[time] Allow timer to be selected at runtime

Allow the active timer (providing udelay() and currticks()) to be
selected at runtime based on probing during the INIT_EARLY stage of
initialisation.

TICKS_PER_SEC is now a fixed compile-time constant for all builds, and
is independent of the underlying clock tick rate.  We choose the value
1024 to allow multiplications and divisions on seconds to be converted
to bit shifts.

TICKS_PER_MS is defined as 1, allowing multiplications and divisions
on milliseconds to be omitted entirely.  The 2% inaccuracy in this
definition is negligible when using the standard BIOS timer (running
at around 18.2Hz).

TIMER_RDTSC now checks for a constant TSC before claiming to be a
usable timer.  (This timer can be tested in KVM via the command-line
option "-cpu host,+invtsc".)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 48 insertions, 0 deletions

diff --git a/src/config/config_timer.c b/src/config/config_timer.c
new file mode 100644
index 00000000..a462970c
--- /dev/null
+++ b/src/config/config_timer.c
@@ -0,0 +1,48 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <config/timer.h>
+
+/** @file
+ *
+ * Timer configuration options
+ *
+ */
+
+PROVIDE_REQUIRING_SYMBOL();
+
+/*
+ * Drag in timers
+ */
+#ifdef TIMER_PCBIOS
+REQUIRE_OBJECT ( bios_timer );
+#endif
+#ifdef TIMER_RDTSC
+REQUIRE_OBJECT ( rdtsc_timer );
+#endif
+#ifdef TIMER_EFI
+REQUIRE_OBJECT ( efi_timer );
+#endif
+#ifdef TIMER_LINUX
+REQUIRE_OBJECT ( linux_timer );
+#endif