[SeaBIOS] [PATCH] Increase precision of usec timer calculation

[Kevin O'Connor]

On Wed, Nov 01, 2017 at 10:40:57AM -0400, Stefan Berger wrote:
> When timer_calc_usec() is used with large timeout falues, such as 60s,
> the function lacks precision and produces different results than when
> using timer_calc(time / 1000) for the same timeout. This patch fixes
> the precision issue by falling back to timer_calc(time / 1000) for
> usec values over one second.

Makes sense.  The code would need to use DIV_ROUND_UP though (it's
okay to go slightly longer, but it must not calculate a shorter time).
Also, I think the check should be extended to
nsleep/usleep/ndelay/udelay as well - see the patch below.

Is this needed for the current code, or is this only an issue for the
future TPM CRB patches?  I was planning to make a release this week -
I'd prefer to add this to the next release unless it fixes an active
bug.

-Kevin


--- a/src/hw/timer.c
+++ b/src/hw/timer.c
@@ -167,53 +167,60 @@ timer_check(u32 end)
 }
 
 static void
-timer_delay(u32 diff)
+timer_delay(u32 end)
 {
-    u32 start = timer_read();
-    u32 end = start + diff;
     while (!timer_check(end))
         cpu_relax();
 }
 
 static void
-timer_sleep(u32 diff)
+timer_sleep(u32 end)
 {
-    u32 start = timer_read();
-    u32 end = start + diff;
     while (!timer_check(end))
         yield();
 }
 
+// Return the TSC value that is 'msecs' time in the future.
+u32
+timer_calc(u32 msecs)
+{
+    return timer_read() + (GET_GLOBAL(TimerKHz) * msecs);
+}
+u32
+timer_calc_usec(u32 usecs)
+{
+    u32 cur = timer_read(), khz = GET_GLOBAL(TimerKHz);
+    if (usecs > 500000)
+        return cur + DIV_ROUND_UP(usecs, 1000) * khz;
+    return cur + DIV_ROUND_UP(usecs * khz, 1000);
+}
+static u32
+timer_calc_nsec(u32 nsecs)
+{
+    u32 cur = timer_read(), khz = GET_GLOBAL(TimerKHz);
+    if (nsecs > 500000)
+        return cur + DIV_ROUND_UP(nsecs, 1000000) * khz;
+    return cur + DIV_ROUND_UP(nsecs * khz, 1000000);
+}
+
 void ndelay(u32 count) {
-    timer_delay(DIV_ROUND_UP(count * GET_GLOBAL(TimerKHz), 1000000));
+    timer_delay(timer_calc_nsec(count));
 }
 void udelay(u32 count) {
-    timer_delay(DIV_ROUND_UP(count * GET_GLOBAL(TimerKHz), 1000));
+    timer_delay(timer_calc_usec(count));
 }
 void mdelay(u32 count) {
-    timer_delay(count * GET_GLOBAL(TimerKHz));
+    timer_delay(timer_calc(count));
 }
 
 void nsleep(u32 count) {
-    timer_sleep(DIV_ROUND_UP(count * GET_GLOBAL(TimerKHz), 1000000));
+    timer_sleep(timer_calc_nsec(count));
 }
 void usleep(u32 count) {
-    timer_sleep(DIV_ROUND_UP(count * GET_GLOBAL(TimerKHz), 1000));
+    timer_sleep(timer_calc_usec(count));
 }
 void msleep(u32 count) {
-    timer_sleep(count * GET_GLOBAL(TimerKHz));
-}
-
-// Return the TSC value that is 'msecs' time in the future.
-u32
-timer_calc(u32 msecs)
-{
-    return timer_read() + (GET_GLOBAL(TimerKHz) * msecs);
-}
-u32
-timer_calc_usec(u32 usecs)
-{
-    return timer_read() + DIV_ROUND_UP(GET_GLOBAL(TimerKHz) * usecs, 1000);
+    timer_sleep(timer_calc(count));
 }