drm/amd/display: Fix NULL sink dereference in USB4 bandwidth functions

allocate_usb4_bandwidth_for_stream() and allocate_usb4_bandwidth() both
use ASSERT(stream->sink) before dereferencing stream->sink->link. ASSERT
is compiled out in production builds, so a NULL stream->sink causes a
NULL pointer dereference.

link_set_dpms_off() already guards two earlier accesses to stream->sink
with explicit NULL checks, confirming that stream->sink can legitimately
be NULL. The subsequent call to deallocate_usb4_bandwidth() had no
equivalent guard.

Replace the ASSERT() calls with proper NULL checks that return false
early, and add a matching stream->sink guard at the call site in
link_set_dpms_off() consistent with the surrounding pattern.

v2: Keep the existing ASSERT() calls for diagnostics (to report
    unexpected NULL sink), and add explicit NULL checks that return false
    before dereferencing stream->sink. Also add a matching stream->sink
    guard at the deallocation call site in link_set_dpms_off(). (Alex)

Fixes: 8fa813b7fd ("drm/amd/display: Remove sink usage from DPMS")
Reported-by: Dan Carpenter <[email protected]>
Cc: Nicholas Kazlauskas <[email protected]>
Cc: Dominik Kaszewski <[email protected]>
Cc: George Zhang <[email protected]>
Cc: Aurabindo Pillai <[email protected]>
Cc: Roman Li <[email protected]>
Cc: Ivan Lipski <[email protected]>
Cc: Dan Wheeler <[email protected]>
Cc: Alex Hung <[email protected]>
Cc: Tom Chung <[email protected]>
Signed-off-by: Srinivasan Shanmugam <[email protected]>
Tested-by: Dan Wheeler <[email protected]>
Reviewed-by: Roman Li <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>
(cherry picked from commit 98029a0ce61feb52c563f65349dd9b25f672e355)
Cc: [email protected]

drivers/gpu/drm/amd/display/dc/link/link_dpms.c

@@ -2306,9 +2306,14 @@ static bool allocate_usb4_bandwidth_for_stream(struct dc_stream_state *stream, i
 {
 	ASSERT(stream->sink);
 
-	struct dc_link *link = stream->sink->link;
+	struct dc_link *link;
 	int req_bw = stream_bw;
 
+	if (!stream->sink)
+		return false;
+
+	link = stream->sink->link;
+
 	DC_LOGGER_INIT(link->ctx->logger);
 
 	if (!link->dpia_bw_alloc_config.bw_alloc_enabled)
@@ -2355,9 +2360,13 @@ static bool allocate_usb4_bandwidth(struct dc_stream_state *stream)
 	ASSERT(stream->sink);
 
 	bool ret;
+	int bw;
 
-	int bw = dc_bandwidth_in_kbps_from_timing(&stream->timing,
-			dc_link_get_highest_encoding_format(stream->sink->link));
+	if (!stream->sink)
+		return false;
+
+	bw = dc_bandwidth_in_kbps_from_timing(&stream->timing,
+					      dc_link_get_highest_encoding_format(stream->sink->link));
 
 	ret = allocate_usb4_bandwidth_for_stream(stream, bw);
 
@@ -2419,7 +2428,8 @@ enum dc_status link_set_dpms_off(struct pipe_ctx *pipe_ctx)
 	update_psp_stream_config(pipe_ctx, true);
 	dc->hwss.blank_stream(pipe_ctx);
 
-	if (pipe_ctx->link_config.dp_tunnel_settings.should_use_dp_bw_allocation)
+	if (pipe_ctx->link_config.dp_tunnel_settings.should_use_dp_bw_allocation &&
+	    stream->sink)
 		deallocate_usb4_bandwidth(stream);
 
 	if (stream->signal == SIGNAL_TYPE_DISPLAY_PORT_MST)