drm/i915: Optimization to reduce the sampling time of GuC log buffer

GuC firmware sends an interrupt to flush the log buffer when it becomes
half full, so Driver doesn't really need to sample the complete buffer
and can just copy only the newly written data by GuC into the local
buffer, i.e. as per the read & write pointer values.
Moreover the flush interrupt would generally come for one type of log
buffer, when it becomes half full, so at that time the other 2 types of
log buffer would comparatively have much lesser unread data in them.
In case of overflow reported by GuC, Driver do need to copy the entire
buffer as the whole buffer would contain the unread data.

v2: Rebase.

v3: Fix the blooper of doing the copy twice. (Tvrtko)

v4: Add curlies for 'else' case also, matching the 'if'. (Tvrtko)
Signed-off-by: Akash Goel <akash.goel@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

@@ -1057,11 +1057,12 @@ static unsigned int guc_get_log_buffer_size(enum guc_log_buffer_type type)
 
 static void guc_read_update_log_buffer(struct intel_guc *guc)
 {
+	unsigned int buffer_size, read_offset, write_offset, bytes_to_copy, full_cnt;
 	struct guc_log_buffer_state *log_buf_state, *log_buf_snapshot_state;
 	struct guc_log_buffer_state log_buf_state_local;
-	unsigned int buffer_size, write_offset, full_cnt;
 	enum guc_log_buffer_type type;
 	void *src_data, *dst_data;
+	bool new_overflow;
 
 	if (WARN_ON(!guc->log.buf_addr))
 		return;
@@ -1084,12 +1085,13 @@ static void guc_read_update_log_buffer(struct intel_guc *guc)
 		memcpy(&log_buf_state_local, log_buf_state,
 		       sizeof(struct guc_log_buffer_state));
 		buffer_size = guc_get_log_buffer_size(type);
+		read_offset = log_buf_state_local.read_ptr;
 		write_offset = log_buf_state_local.sampled_write_ptr;
 		full_cnt = log_buf_state_local.buffer_full_cnt;
 
 		/* Bookkeeping stuff */
 		guc->log.flush_count[type] += log_buf_state_local.flush_to_file;
-		guc_check_log_buf_overflow(guc, type, full_cnt);
+		new_overflow = guc_check_log_buf_overflow(guc, type, full_cnt);
 
 		/* Update the state of shared log buffer */
 		log_buf_state->read_ptr = write_offset;
@@ -1112,7 +1114,27 @@ static void guc_read_update_log_buffer(struct intel_guc *guc)
 		log_buf_snapshot_state++;
 
 		/* Now copy the actual logs. */
-		memcpy(dst_data, src_data, buffer_size);
+		if (unlikely(new_overflow)) {
+			/* copy the whole buffer in case of overflow */
+			read_offset = 0;
+			write_offset = buffer_size;
+		} else if (unlikely((read_offset > buffer_size) ||
+				    (write_offset > buffer_size))) {
+			DRM_ERROR("invalid log buffer state\n");
+			/* copy whole buffer as offsets are unreliable */
+			read_offset = 0;
+			write_offset = buffer_size;
+		}
+
+		/* Just copy the newly written data */
+		if (read_offset > write_offset) {
+			memcpy(dst_data, src_data, write_offset);
+			bytes_to_copy = buffer_size - read_offset;
+		} else {
+			bytes_to_copy = write_offset - read_offset;
+		}
+		memcpy(dst_data + read_offset,
+		       src_data + read_offset, bytes_to_copy);
 
 		src_data += buffer_size;
 		dst_data += buffer_size;