drm/i915: Reject excessive SAGV block time

If the mailbox returns an exceesively large SAGV block time let's just
reject it. This avoids having to worry about overflows when we add the
SAGV block time to the wm0 latency.

We shall put the limit arbitrarily at U16_MAX. >65msec latency
doesn't really make sense to me in any case.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220309164948.10671-5-ville.syrjala@linux.intel.comReviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -3716,6 +3716,12 @@ static void intel_sagv_init(struct drm_i915_private *i915)
 	drm_dbg_kms(&i915->drm, "SAGV supported: %s, original SAGV block time: %u us\n",
 		    str_yes_no(intel_has_sagv(i915)), i915->sagv_block_time_us);
 
+	/* avoid overflow when adding with wm0 latency/etc. */
+	if (drm_WARN(&i915->drm, i915->sagv_block_time_us > U16_MAX,
+		     "Excessive SAGV block time %u, ignoring\n",
+		     i915->sagv_block_time_us))
+		i915->sagv_block_time_us = 0;
+
 	if (!intel_has_sagv(i915))
 		i915->sagv_block_time_us = 0;
 }