drm/i915/gt: Fix CCS id's calculation for CCS mode setting

The whole point of the previous fixes has been to change the CCS
hardware configuration to generate only one stream available to
the compute users. We did this by changing the info.engine_mask
that is set during device probe, reset during the detection of
the fused engines, and finally reset again when choosing the CCS
mode.

We can't use the engine_mask variable anymore, as with the
current configuration, it imposes only one CCS no matter what the
hardware configuration is.

Before changing the engine_mask for the third time, save it and
use it for calculating the CCS mode.

After the previous changes, the user reported a performance drop
to around 1/4. We have tested that the compute operations, with
the current patch, have improved by the same factor.

Fixes: 6db31251 ("drm/i915/gt: Enable only one CCS for compute workload")
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Chris Wilson <chris.p.wilson@linux.intel.com>
Cc: Gnattu OC <gnattuoc@me.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Tested-by: Jian Ye <jian.ye@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Tested-by: Gnattu OC <gnattuoc@me.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517090616.242529-1-andi.shyti@linux.intel.com

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

@@ -885,6 +885,12 @@ static intel_engine_mask_t init_engine_mask(struct intel_gt *gt)
 	if (IS_DG2(gt->i915)) {
 		u8 first_ccs = __ffs(CCS_MASK(gt));
 
+		/*
+		 * Store the number of active cslices before
+		 * changing the CCS engine configuration
+		 */
+		gt->ccs.cslices = CCS_MASK(gt);
+
 		/* Mask off all the CCS engine */
 		info->engine_mask &= ~GENMASK(CCS3, CCS0);
 		/* Put back in the first CCS engine */

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

@@ -19,7 +19,7 @@ unsigned int intel_gt_apply_ccs_mode(struct intel_gt *gt)
 
 	/* Build the value for the fixed CCS load balancing */
 	for (cslice = 0; cslice < I915_MAX_CCS; cslice++) {
-		if (CCS_MASK(gt) & BIT(cslice))
+		if (gt->ccs.cslices & BIT(cslice))
 			/*
 			 * If available, assign the cslice
 			 * to the first available engine...

drivers/gpu/drm/i915/gt/intel_gt_types.h

@@ -207,6 +207,14 @@ struct intel_gt {
 					    [MAX_ENGINE_INSTANCE + 1];
 	enum intel_submission_method submission_method;
 
+	struct {
+		/*
+		 * Mask of the non fused CCS slices
+		 * to be used for the load balancing
+		 */
+		intel_engine_mask_t cslices;
+	} ccs;
+
 	/*
 	 * Default address space (either GGTT or ppGTT depending on arch).
 	 *