drm/amd/display: Add HBlank reduction DPCD write to DPMS sequence

[Why]
Certain small HBlank timings may not have a large enough HBlank to
support audio when low bpp DSC is enabled. HBlank expansion by the
source can solve this problem, but requires the branch/sink to support
HBlank reduction.

[How]
Update DPMS sequence to call DM to perform DPCD write to enable HBlank
reduction on the branch/sink. Add stub in dm_helpers to be implemented
later.

Reviewed-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c

@@ -885,6 +885,12 @@ bool dm_helpers_dp_write_dsc_enable(
 	return ret;
 }
 
+bool dm_helpers_dp_write_hblank_reduction(struct dc_context *ctx, const struct dc_stream_state *stream)
+{
+	// TODO
+	return false;
+}
+
 bool dm_helpers_is_dp_sink_present(struct dc_link *link)
 {
 	bool dp_sink_present;

drivers/gpu/drm/amd/display/dc/dm_helpers.h

@@ -158,6 +158,11 @@ bool dm_helpers_dp_write_dsc_enable(
 		const struct dc_stream_state *stream,
 		bool enable
 );
+
+bool dm_helpers_dp_write_hblank_reduction(
+		struct dc_context *ctx,
+		const struct dc_stream_state *stream);
+
 bool dm_helpers_is_dp_sink_present(
 		struct dc_link *link);
 

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

@@ -772,6 +772,20 @@ static bool dp_set_dsc_on_rx(struct pipe_ctx *pipe_ctx, bool enable)
 	return result;
 }
 
+static bool dp_set_hblank_reduction_on_rx(struct pipe_ctx *pipe_ctx)
+{
+	struct dc *dc = pipe_ctx->stream->ctx->dc;
+	struct dc_stream_state *stream = pipe_ctx->stream;
+	bool result = false;
+
+	if (dc_is_virtual_signal(stream->signal))
+		result = true;
+	else
+		result = dm_helpers_dp_write_hblank_reduction(dc->ctx, stream);
+	return result;
+}
+
+
 /* The stream with these settings can be sent (unblanked) only after DSC was enabled on RX first,
  * i.e. after dp_enable_dsc_on_rx() had been called
  */
@@ -2599,6 +2613,9 @@ void link_set_dpms_on(
 		}
 	}
 
+	if (dc_is_dp_signal(pipe_ctx->stream->signal))
+		dp_set_hblank_reduction_on_rx(pipe_ctx);
+
 	if (pipe_ctx->stream->link->ep_type == DISPLAY_ENDPOINT_USB4_DPIA)
 		allocate_usb4_bandwidth(pipe_ctx->stream);