selftests/resctrl: Fix checking for < 0 for unsigned values

Dan reported following static checker warnings

tools/testing/selftests/resctrl/resctrl_val.c:545 measure_vals()
warn: 'bw_imc' unsigned <= 0

tools/testing/selftests/resctrl/resctrl_val.c:549 measure_vals()
warn: 'bw_resc_end' unsigned <= 0

These warnings are reported because
1. measure_vals() declares 'bw_imc' and 'bw_resc_end' as unsigned long
variables
2. Return value of get_mem_bw_imc() and get_mem_bw_resctrl() are assigned
to 'bw_imc' and 'bw_resc_end' respectively
3. The returned values are checked for <= 0 to see if the calls failed

Checking for < 0 for an unsigned value doesn't make any sense.

Fix this issue by changing the implementation of get_mem_bw_imc() and
get_mem_bw_resctrl() such that they now accept reference to a variable
and set the variable appropriately upon success and return 0, else return
< 0 on error.

Reported-by: Dan Carpenter <[e-mail]>
Tested-by: Babu Moger <[e-mail]>
Signed-off-by: Fenghua Yu <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/resctrl/resctrl_val.c

···
  * Memory B/W utilized by a process on a socket can be calculated using
  * iMC counters. Perf events are used to read these counters.
  *
- * Return: >= 0 on success. < 0 on failure.
+ * Return: = 0 on success. < 0 on failure.
  */
-static float get_mem_bw_imc(int cpu_no, char *bw_report)
+static int get_mem_bw_imc(int cpu_no, char *bw_report, float *bw_imc)
 {
 	float reads, writes, of_mul_read, of_mul_write;
 	int imc, j, ret;
···
 		close(imc_counters_config[imc][WRITE].fd);
 	}
 
-	if (strcmp(bw_report, "reads") == 0)
-		return reads;
+	if (strcmp(bw_report, "reads") == 0) {
+		*bw_imc = reads;
+		return 0;
+	}
 
-	if (strcmp(bw_report, "writes") == 0)
-		return writes;
+	if (strcmp(bw_report, "writes") == 0) {
+		*bw_imc = writes;
+		return 0;
+	}
 
-	return (reads + writes);
+	*bw_imc = reads + writes;
+	return 0;
 }
 
 void set_mbm_path(const char *ctrlgrp, const char *mongrp, int resource_id)
···
  * 1. If con_mon grp is given, then read from it
  * 2. If con_mon grp is not given, then read from root con_mon grp
  */
-static unsigned long get_mem_bw_resctrl(void)
+static int get_mem_bw_resctrl(unsigned long *mbm_total)
 {
-	unsigned long mbm_total = 0;
 	FILE *fp;
 
 	fp = fopen(mbm_total_path, "r");
···
 
 		return -1;
 	}
-	if (fscanf(fp, "%lu", &mbm_total) <= 0) {
+	if (fscanf(fp, "%lu", mbm_total) <= 0) {
 		perror("Could not get mbm local bytes");
 		fclose(fp);
 
···
 	}
 	fclose(fp);
 
-	return mbm_total;
+	return 0;
 }
 
 pid_t bm_pid, ppid;
···
 static int
 measure_vals(struct resctrl_val_param *param, unsigned long *bw_resc_start)
 {
-	unsigned long bw_imc, bw_resc, bw_resc_end;
+	unsigned long bw_resc, bw_resc_end;
+	float bw_imc;
 	int ret;
 
 	/*
···
 	 * Compare the two values to validate resctrl value.
 	 * It takes 1sec to measure the data.
 	 */
-	bw_imc = get_mem_bw_imc(param->cpu_no, param->bw_report);
-	if (bw_imc <= 0)
-		return bw_imc;
+	ret = get_mem_bw_imc(param->cpu_no, param->bw_report, &bw_imc);
+	if (ret < 0)
+		return ret;
 
-	bw_resc_end = get_mem_bw_resctrl();
-	if (bw_resc_end <= 0)
-		return bw_resc_end;
+	ret = get_mem_bw_resctrl(&bw_resc_end);
+	if (ret < 0)
+		return ret;
 
 	bw_resc = (bw_resc_end - *bw_resc_start) / MB;
 	ret = print_results_bw(param->filename, bm_pid, bw_imc, bw_resc);