Commit d0a981b2 authored by Paul Brook's avatar Paul Brook
Browse files

Avoid rounding problems in ptimer_get_count 



Signed-off-by: default avatarPaul Brook <paul@codesourcery.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6961 c046a42c-6fe2-441c-8c8c-71466251a162
parent bbeea539

hw/ptimer.c

+31 −1
Original line number Diff line number Diff line
@@ -7,7 +7,7 @@ 
 */

#include "hw.h"

#include "qemu-timer.h"



#include "host-utils.h"



struct ptimer_state

{
@@ -78,9 +78,39 @@ uint64_t ptimer_get_count(ptimer_state *s) 
        } else {

            uint64_t rem;

            uint64_t div;

            uint32_t frac;

            int clz1, clz2;

            int shift;



            /* We need to divide time by period, where time is stored in

               rem (64-bit integer) and period is stored in period/period_frac

               (64.32 fixed point).

              

               Doing full precision division is hard, so scale values and

               do a 64-bit division.  The result should be rounded down,

               so that the rounding error never causes the timer to go

               backwards.

            */



            rem = s->next_event - now;

            div = s->period;



            clz1 = clz64(rem);

            clz2 = clz64(div);

            shift = clz1 < clz2 ? clz1 : clz2;



            rem <<= shift;

            div <<= shift;

            if (shift >= 32) {

                div |= ((uint64_t)s->period_frac << (shift - 32));

            } else {

                if (shift != 0)

                    div |= (s->period_frac >> (32 - shift));

                /* Look at remaining bits of period_frac and round div up if 

                   necessary.  */

                if ((uint32_t)(s->period_frac << shift))

                    div += 1;

            }

            counter = rem / div;

        }

    } else {