Timing self test: increased duration

Increase the duration of the self test, otherwise it tends to fail on
a busy machine even with the recently upped tolerance. But run the
loop only once, it's enough for a simple smoke test.

ChangeLog

@@ -6,7 +6,8 @@ Features
    * Allow comments in test data files.
    * The selftest program can execute a subset of the tests based on command
      line arguments.
-   * New unit tests for timing.
+   * New unit tests for timing. Improve the self-test to be more robust
+     when run on a heavily-loaded machine.
 
 Bugfix
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.

library/timing.c

@@ -444,28 +444,22 @@ int mbedtls_timing_self_test( int verbose )
     if( verbose != 0 )
         mbedtls_printf( "  TIMING test #2 (set/get_delay        ): " );
 
-    for( a = 200; a <= 400; a += 200 )
     {
-        for( b = 200; b <= 400; b += 200 )
-        {
-            mbedtls_timing_set_delay( &ctx, a, a + b );
-
-            busy_msleep( a - a / 4 );
-            if( mbedtls_timing_get_delay( &ctx ) != 0 )
-                FAIL;
+        a = 800;
+        b = 400;
+        mbedtls_timing_set_delay( &ctx, a, a + b );          /* T = 0 */
 
-            busy_msleep( a / 2 );
-            if( mbedtls_timing_get_delay( &ctx ) != 1 )
-                FAIL;
+        busy_msleep( a - a / 4 );                      /* T = a - a/4 */
+        if( mbedtls_timing_get_delay( &ctx ) != 0 )
+            FAIL;
 
-            busy_msleep( b - a / 4 - b / 4 );
-            if( mbedtls_timing_get_delay( &ctx ) != 1 )
-                FAIL;
+        busy_msleep( a / 4 + b / 4 );                  /* T = a + b/4 */
+        if( mbedtls_timing_get_delay( &ctx ) != 1 )
+            FAIL;
 
-            busy_msleep( b / 2 );
-            if( mbedtls_timing_get_delay( &ctx ) != 2 )
-                FAIL;
-        }
+        busy_msleep( b );                          /* T = a + b + b/4 */
+        if( mbedtls_timing_get_delay( &ctx ) != 2 )
+            FAIL;
     }
 
     mbedtls_timing_set_delay( &ctx, 0, 0 );