drm/i915: drop one bit on the hw_id when using guc

We currently using GuC as a proxy to the hardware. When Guc is used in
such mode, it consumes the bit 20 of the hw_id to indicate that the
workload was submitted by proxy.

So far we probably haven't seen the issue because we need to allocate
1048576+ contexts to hit this issue. Still, we should avoid allocating
the hw_id on that bit and restriction to bits [0:19] (i.e 20bits
instead of 21).

v2: Leave the max hw_id computation in i915_gem_context.c (Michel)

v3: Be consistent on if/else usage (Chris)
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
BSpec: 1237
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180602112946.30803-2-lionel.g.landwerlin@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -1841,6 +1841,7 @@ struct drm_i915_private {
 		 */
 		struct ida hw_ida;
 #define MAX_CONTEXT_HW_ID (1<<21) /* exclusive */
+#define MAX_GUC_CONTEXT_HW_ID (1 << 20) /* exclusive */
 #define GEN11_MAX_CONTEXT_HW_ID (1<<11) /* exclusive */
 	} contexts;
 

drivers/gpu/drm/i915/i915_gem_context.c

@@ -208,10 +208,19 @@ static int assign_hw_id(struct drm_i915_private *dev_priv, unsigned *out)
 	int ret;
 	unsigned int max;
 
-	if (INTEL_GEN(dev_priv) >= 11)
+	if (INTEL_GEN(dev_priv) >= 11) {
 		max = GEN11_MAX_CONTEXT_HW_ID;
+	} else {
+		/*
+		 * When using GuC in proxy submission, GuC consumes the
+		 * highest bit in the context id to indicate proxy submission.
+		 */
+		if (USES_GUC_SUBMISSION(dev_priv))
+			max = MAX_GUC_CONTEXT_HW_ID;
 		else
 			max = MAX_CONTEXT_HW_ID;
+	}
+
 
 	ret = ida_simple_get(&dev_priv->contexts.hw_ida,
 			     0, max, GFP_KERNEL);

drivers/gpu/drm/i915/intel_lrc.c

@@ -200,7 +200,7 @@ static inline bool need_preempt(const struct intel_engine_cs *engine,
  *
  *      bits  0-11:    flags, GEN8_CTX_* (cached in ctx->desc_template)
  *      bits 12-31:    LRCA, GTT address of (the HWSP of) this context
- *      bits 32-52:    ctx ID, a globally unique tag
+ *      bits 32-52:    ctx ID, a globally unique tag (highest bit used by GuC)
  *      bits 53-54:    mbz, reserved for use by hardware
  *      bits 55-63:    group ID, currently unused and set to 0
  *