Re: [Qemu-devel] [PATCH v1 3/3] armv7m_systick: abort instead of locking on a bad rate

[KONRAD Frederic]

On 06/29/2017 03:02 PM, Philippe Mathieu-Daudé wrote:
> On 06/29/2017 09:43 AM, Peter Maydell wrote:
> > On 29 June 2017 at 13:35, Philippe Mathieu-Daudé 
> > <address@hidden> wrote:
> > > This is true it is better to abort here than risking a deadlock.
> > >
> > > However it seems to me they are 3 issues here:
> > > - the deadlock pattern is caused by using a global variable,
> > > - stellaris:ssys_calculate_system_clock() no checking 
> > > RCC.SYSDIV and
> > > RCC2.SYSDIV2 values <= 2 are reserved (GUEST_ERROR)
> > > - stellaris:ssys_write(RCC2) not overrides correctly RCC
> >
> > Stellaris works fine. It's other ARMv7M boards (which might forget
> > to set system_clock_scale) which don't work.
>
> Oh I misread ssys_calculate_system_clock(), indeed 
> system_clock_scale can not get below 5 so no deadlock on Stellaris.
>
> > > I'd rather take this opportunity to fix the deadlock pattern 
> > > using a
> > > getter/setter on system_clock_scale, doing the zero check in 
> > > the setter and
> > > eventually aborting in the getter, what do you think?
> >
> > We should be using a clock property on the CPU instead of 
> > system_clock_scale.
> > Unfortunately Konrad's series attempting to add that 
> > infrastructure
> > is still in the "trying to sort out the right API in code review"
> > stage. I don't think it's worth trying to fiddle with the API
> > for it before we have the right eventual infrastructure in place.
>
> I see. I'd rather move the comment and assert() in systick_scale().

Makes sense, I missed the potential divide-by-zero exception
which might happen in SysTick Current Value:

val = ((s->tick - (t + 1)) / systick_scale(s)) + 1;

I'll do the change and re-post a V2 when there will be some input
on the two first patches.

Thanks,
Fred

> > (What system_clock_scale is actually doing is setting the
> > emulated frequency of the CPU, since that affects the
> > frequency of the timer.)