Re: [RFC 01/10] hw/mos6522: Remove get_load_time() methods and functions

[Finn Thain <fthain@linux-m68k.org>]

On Wed, 25 Aug 2021, Mark Cave-Ayland wrote:

> On 24/08/2021 11:09, Finn Thain wrote:
> 
> > This code appears to be unnecessary.
> > 
> > Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> > ---
> >   hw/misc/mos6522.c | 22 +---------------------
> >   1 file changed, 1 insertion(+), 21 deletions(-)
> > 
> > diff --git a/hw/misc/mos6522.c b/hw/misc/mos6522.c
> > index 1c57332b40..a478c1ca43 100644
> > --- a/hw/misc/mos6522.c
> > +++ b/hw/misc/mos6522.c
> > @@ -63,17 +63,6 @@ static uint64_t get_counter_value(MOS6522State *s,
> > MOS6522Timer *ti)
> >       }
> >   }
> >   -static uint64_t get_load_time(MOS6522State *s, MOS6522Timer *ti)
> > -{
> > -    MOS6522DeviceClass *mdc = MOS6522_GET_CLASS(s);
> > -
> > -    if (ti->index == 0) {
> > -        return mdc->get_timer1_load_time(s, ti);
> > -    } else {
> > -        return mdc->get_timer2_load_time(s, ti);
> > -    }
> > -}
> > -
> >   static unsigned int get_counter(MOS6522State *s, MOS6522Timer *ti)
> >   {
> >       int64_t d;
> > @@ -98,7 +87,7 @@ static unsigned int get_counter(MOS6522State *s,
> > MOS6522Timer *ti)
> >   static void set_counter(MOS6522State *s, MOS6522Timer *ti, unsigned int
> > val)
> >   {
> >       trace_mos6522_set_counter(1 + ti->index, val);
> > -    ti->load_time = get_load_time(s, ti);
> > +    ti->load_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> >       ti->counter_value = val;
> >       if (ti->index == 0) {
> >           mos6522_timer1_update(s, ti, ti->load_time);
> > @@ -208,13 +197,6 @@ static uint64_t mos6522_get_counter_value(MOS6522State
> > *s, MOS6522Timer *ti)
> >                       ti->frequency, NANOSECONDS_PER_SECOND);
> >   }
> >   -static uint64_t mos6522_get_load_time(MOS6522State *s, MOS6522Timer *ti)
> > -{
> > -    uint64_t load_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> > -
> > -    return load_time;
> > -}
> > -
> >   static void mos6522_portA_write(MOS6522State *s)
> >   {
> >       qemu_log_mask(LOG_UNIMP, "portA_write unimplemented\n");
> > @@ -518,8 +500,6 @@ static void mos6522_class_init(ObjectClass *oc, void
> > *data)
> >       mdc->update_irq = mos6522_update_irq;
> >       mdc->get_timer1_counter_value = mos6522_get_counter_value;
> >       mdc->get_timer2_counter_value = mos6522_get_counter_value;
> > -    mdc->get_timer1_load_time = mos6522_get_load_time;
> > -    mdc->get_timer2_load_time = mos6522_get_load_time;
> >   }
> >     static const TypeInfo mos6522_type_info = {
> 
> Both the get_counter_value() and get_load_time() callbacks are used as part of
> the CUDA emulation in hw/misc/macio/cuda.c as per the comment:
> 
> /* MacOS uses timer 1 for calibration on startup, so we use
>  * the timebase frequency and cuda_get_counter_value() with
>  * cuda_get_load_time() to steer MacOS to calculate calibrate its timers
>  * correctly for both TCG and KVM (see commit b981289c49 "PPC: Cuda: Use cuda
>  * timer to expose tbfreq to guest" for more information) */
> 
> Certainly for the 6522 device it is worth configuring with
> --target-list="ppc-softmmu m68k-softmmu" to make sure that you don't
> inadvertently break anything in the PPC world.
> 

No build failure here. Maybe your tree is different?

> A bit of history here: the original mos6522.c was extracted from
> hw/misc/macio/cuda.c when Laurent presented his initial q800 patches since
> they also had their own implementation of the 6522, and it was better to move
> the implementation into a separate QEMU device so that the logic could be
> shared.
> 
> The Darwin kernel timer calibration loop is quite hard to get right: see
> https://opensource.apple.com/source/xnu/xnu-123.5/pexpert/ppc/pe_clock_speed_asm.s.auto.html
> and
> https://opensource.apple.com/source/xnu/xnu-123.5/pexpert/ppc/pe_clock_speed.c.auto.html.
> Ben/Alex came up with the current mechanism to fool the calibration routine,
> and I simply added in those callbacks to allow it to be implemented as part of
> the now-generic 6522 device.
> 

I didn't find any references to these methods (get_timer1_counter_value, 
get_timer2_counter_value, get_timer1_load_time and get_timer2_load_time).
It appears to be dead code, and it adds complexity and harms readability. 
The Linux kernel project has a policy that no code is added if it lacks 
any in-tree usage. Does QEMU have the same policy?