drm/i915/guc: refactor doorbell management code

This patch refactors the driver's handling and tracking of doorbells, in
preparation for a later one which will resolve a suspend-resume issue.

There are three resources to be managed:
1. Cachelines: a single line within the client-object's page 0
   is snooped by doorbell hardware for writes from the host.
2. Doorbell registers: each defines one cacheline to be snooped.
3. Bitmap: tracks which doorbell registers are in use.

The doorbell setup/teardown protocol starts with:
1. Pick a cacheline: select_doorbell_cacheline()
2. Find an available doorbell register: assign_doorbell()
(These values are passed to the GuC via the shared context
descriptor; this part of the sequence remains unchanged).

3. Update the bitmap to reflect registers-in-use
4. Prepare the cacheline for use by setting its status to ENABLED
5. Ask the GuC to program the doorbell to snoop the cacheline

and of course teardown is very similar:
6. Set the cacheline to DISABLED
7. Ask the GuC to reprogram the doorbell to stop snooping
8. Record that the doorbell is not in use.

Operations 6-8 (guc_disable_doorbell(), host2guc_release_doorbell(), and
release_doorbell()) were called in sequence from guc_client_free(), but
are now moved into the teardown phase of the common function.

Steps 4-5 (guc_init_doorbell() and host2guc_allocate_doorbell()) were
similarly done as sequential steps in guc_client_alloc(), but since it
turns out that we don't need to be able to do them separately they're
now collected into the setup phase of the common function.

The only new code (and new capability) is the block tagged
    /* Update the GuC's idea of the doorbell ID */
i.e. we can now *change* the doorbell register used by an existing
client, whereas previously it was set once for the entire lifetime
of the client. We will use this new feature in the next patch.

v2: Trivial independent fixes pushed ahead as separate patches.
    MUCH longer commit message :) [Tvrtko Ursulin]
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -174,31 +174,59 @@ static int host2guc_sample_forcewake(struct intel_guc *guc,
  * client object which contains the page being used for the doorbell
  */
 
-static void guc_init_doorbell(struct intel_guc *guc,
-			      struct i915_guc_client *client)
+static int guc_update_doorbell_id(struct intel_guc *guc,
+				  struct i915_guc_client *client,
+				  u16 new_id)
 {
+	struct sg_table *sg = guc->ctx_pool_obj->pages;
+	void *doorbell_bitmap = guc->doorbell_bitmap;
 	struct guc_doorbell_info *doorbell;
+	struct guc_context_desc desc;
+	size_t len;
 
 	doorbell = client->client_base + client->doorbell_offset;
 
-	doorbell->db_status = GUC_DOORBELL_ENABLED;
+	if (client->doorbell_id != GUC_INVALID_DOORBELL_ID &&
+	    test_bit(client->doorbell_id, doorbell_bitmap)) {
+		/* Deactivate the old doorbell */
+		doorbell->db_status = GUC_DOORBELL_DISABLED;
+		(void)host2guc_release_doorbell(guc, client);
+		__clear_bit(client->doorbell_id, doorbell_bitmap);
+	}
+
+	/* Update the GuC's idea of the doorbell ID */
+	len = sg_pcopy_to_buffer(sg->sgl, sg->nents, &desc, sizeof(desc),
+			     sizeof(desc) * client->ctx_index);
+	if (len != sizeof(desc))
+		return -EFAULT;
+	desc.db_id = new_id;
+	len = sg_pcopy_from_buffer(sg->sgl, sg->nents, &desc, sizeof(desc),
+			     sizeof(desc) * client->ctx_index);
+	if (len != sizeof(desc))
+		return -EFAULT;
+
+	client->doorbell_id = new_id;
+	if (new_id == GUC_INVALID_DOORBELL_ID)
+		return 0;
+
+	/* Activate the new doorbell */
+	__set_bit(new_id, doorbell_bitmap);
 	doorbell->cookie = 0;
+	doorbell->db_status = GUC_DOORBELL_ENABLED;
+	return host2guc_allocate_doorbell(guc, client);
+}
+
+static int guc_init_doorbell(struct intel_guc *guc,
+			      struct i915_guc_client *client,
+			      uint16_t db_id)
+{
+	return guc_update_doorbell_id(guc, client, db_id);
 }
 
