drm/amd/display: prevent hang on link training fail

[Why]
When link training fails, the phy clock will be disabled. However, in
enable_streams, it is assumed that link training succeeded and the
mux selects the phy clock, causing a hang when a register write is made.

[How]
When enable_stream is hit, check if link training failed. If it did, fall
back to the ref clock to avoid a hang and keep the system in a recoverable
state.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Brendan Tam <Brendan.Tam@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

Author: Brendan Tam

drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c

@@ -3033,7 +3033,11 @@ void dcn20_enable_stream(struct pipe_ctx *pipe_ctx)
 		dccg->funcs->set_dpstreamclk(dccg, DTBCLK0, tg->inst, dp_hpo_inst);
 
 		phyd32clk = get_phyd32clk_src(link);
-		dccg->funcs->enable_symclk32_se(dccg, dp_hpo_inst, phyd32clk);
+		if (link->cur_link_settings.link_rate == LINK_RATE_UNKNOWN) {
+			dccg->funcs->disable_symclk32_se(dccg, dp_hpo_inst);
+		} else {
+			dccg->funcs->enable_symclk32_se(dccg, dp_hpo_inst, phyd32clk);
+		}
 	} else {
 		if (dccg->funcs->enable_symclk_se)
 			dccg->funcs->enable_symclk_se(dccg, stream_enc->stream_enc_inst,

drivers/gpu/drm/amd/display/dc/hwss/dcn401/dcn401_hwseq.c

@@ -936,8 +936,11 @@ void dcn401_enable_stream(struct pipe_ctx *pipe_ctx)
 	if (dc_is_dp_signal(pipe_ctx->stream->signal) || dc_is_virtual_signal(pipe_ctx->stream->signal)) {
 		if (dc->link_srv->dp_is_128b_132b_signal(pipe_ctx)) {
 			dccg->funcs->set_dpstreamclk(dccg, DPREFCLK, tg->inst, dp_hpo_inst);
-
-			dccg->funcs->enable_symclk32_se(dccg, dp_hpo_inst, phyd32clk);
+			if (link->cur_link_settings.link_rate == LINK_RATE_UNKNOWN) {
+				dccg->funcs->disable_symclk32_se(dccg, dp_hpo_inst);
+			} else {
+				dccg->funcs->enable_symclk32_se(dccg, dp_hpo_inst, phyd32clk);
+			}
 		} else {
 			dccg->funcs->enable_symclk_se(dccg, stream_enc->stream_enc_inst,
 					link_enc->transmitter - TRANSMITTER_UNIPHY_A);