maint: make it clear when to add speeds to the speedgen list

Add a comment making it clear that it is perfectly safe to add
additional speeds to the canned list in the speedgen script, and under
what conditions it is appropriate to do so.

Signed-off-by: "H. Peter Anvin" (Intel) <hpa@zytor.com>

diff --git a/src/speedgen b/src/speedgen
index f1647d9f0acf9e8b9eef144a949386a119754ad2..56998d6a72c72d6113ac737704991acf89a249fa 100755 (executable)
--- a/src/speedgen
+++ b/src/speedgen
@@ -13,7 +13,19 @@ s='[[:space:]]'                      # For brevity's sake
 trap "rm -f '$tmp'" EXIT
 trap "rm -f '$tmp' '$out'" ERR HUP INT QUIT TERM
 
-# Fallback list of speeds that are always tested for
+# Fallback list of speeds that are always tested for.
+#
+# It is safe to add additional speeds to this list; speeds not defined
+# on any particular platform are simply ignored.  It is specifically
+# encouraged to add additional speeds here if some are found to be
+# defined on a platform for which:
+#
+# - the C compiler(s) normally used does not support the -dM option
+#   (gcc, clang, and derived compilers do support this option), AND
+# - there are baud rate constants (B<num>) defined in <termios.h>
+#   that are not identity-mapped (i.e., B<num> != <num> for at least
+#   some <num>).
+#
 defspeeds="0 50 75 110 134 150 200 300 600 1200 1800 2400 4800 7200 9600 \
 14400 19200 28800 33600 38400 57600 76800 115200 153600 230400 307200 \
 460800 500000 576000 614400 921600 1000000 1152000 1500000 \