The CPU vendor IDs are required to be unique bits because they're used
for vendor_specific bitmask in the struct resctrl_test.
Consider for example their usage in test_vendor_specific_check():
	return get_vendor() & test->vendor_specific

However, the definitions of CPU vendor IDs in file resctrl.h is quite
subtle as a bitmask value:
  #define ARCH_INTEL     1
  #define ARCH_AMD       2

A clearer and more maintainable approach is to define these CPU vendor
IDs using BIT(). This ensures each vendor corresponds to a distinct bit
and makes it obvious when adding new vendor IDs.

Accordingly, update the return types of detect_vendor() and get_vendor()
from 'int' to 'unsigned int' to align with their usage as bitmask values
and to prevent potentially risky type conversions.

Furthermore, introduce a bool flag 'initialized' to simplify the
get_vendor() -> detect_vendor() logic. This ensures the vendor ID is
detected only once and resolves the ambiguity of using the same variable
'vendor' both as a value and as a state.

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Suggested-by: Fenghua Yu <fenghuay@nvidia.com>
Signed-off-by: Xiaochen Shen <shenxiaochen@open-hieco.net>
---
 tools/testing/selftests/resctrl/resctrl.h     |  7 ++---
 .../testing/selftests/resctrl/resctrl_tests.c | 26 +++++++++++++------
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
index 3c51bdac2dfa..4f9c7d04c98d 100644
--- a/tools/testing/selftests/resctrl/resctrl.h
+++ b/tools/testing/selftests/resctrl/resctrl.h
@@ -23,6 +23,7 @@
 #include <asm/unistd.h>
 #include <linux/perf_event.h>
 #include <linux/compiler.h>
+#include <linux/bits.h>
 #include "kselftest.h"
 
 #define MB			(1024 * 1024)
@@ -36,8 +37,8 @@
  * Define as bits because they're used for vendor_specific bitmask in
  * the struct resctrl_test.
  */
-#define ARCH_INTEL     1
-#define ARCH_AMD       2
+#define ARCH_INTEL	BIT(0)
+#define ARCH_AMD	BIT(1)
 
 #define END_OF_TESTS	1
 
@@ -163,7 +164,7 @@ extern int snc_unreliable;
 extern char llc_occup_path[1024];
 
 int snc_nodes_per_l3_cache(void);
-int get_vendor(void);
+unsigned int get_vendor(void);
 bool check_resctrlfs_support(void);
 int filter_dmesg(void);
 int get_domain_id(const char *resource, int cpu_no, int *domain_id);
diff --git a/tools/testing/selftests/resctrl/resctrl_tests.c b/tools/testing/selftests/resctrl/resctrl_tests.c
index 5154ffd821c4..980ecf2bcf10 100644
--- a/tools/testing/selftests/resctrl/resctrl_tests.c
+++ b/tools/testing/selftests/resctrl/resctrl_tests.c
@@ -23,16 +23,24 @@ static struct resctrl_test *resctrl_tests[] = {
 	&l2_noncont_cat_test,
 };
 
-static int detect_vendor(void)
+static unsigned int detect_vendor(void)
 {
-	FILE *inf = fopen("/proc/cpuinfo", "r");
-	int vendor_id = 0;
+	FILE *inf;
+	static unsigned int vendor_id;
 	char *s = NULL;
 	char *res;
+	static bool initialized;
 
-	if (!inf)
+	if (initialized)
 		return vendor_id;
 
+	inf = fopen("/proc/cpuinfo", "r");
+	if (!inf) {
+		vendor_id = 0;
+		initialized = true;
+		return vendor_id;
+	}
+
 	res = fgrep(inf, "vendor_id");
 
 	if (res)
@@ -45,15 +53,17 @@ static int detect_vendor(void)
 
 	fclose(inf);
 	free(res);
+
+	initialized = true;
 	return vendor_id;
 }
 
-int get_vendor(void)
+unsigned int get_vendor(void)
 {
-	static int vendor = -1;
+	unsigned int vendor;
+
+	vendor = detect_vendor();
 
-	if (vendor == -1)
-		vendor = detect_vendor();
 	if (vendor == 0)
 		ksft_print_msg("Can not get vendor info...\n");
 
-- 
2.47.3