drm/i915/guc: Avoid wasting memory on incorrect GuC pin bias

It would appear that the calculated GuC pin bias was larger than it should
be, as the GuC address space does NOT contain the "HW contexts RSVD" part
of the WOPCM. Thus, the GuC pin bias is simply the GuC WOPCM size.

v5:
Clarify the diagram to better represent the GuC address space.
Since we now don't use guc.base for the pin bias there's no need to
validate it. It also has already been verified in WOPCM init.

Bspec: 1180
Signed-off-by: Jakub Bartmiński <jakub.bartminski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180727141148.30874-1-jakub.bartminski@intel.com

drivers/gpu/drm/i915/intel_guc.c

@@ -585,50 +585,41 @@ int intel_guc_resume(struct intel_guc *guc)
  *
  * ::
  *
- *     +==============> +====================+ <== GUC_GGTT_TOP
- *     ^                |                    |
- *     |                |                    |
- *     |                |        DRAM        |
- *     |                |       Memory       |
- *     |                |                    |
- *    GuC               |                    |
- *  Address  +========> +====================+ <== WOPCM Top
- *   Space   ^          |   HW contexts RSVD |
- *     |     |          |        WOPCM       |
- *     |     |     +==> +--------------------+ <== GuC WOPCM Top
- *     |    GuC    ^    |                    |
- *     |    GGTT   |    |                    |
- *     |    Pin   GuC   |        GuC         |
- *     |    Bias WOPCM  |       WOPCM        |
- *     |     |    Size  |                    |
- *     |     |     |    |                    |
- *     v     v     v    |                    |
- *     +=====+=====+==> +====================+ <== GuC WOPCM Base
- *                      |   Non-GuC WOPCM    |
- *                      |   (HuC/Reserved)   |
- *                      +====================+ <== WOPCM Base
+ *     +===========> +====================+ <== FFFF_FFFF
+ *     ^             |      Reserved      |
+ *     |             +====================+ <== GUC_GGTT_TOP
+ *     |             |                    |
+ *     |             |        DRAM        |
+ *    GuC            |                    |
+ *  Address    +===> +====================+ <== GuC ggtt_pin_bias
+ *   Space     ^     |                    |
+ *     |       |     |                    |
+ *     |      GuC    |        GuC         |
+ *     |     WOPCM   |       WOPCM        |
+ *     |      Size   |                    |
+ *     |       |     |                    |
+ *     v       v     |                    |
+ *     +=======+===> +====================+ <== 0000_0000
  *
- * The lower part of GuC Address Space [0, ggtt_pin_bias) is mapped to WOPCM
+ * The lower part of GuC Address Space [0, ggtt_pin_bias) is mapped to GuC WOPCM
  * while upper part of GuC Address Space [ggtt_pin_bias, GUC_GGTT_TOP) is mapped
- * to DRAM. The value of the GuC ggtt_pin_bias is determined by WOPCM size and
- * actual GuC WOPCM size.
+ * to DRAM. The value of the GuC ggtt_pin_bias is the GuC WOPCM size.
  */
 
 /**
  * guc_init_ggtt_pin_bias() - Initialize the GuC ggtt_pin_bias value.
  * @guc: intel_guc structure.
  *
- * This function will calculate and initialize the ggtt_pin_bias value based on
- * overall WOPCM size and GuC WOPCM size.
+ * This function will calculate and initialize the ggtt_pin_bias value
+ * based on the GuC WOPCM size.
  */
 static void guc_init_ggtt_pin_bias(struct intel_guc *guc)
 {
 	struct drm_i915_private *i915 = guc_to_i915(guc);
 
 	GEM_BUG_ON(!i915->wopcm.size);
-	GEM_BUG_ON(i915->wopcm.size < i915->wopcm.guc.base);
 
-	guc->ggtt_pin_bias = i915->wopcm.size - i915->wopcm.guc.base;
+	guc->ggtt_pin_bias = i915->wopcm.guc.size;
 }
 
 /**