drm/i915/gsc: Disable GSC engine and power well if FW is not selected

The GSC CS is only used for communicating with the GSC FW, so no need to
initialize it if we're not going to use the FW. If we're not using
neither the engine nor the microcontoller, then we can also disable the
power well.

IMPORTANT: lack of GSC FW breaks media C6 due to opposing requirements
between CS setup and forcewake idleness. See in-code comment for detail.
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: John C Harrison <John.C.Harrison@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208200521.2928378-6-daniele.ceraolospurio@intel.com

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -892,6 +892,24 @@ static intel_engine_mask_t init_engine_mask(struct intel_gt *gt)
 	engine_mask_apply_compute_fuses(gt);
 	engine_mask_apply_copy_fuses(gt);
 
+	/*
+	 * The only use of the GSC CS is to load and communicate with the GSC
+	 * FW, so we have no use for it if we don't have the FW.
+	 *
+	 * IMPORTANT: in cases where we don't have the GSC FW, we have a
+	 * catch-22 situation that breaks media C6 due to 2 requirements:
+	 * 1) once turned on, the GSC power well will not go to sleep unless the
+	 *    GSC FW is loaded.
+	 * 2) to enable idling (which is required for media C6) we need to
+	 *    initialize the IDLE_MSG register for the GSC CS and do at least 1
+	 *    submission, which will wake up the GSC power well.
+	 */
+	if (__HAS_ENGINE(info->engine_mask, GSC0) && !intel_uc_wants_gsc_uc(&gt->uc)) {
+		drm_notice(&gt->i915->drm,
+			   "No GSC FW selected, disabling GSC CS and media C6\n");
+		info->engine_mask &= ~BIT(GSC0);
+	}
+
 	return info->engine_mask;
 }
 

drivers/gpu/drm/i915/intel_uncore.c

@@ -2700,6 +2700,9 @@ void intel_uncore_prune_engine_fw_domains(struct intel_uncore *uncore,
 		if (fw_domains & BIT(domain_id))
 			fw_domain_fini(uncore, domain_id);
 	}
+
+	if ((fw_domains & BIT(FW_DOMAIN_ID_GSC)) && !HAS_ENGINE(gt, GSC0))
+		fw_domain_fini(uncore, FW_DOMAIN_ID_GSC);
 }
 
 /*