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: 4c4dbb4a73 ("net/mlx5e: Move dynamic interrupt coalescing code to include/linux")
Fixes: cb3c7fd4f8 ("net/mlx5e: Support adaptive RX coalescing")
Signed-off-by: Roy Novich <royno@nvidia.com>
Reviewed-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Link: https://lore.kernel.org/r/20230507135743.138993-1-tariqt@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/dim.h

@@ -236,8 +236,9 @@ void dim_park_tired(struct dim *dim);
  *
  * 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

@@ -54,7 +54,7 @@ void dim_park_tired(struct dim *dim)
 }
 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 */
@@ -66,7 +66,7 @@ void dim_calc_stats(struct dim_sample *start, struct dim_sample *end,
 			     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);
@@ -79,5 +79,6 @@ void dim_calc_stats(struct dim_sample *start, struct dim_sample *end,
 	else
 		curr_stats->cpe_ratio = 0;
 
+	return true;
 }
 EXPORT_SYMBOL(dim_calc_stats);

lib/dim/net_dim.c

@@ -227,7 +227,8 @@ void net_dim(struct dim *dim, struct dim_sample end_sample)
 				  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

@@ -88,7 +88,8 @@ void rdma_dim(struct dim *dim, u64 completions)
 		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);