r300g: add workaround for multiple contexts

src/gallium/drivers/r300/r300_context.c

@@ -121,7 +121,7 @@ static void r300_destroy_context(struct pipe_context* context)
     FREE(r300);
 }
 
-static void r300_flush_cb(void *data)
+void r300_flush_cb(void *data)
 {
     struct r300_context* const cs_context_copy = data;
 

src/gallium/drivers/r300/r300_context.h

@@ -564,6 +564,7 @@ struct pipe_context* r300_create_context(struct pipe_screen* screen,
 
 boolean r300_check_cs(struct r300_context *r300, unsigned size);
 void r300_finish(struct r300_context *r300);
+void r300_flush_cb(void *data);
 
 /* Context initialization. */
 struct draw_stage* r300_draw_stage(struct r300_context* r300);

src/gallium/drivers/r300/r300_screen_buffer.c

@@ -178,7 +178,14 @@ r300_buffer_transfer_map( struct pipe_context *pipe,
 	    }
 	}
     }
+
 just_map:
+    /* XXX buffer_map might flush.
+     * We cannot flush here because there is a buffer manager between
+     * the context and winsys, and it does some magic to make the driver
+     * fast. This is a workaround for the case of multiple contexts. */
+    rws->set_flush_cb(rws, r300_flush_cb, pipe);
+
     map = rws->buffer_map(rws, rbuf->buf, transfer->usage);
 
     if (map == NULL)