 static void guc_disable_doorbell(struct intel_guc *guc,
 				 struct i915_guc_client *client)
 {
-	struct drm_i915_private *dev_priv = guc_to_i915(guc);
-	struct guc_doorbell_info *doorbell;
-	i915_reg_t drbreg = GEN8_DRBREGL(client->doorbell_id);
-	int value;
-
-	doorbell = client->client_base + client->doorbell_offset;
-
-	doorbell->db_status = GUC_DOORBELL_DISABLED;
-
-	value = I915_READ(drbreg);
-	WARN_ON((value & GEN8_DRB_VALID) != 0);
+	(void)guc_update_doorbell_id(guc, client, GUC_INVALID_DOORBELL_ID);
 
 	/* XXX: wait for any interrupts */
 	/* XXX: wait for workqueue to drain */
@@ -254,11 +282,6 @@ static uint16_t assign_doorbell(struct intel_guc *guc, uint32_t priority)
 	return id;
 }
 
-static void release_doorbell(struct intel_guc *guc, uint16_t id)
-{
-	__clear_bit(id, guc->doorbell_bitmap);
-}
-
 /*
  * Initialise the process descriptor shared with the GuC firmware.
  */
@@ -651,21 +674,11 @@ guc_client_free(struct drm_i915_private *dev_priv,
 	 */
 
 	if (client->client_base) {
-		uint16_t db_id = client->doorbell_id;
-
 		/*
-		 * If we got as far as setting up a doorbell, make sure
-		 * we shut it down before unmapping & deallocating the
-		 * memory. So first disable the doorbell, then tell the
-		 * GuC that we've finished with it, finally deallocate
-		 * it in our bitmap
+		 * If we got as far as setting up a doorbell, make sure we
+		 * shut it down before unmapping & deallocating the memory.
 		 */
-		if (db_id != GUC_INVALID_DOORBELL_ID) {
 		guc_disable_doorbell(guc, client);
-			if (test_bit(db_id, guc->doorbell_bitmap))
-				host2guc_release_doorbell(guc, client);
-			release_doorbell(guc, db_id);
-		}
 
 		kunmap(kmap_to_page(client->client_base));
 	}
@@ -700,6 +713,7 @@ guc_client_alloc(struct drm_i915_private *dev_priv,
 	struct i915_guc_client *client;
 	struct intel_guc *guc = &dev_priv->guc;
 	struct drm_i915_gem_object *obj;
+	uint16_t db_id;
 
 	client = kzalloc(sizeof(*client), GFP_KERNEL);
 	if (!client)
@@ -740,22 +754,20 @@ guc_client_alloc(struct drm_i915_private *dev_priv,
 	else
 		client->proc_desc_offset = (GUC_DB_SIZE / 2);
 
-	client->doorbell_id = assign_doorbell(guc, client->priority);
-	if (client->doorbell_id == GUC_INVALID_DOORBELL_ID)
+	db_id = assign_doorbell(guc, client->priority);
+	if (db_id == GUC_INVALID_DOORBELL_ID)
 		/* XXX: evict a doorbell instead */
 		goto err;
 
 	guc_init_proc_desc(guc, client);
 	guc_init_ctx_desc(guc, client);
-	guc_init_doorbell(guc, client);
-
-	/* XXX: Any cache flushes needed? General domain mgmt calls? */
-
-	if (host2guc_allocate_doorbell(guc, client))
+	if (guc_init_doorbell(guc, client, db_id))
 		goto err;
 
-	DRM_DEBUG_DRIVER("new priority %u client %p: ctx_index %u db_id %u\n",
-		priority, client, client->ctx_index, client->doorbell_id);
+	DRM_DEBUG_DRIVER("new priority %u client %p: ctx_index %u\n",
+		priority, client, client->ctx_index);
+	DRM_DEBUG_DRIVER("doorbell id %u, cacheline offset 0x%x\n",
+		client->doorbell_id, client->doorbell_offset);
 
 	return client;