Re: [Qemu-devel] [PATCH v3 4/9] mips: remove muldiv64()

[Laurent Vivier <lvivier@redhat.com>]

ping ?

On 27/08/2015 21:33, Laurent Vivier wrote:
> Originally, timers were ticks based, and it made sense to
> add ticks to current time to know when to trigger an alarm.
> 
> But since commit:
> 
> 7447545 change all other clock references to use nanosecond resolution accessors
> 
> All timers use nanoseconds and we need to convert ticks to nanoseconds, by
> doing something like:
> 
>     y = muldiv64(x, get_ticks_per_sec(), TIMER_FREQ)
> 
> where x is the number of device ticks and y the number of system ticks.
> 
> y is used as nanoseconds in timer functions,
> it works because 1 tick is 1 nanosecond.
> (get_ticks_per_sec() is 10^9)
> 
> But as MIPS timer frequency is 100 MHz, we can also do:
> 
>     y = x * 10; /* 100 MHz period is 10 ns */
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  hw/mips/cputimer.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/hw/mips/cputimer.c b/hw/mips/cputimer.c
> index 577c9ae..a74ae5c 100644
> --- a/hw/mips/cputimer.c
> +++ b/hw/mips/cputimer.c
> @@ -25,7 +25,7 @@
>  #include "qemu/timer.h"
>  #include "sysemu/kvm.h"
>  
> -#define TIMER_FREQ	100 * 1000 * 1000
> +#define TIMER_PERIOD 10 /* 10 ns period for 100 Mhz frequency */
>  
>  /* XXX: do not use a global */
>  uint32_t cpu_mips_get_random (CPUMIPSState *env)
> @@ -49,9 +49,8 @@ static void cpu_mips_timer_update(CPUMIPSState *env)
>      uint32_t wait;
>  
>      now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> -    wait = env->CP0_Compare - env->CP0_Count -
> -	    (uint32_t)muldiv64(now, TIMER_FREQ, get_ticks_per_sec());
> -    next = now + muldiv64(wait, get_ticks_per_sec(), TIMER_FREQ);
> +    wait = env->CP0_Compare - env->CP0_Count - (uint32_t)(now / TIMER_PERIOD);
> +    next = now + (uint64_t)wait * TIMER_PERIOD;
>      timer_mod(env->timer, next);
>  }
>  
> @@ -79,8 +78,7 @@ uint32_t cpu_mips_get_count (CPUMIPSState *env)
>              cpu_mips_timer_expire(env);
>          }
>  
> -        return env->CP0_Count +
> -            (uint32_t)muldiv64(now, TIMER_FREQ, get_ticks_per_sec());
> +        return env->CP0_Count + (uint32_t)(now / TIMER_PERIOD);
>      }
>  }
>  
> @@ -95,9 +93,8 @@ void cpu_mips_store_count (CPUMIPSState *env, uint32_t count)
>          env->CP0_Count = count;
>      else {
>          /* Store new count register */
> -        env->CP0_Count =
> -            count - (uint32_t)muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
> -                                       TIMER_FREQ, get_ticks_per_sec());
> +        env->CP0_Count = count -
> +               (uint32_t)(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) / TIMER_PERIOD);
>          /* Update timer timer */
>          cpu_mips_timer_update(env);
>      }
> @@ -121,8 +118,8 @@ void cpu_mips_start_count(CPUMIPSState *env)
>  void cpu_mips_stop_count(CPUMIPSState *env)
>  {
>      /* Store the current value */
> -    env->CP0_Count += (uint32_t)muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
> -                                         TIMER_FREQ, get_ticks_per_sec());
> +    env->CP0_Count += (uint32_t)(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) /
> +                                 TIMER_PERIOD);
>  }
>  
>  static void mips_timer_cb (void *opaque)
>