common.c

@@ -161,6 +161,9 @@ bool r600_init_resource(struct r600_common_screen *rscreen,
 		flags |= RADEON_FLAG_NO_CPU_ACCESS;
 	}
 
+	if (rscreen->debug_flags & DBG_NO_WC)
+		flags &= ~RADEON_FLAG_GTT_WC;
+
 	/* Allocate a new resource. */
 	new_buf = rscreen->ws->buffer_create(rscreen->ws, size, alignment,
 					     use_reusable_pool,

src/gallium/drivers/radeon/r600_pipe_common.c

@@ -355,6 +355,7 @@ static const struct debug_named_value common_debug_options[] = {
 	{ "switch_on_eop", DBG_SWITCH_ON_EOP, "Program WD/IA to switch on end-of-packet." },
 	{ "forcedma", DBG_FORCE_DMA, "Use asynchronous DMA for all operations when possible." },
 	{ "precompile", DBG_PRECOMPILE, "Compile one shader variant at shader creation." },
+	{ "nowc", DBG_NO_WC, "Disable GTT write combining" },
 
 	DEBUG_NAMED_VALUE_END /* must be last */
 };

src/gallium/drivers/radeon/r600_pipe_common.h

@@ -97,6 +97,7 @@
 #define DBG_FORCE_DMA		(1llu << 38)
 #define DBG_PRECOMPILE		(1llu << 39)
 #define DBG_INFO		(1llu << 40)
+#define DBG_NO_WC		(1llu << 41)
 
 #define R600_MAP_BUFFER_ALIGNMENT 64