cmd: mem: Add bitflip memory test to alternate mtest

This additional bitflip memory test is inspired by the bitflip test
in memtester v4.3.0. It show some errors on some problematic GARDENA
MT7688 based boards. The other memory tests usually don't show any
errors here.
Signed-off-by: NStefan Roese <sr@denx.de>

cmd/mem.c

@@ -801,6 +801,59 @@ static ulong mem_test_alt(vu_long *buf, ulong start_addr, ulong end_addr,
 	return errs;
 }
 
+static int compare_regions(volatile unsigned long *bufa,
+			   volatile unsigned long *bufb, size_t count)
+{
+	volatile unsigned long  *p1 = bufa;
+	volatile unsigned long  *p2 = bufb;
+	int errs = 0;
+	size_t i;
+
+	for (i = 0; i < count; i++, p1++, p2++) {
+		if (*p1 != *p2) {
+			printf("FAILURE: 0x%08lx != 0x%08lx (delta=0x%08lx -> bit %ld) at offset 0x%08lx\n",
+			       (unsigned long)*p1, (unsigned long)*p2,
+			       *p1 ^ *p2, __ffs(*p1 ^ *p2),
+				(unsigned long)(i * sizeof(unsigned long)));
+			errs++;
+		}
+	}
+
+	return errs;
+}
+
+static ulong test_bitflip_comparison(volatile unsigned long *bufa,
+				     volatile unsigned long *bufb, size_t count)
+{
+	volatile unsigned long *p1 = bufa;
+	volatile unsigned long *p2 = bufb;
+	unsigned int j, k;
+	unsigned long q;
+	size_t i;
+	int max;
+	int errs = 0;
+
+	max = sizeof(unsigned long) * 8;
+	for (k = 0; k < max; k++) {
+		q = 0x00000001L << k;
+		for (j = 0; j < 8; j++) {
+			WATCHDOG_RESET();
+			q = ~q;
+			p1 = (volatile unsigned long *)bufa;
+			p2 = (volatile unsigned long *)bufb;
+			for (i = 0; i < count; i++)
+				*p1++ = *p2++ = (i % 2) == 0 ? q : ~q;
+
+			errs += compare_regions(bufa, bufb, count);
+		}
+
+		if (ctrlc())
+			return -1UL;
+	}
+
+	return errs;
+}
+
 static ulong mem_test_quick(vu_long *buf, ulong start_addr, ulong end_addr,
 			    vu_long pattern, int iteration)
 {
@@ -918,6 +971,13 @@ static int do_mem_mtest(cmd_tbl_t *cmdtp, int flag, int argc,
 		debug("\n");
 		if (IS_ENABLED(CONFIG_SYS_ALT_MEMTEST)) {
 			errs = mem_test_alt(buf, start, end, dummy);
+			if (errs == -1UL)
+				break;
+			count += errs;
+			errs = test_bitflip_comparison(buf,
+						       buf + (end - start) / 2,
+						       (end - start) /
+						       sizeof(unsigned long));
 		} else {
 			errs = mem_test_quick(buf, start, end, pattern,
 					      iteration);