drm/i915/guc: Add CT size delay helper

As of now, there is no mechanism for tracking a given request's
progress through the queue.  Instead, add a helper that returns
an estimated maximum time the queue should take to drain if
completely full.

Suggested-by: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231017180806.3054290-3-jonathan.cavitt@intel.com

Author: Jonathan-Cavitt

drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c

@@ -103,6 +103,33 @@ enum { CTB_SEND = 0, CTB_RECV = 1 };
 
 enum { CTB_OWNER_HOST = 0 };
 
+/*
+ * Some H2G commands involve a synchronous response that the driver needs
+ * to wait for. In such cases, a timeout is required to prevent the driver
+ * from waiting forever in the case of an error (either no error response
+ * is defined in the protocol or something has died and requires a reset).
+ * The specific command may be defined as having a time bound response but
+ * the CT is a queue and that time guarantee only starts from the point
+ * when the command reaches the head of the queue and is processed by GuC.
+ *
+ * Ideally there would be a helper to report the progress of a given
+ * command through the CT. However, that would require a significant
+ * amount of work in the CT layer. In the meantime, provide a reasonable
+ * estimation of the worst case latency it should take for the entire
+ * queue to drain. And therefore, how long a caller should wait before
+ * giving up on their request. The current estimate is based on empirical
+ * measurement of a test that fills the buffer with context creation and
+ * destruction requests as they seem to be the slowest operation.
+ */
+long intel_guc_ct_max_queue_time_jiffies(void)
+{
+	/*
+	 * A 4KB buffer full of context destroy commands takes a little
+	 * over a second to process so bump that to 2s to be super safe.
+	 */
+	return (CTB_H2G_BUFFER_SIZE * HZ) / SZ_2K;
+}
+
 static void ct_receive_tasklet_func(struct tasklet_struct *t);
 static void ct_incoming_request_worker_func(struct work_struct *w);
 

drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h

@@ -104,6 +104,8 @@ struct intel_guc_ct {
 #endif
 };
 
+long intel_guc_ct_max_queue_time_jiffies(void);
+
 void intel_guc_ct_init_early(struct intel_guc_ct *ct);
 int intel_guc_ct_init(struct intel_guc_ct *ct);
 void intel_guc_ct_fini(struct intel_guc_ct *ct);