Add support for storing which time base a USF file uses

Store which time base a USF file uses in its header. Old files will
default to 'accesses', which is the typical case. Future applications
may want to store cycles or instructions instead of accesses, enable
that by reserving two bits in the header to represent the time base.
Also, update the usfdump utility to display which time base is used.

include/uart/usf.h

@@ -71,6 +71,19 @@ typedef uint16_t usf_compression_t;
 /** File contains instruction samples */
 #define USF_FLAG_INSTRUCTIONS (1 << 3)
 
+/**
+ * Reserve two bits for the time base, note that these may not be
+ * contiguous in future releases.
+ */
+#define USF_FLAG_TIME_MASK ((1 << 4) | (1 << 5))
+
+/** Time in accesses */
+#define USF_FLAG_TIME_ACCESSES (0 << 4)
+/** Time in instructions */
+#define USF_FLAG_TIME_INSTRUCTIONS (1 << 4)
+/** Time in cycles */
+#define USF_FLAG_TIME_CYCLES (3 << 4)
+
 /* @{ */
 /**
  * Always set the native endian flag when creating a file. If the

tools/usfdump.c

@@ -150,6 +150,22 @@ print_header(const usf_header_t *h)
         if (h->flags & flag_names[i].flag)
             printf("\t\t%s\n", flag_names[i].name);
     }
+
+    printf("\tTime base: ");
+    switch (h->flags & USF_FLAG_TIME_MASK) {
+    case USF_FLAG_TIME_ACCESSES:
+        printf("accesses\n");
+        break;
+    case USF_FLAG_TIME_INSTRUCTIONS:
+        printf("instructions\n");
+        break;
+    case USF_FLAG_TIME_CYCLES:
+        printf("cycles\n");
+        break;
+    default:
+        printf("unknown (0x%x)\n", h->flags & USF_FLAG_TIME_MASK);
+        break;
+    }
 }
 
 static int