drm/i915/uapi: convert i915_query and friend to kernel doc

Add a note about the two-step process.

v2(Tvrtko):
  - Also document the other method of just passing in a buffer which is
    large enough, which avoids two ioctl calls. Can make sense for
    smaller query items.
v3: prefer kernel-doc references for structs and members
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419105741.27844-4-matthew.auld@intel.com

include/uapi/drm/i915_drm.h

@@ -2218,26 +2218,38 @@ struct drm_i915_perf_oa_config {
 	__u64 flex_regs_ptr;
 };
 
+/**
+ * struct drm_i915_query_item - An individual query for the kernel to process.
+ *
+ * The behaviour is determined by the @query_id. Note that exactly what
+ * @data_ptr is also depends on the specific @query_id.
+ */
 struct drm_i915_query_item {
+	/** @query_id: The id for this query */
 	__u64 query_id;
 #define DRM_I915_QUERY_TOPOLOGY_INFO    1
 #define DRM_I915_QUERY_ENGINE_INFO	2
 #define DRM_I915_QUERY_PERF_CONFIG      3
 /* Must be kept compact -- no holes and well documented */
 
-	/*
+	/**
+	 * @length:
+	 *
 	 * When set to zero by userspace, this is filled with the size of the
-	 * data to be written at the data_ptr pointer. The kernel sets this
+	 * data to be written at the @data_ptr pointer. The kernel sets this
 	 * value to a negative value to signal an error on a particular query
 	 * item.
 	 */
 	__s32 length;
 
-	/*
+	/**
+	 * @flags:
+	 *
 	 * When query_id == DRM_I915_QUERY_TOPOLOGY_INFO, must be 0.
 	 *
 	 * When query_id == DRM_I915_QUERY_PERF_CONFIG, must be one of the
-	 * following :
+	 * following:
+	 *
 	 *	- DRM_I915_QUERY_PERF_CONFIG_LIST
 	 *      - DRM_I915_QUERY_PERF_CONFIG_DATA_FOR_UUID
 	 *      - DRM_I915_QUERY_PERF_CONFIG_FOR_UUID
@@ -2247,24 +2259,53 @@ struct drm_i915_query_item {
 #define DRM_I915_QUERY_PERF_CONFIG_DATA_FOR_UUID 2
 #define DRM_I915_QUERY_PERF_CONFIG_DATA_FOR_ID   3
 
-	/*
-	 * Data will be written at the location pointed by data_ptr when the
-	 * value of length matches the length of the data to be written by the
+	/**
+	 * @data_ptr:
+	 *
+	 * Data will be written at the location pointed by @data_ptr when the
+	 * value of @length matches the length of the data to be written by the
 	 * kernel.
 	 */
 	__u64 data_ptr;
 };
 
+/**
+ * struct drm_i915_query - Supply an array of struct drm_i915_query_item for the
+ * kernel to fill out.
+ *
+ * Note that this is generally a two step process for each struct
+ * drm_i915_query_item in the array:
+ *
+ * 1. Call the DRM_IOCTL_I915_QUERY, giving it our array of struct
+ *    drm_i915_query_item, with &drm_i915_query_item.length set to zero. The
+ *    kernel will then fill in the size, in bytes, which tells userspace how
+ *    memory it needs to allocate for the blob(say for an array of properties).
+ *
+ * 2. Next we call DRM_IOCTL_I915_QUERY again, this time with the
+ *    &drm_i915_query_item.data_ptr equal to our newly allocated blob. Note that
+ *    the &drm_i915_query_item.length should still be the same as what the
+ *    kernel previously set. At this point the kernel can fill in the blob.
+ *
+ * Note that for some query items it can make sense for userspace to just pass
+ * in a buffer/blob equal to or larger than the required size. In this case only
+ * a single ioctl call is needed. For some smaller query items this can work
+ * quite well.
+ *
+ */
 struct drm_i915_query {
+	/** @num_items: The number of elements in the @items_ptr array */
 	__u32 num_items;
 
-	/*
-	 * Unused for now. Must be cleared to zero.
+	/**
+	 * @flags: Unused for now. Must be cleared to zero.
 	 */
 	__u32 flags;
 
-	/*
-	 * This points to an array of num_items drm_i915_query_item structures.
+	/**
+	 * @items_ptr:
+	 *
+	 * Pointer to an array of struct drm_i915_query_item. The number of
+	 * array elements is @num_items.
 	 */
 	__u64 items_ptr;
 };