linux/dim: Do nothing if no time delta between samples

Add return value for dim_calc_stats. This is an indication for the
caller if curr_stats was assigned by the function. Avoid using
curr_stats uninitialized over {rdma/net}_dim, when no time delta between
samples. Coverity reported this potential use of an uninitialized
variable.

Fixes: 4c4dbb4a7363 ("net/mlx5e: Move dynamic interrupt coalescing code to include/linux")
Fixes: cb3c7fd4f839 ("net/mlx5e: Support adaptive RX coalescing")
Signed-off-by: Roy Novich <[e-mail]>
Reviewed-by: Aya Levin <[e-mail]>
Reviewed-by: Saeed Mahameed <[e-mail]>
Signed-off-by: Tariq Toukan <[e-mail]>
Reviewed-by: Leon Romanovsky <[e-mail]>
Reviewed-by: Michal Kubiak <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/linux/dim.h

···
  *
  * Calculate the delta between two samples (in data rates).
  * Takes into consideration counter wrap-around.
+ * Returned boolean indicates whether curr_stats are reliable.
  */
-void dim_calc_stats(struct dim_sample *start, struct dim_sample *end,
+bool dim_calc_stats(struct dim_sample *start, struct dim_sample *end,
 		    struct dim_stats *curr_stats);
 
 /**

lib/dim/dim.c

···
 }
 EXPORT_SYMBOL(dim_park_tired);
 
-void dim_calc_stats(struct dim_sample *start, struct dim_sample *end,
+bool dim_calc_stats(struct dim_sample *start, struct dim_sample *end,
 		    struct dim_stats *curr_stats)
 {
 	/* u32 holds up to 71 minutes, should be enough */
···
 			     start->comp_ctr);
 
 	if (!delta_us)
-		return;
+		return false;
 
 	curr_stats->ppms = DIV_ROUND_UP(npkts * USEC_PER_MSEC, delta_us);
 	curr_stats->bpms = DIV_ROUND_UP(nbytes * USEC_PER_MSEC, delta_us);
···
 	else
 		curr_stats->cpe_ratio = 0;
 
+	return true;
 }
 EXPORT_SYMBOL(dim_calc_stats);

lib/dim/net_dim.c

···
 				  dim->start_sample.event_ctr);
 		if (nevents < DIM_NEVENTS)
 			break;
-		dim_calc_stats(&dim->start_sample, &end_sample, &curr_stats);
+		if (!dim_calc_stats(&dim->start_sample, &end_sample, &curr_stats))
+			break;
 		if (net_dim_decision(&curr_stats, dim)) {
 			dim->state = DIM_APPLY_NEW_PROFILE;
 			schedule_work(&dim->work);

lib/dim/rdma_dim.c

···
 		nevents = curr_sample->event_ctr - dim->start_sample.event_ctr;
 		if (nevents < DIM_NEVENTS)
 			break;
-		dim_calc_stats(&dim->start_sample, curr_sample, &curr_stats);
+		if (!dim_calc_stats(&dim->start_sample, curr_sample, &curr_stats))
+			break;
 		if (rdma_dim_decision(&curr_stats, dim)) {
 			dim->state = DIM_APPLY_NEW_PROFILE;
 			schedule_work(&dim->work);