radeonsi: document some subtle details of fence_finish & fence_server_sync

v2: remove the change to si_fence_server_sync, we'll handle that more
    robustly

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

diff --git a/src/gallium/drivers/radeonsi/si_fence.c b/src/gallium/drivers/radeonsi/si_fence.c
index 701e8df9cfc18e43d8e5e8494d628fbd16b6ce32..81007192994e9a974a8f81123b726d2a708f4c07 100644 (file)
--- a/src/gallium/drivers/radeonsi/si_fence.c
+++ b/src/gallium/drivers/radeonsi/si_fence.c
@@ -175,6 +175,28 @@ static boolean si_fence_finish(struct pipe_screen *screen,
        if (rctx &&
            rfence->gfx_unflushed.ctx == rctx &&
            rfence->gfx_unflushed.ib_index == rctx->num_gfx_cs_flushes) {
+               /* Section 4.1.2 (Signaling) of the OpenGL 4.6 (Core profile)
+                * spec says:
+                *
+                *    "If the sync object being blocked upon will not be
+                *     signaled in finite time (for example, by an associated
+                *     fence command issued previously, but not yet flushed to
+                *     the graphics pipeline), then ClientWaitSync may hang
+                *     forever. To help prevent this behavior, if
+                *     ClientWaitSync is called and all of the following are
+                *     true:
+                *
+                *     * the SYNC_FLUSH_COMMANDS_BIT bit is set in flags,
+                *     * sync is unsignaled when ClientWaitSync is called,
+                *     * and the calls to ClientWaitSync and FenceSync were
+                *       issued from the same context,
+                *
+                *     then the GL will behave as if the equivalent of Flush
+                *     were inserted immediately after the creation of sync."
+                *
+                * This means we need to flush for such fences even when we're
+                * not going to wait.
+                */
                rctx->gfx.flush(rctx, timeout ? 0 : RADEON_FLUSH_ASYNC, NULL);
                rfence->gfx_unflushed.ctx = NULL;