drm/amd/display: Add guards for idle on reg read/write

[WHY]
If DCN is in idle then we should not be accessing DCN registers or
it can lead to hangs.

[HOW]
Log the error and return 0 or drop the write if it's in idle.

This is skipped in the exit sequence itself since the boolean flips
before it starts.

It also does not cover accesses from external clients outside of DM/DC
like firmware or the kernel mode driver.
Reviewed-by: Duncan Ma <duncan.ma@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dc_dmub_srv.c

@@ -1270,6 +1270,10 @@ static void dc_dmub_srv_notify_idle(const struct dc *dc, bool allow_idle)
 	/* NOTE: This does not use the "wake" interface since this is part of the wake path. */
 	/* We also do not perform a wait since DMCUB could enter idle after the notification. */
 	dm_execute_dmub_cmd(dc->ctx, &cmd, allow_idle ? DM_DMUB_WAIT_TYPE_NO_WAIT : DM_DMUB_WAIT_TYPE_WAIT);
+
+	/* Register access should stop at this point. */
+	if (allow_idle)
+		dc_dmub_srv->needs_idle_wake = true;
 }
 
 static void dc_dmub_srv_exit_low_power_state(const struct dc *dc)
@@ -1301,6 +1305,11 @@ static void dc_dmub_srv_exit_low_power_state(const struct dc *dc)
 			ips_fw->signals.bits.ips1_commit,
 			ips_fw->signals.bits.ips2_commit);
 
+		/* Note: register access has technically not resumed for DCN here, but we
+		 * need to be message PMFW through our standard register interface.
+		 */
+		dc_dmub_srv->needs_idle_wake = false;
+
 		if (prev_driver_signals.bits.allow_ips2) {
 			DC_LOG_IPS(
 				"wait IPS2 eval (ips1_commit=%d ips2_commit=%d)",

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

@@ -52,6 +52,7 @@ struct dc_dmub_srv {
 	void *dm;
 
 	bool idle_allowed;
+	bool needs_idle_wake;
 };
 
 void dc_dmub_srv_wait_idle(struct dc_dmub_srv *dc_dmub_srv);