[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, 28 insertions, 9 deletions

diff --git a/src/arch/x86/interface/pcbios/bios_timer.c b/src/arch/x86/interface/pcbios/bios_timer.c
index 3299c9aae..49e1d2265 100644
--- a/src/arch/x86/interface/pcbios/bios_timer.c
+++ b/src/arch/x86/interface/pcbios/bios_timer.c
@@ -32,6 +32,18 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/timer.h>
 #include <realmode.h>
 #include <bios.h>
+#include <ipxe/pit8254.h>
+
+/** Number of ticks per day
+ *
+ * This seems to be the normative value, as used by e.g. SeaBIOS to
+ * decide when to set the midnight rollover flag.
+ */
+#define BIOS_TICKS_PER_DAY 0x1800b0
+
+/** Number of ticks per BIOS tick */
+#define TICKS_PER_BIOS_TICK \
+	( ( TICKS_PER_SEC * 60 * 60 * 24 ) / BIOS_TICKS_PER_DAY )
 
 /**
  * Get current system time in ticks
@@ -43,7 +55,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  * of calling timeofday BIOS interrupt.
  */
 static unsigned long bios_currticks ( void ) {
-	static int days = 0;
+	static uint32_t offset;
 	uint32_t ticks;
 	uint8_t midnight;
 
@@ -53,18 +65,25 @@ static unsigned long bios_currticks ( void ) {
 			       "nop\n\t"
 			       "cli\n\t" );
 
-	get_real ( ticks, BDA_SEG, 0x006c );
-	get_real ( midnight, BDA_SEG, 0x0070 );
+	/* Read current BIOS time of day */
+	get_real ( ticks, BDA_SEG, BDA_TICKS );
+	get_real ( midnight, BDA_SEG, BDA_MIDNIGHT );
 
+	/* Handle midnight rollover */
 	if ( midnight ) {
 		midnight = 0;
-		put_real ( midnight, BDA_SEG, 0x0070 );
-		days += 0x1800b0;
+		put_real ( midnight, BDA_SEG, BDA_MIDNIGHT );
+		offset += BIOS_TICKS_PER_DAY;
 	}
+	ticks += offset;
 
-	return ( days + ticks );
+	/* Convert to timer ticks */
+	return ( ticks * TICKS_PER_BIOS_TICK );
 }
 
-PROVIDE_TIMER_INLINE ( pcbios, udelay );
-PROVIDE_TIMER ( pcbios, currticks, bios_currticks );
-PROVIDE_TIMER_INLINE ( pcbios, ticks_per_sec );
+/** BIOS timer */
+struct timer bios_timer __timer ( TIMER_NORMAL ) = {
+	.name = "bios",
+	.currticks = bios_currticks,
+	.udelay = pit8254_udelay,
+};