Re: AMU extension v1 support for cortex A76, A77, A78 CPUs

[Neeraj Upadhyay]

Thanks Marc, Vladimir, Mark, Sudeep for your inputs!


Thanks
Neeraj


On 11/20/2020 3:43 PM, Mark Rutland wrote:
> On Fri, Nov 20, 2020 at 09:09:00AM +0000, Vladimir Murzin wrote:
> > On 11/20/20 8:56 AM, Marc Zyngier wrote:
> > > On 2020-11-20 04:30, Neeraj Upadhyay wrote:
> > > > Hi,
> > > >
> > > > For ARM cortex A76, A77, A78 cores (which as per TRM, support AMU)
> > > > AA64PFR0[47:44] field is not set, and AMU does not get enabled for
> > > > them.
> > > > Can you please provide support for these CPUs in cpufeature.c?
> > >
> > > If that was the case, that'd be an erratum, and it would need to be
> > > documented as such. It could also be that this is an optional feature
> > > for these cores (though the TRM doesn't suggest that).
> > >
> > > Can someone at ARM confirm what is the expected behaviour of these CPUs?
> >
> > Not a confirmation, but IIRC, these are imp def features, while our cpufeatures
> > catches architected one.
>
> We generally don't make use of IMP-DEF featurees because of all the pain
> it brings.
>
> Looking at the Cortex-A76 TRM, the encoding for AMCNTENCLR is:
>
>   Op0: 3  (0b11)
>   Op1: 3  (0b011)
>   CRn: 15 (0b1111)
>   CRm: 9  (0b1001)
>   Op2: 7  (0b111)
>
> ... whereas the architected encoding (from our sysreg.h) is:
>
>   Op0: 3
>   Op1: 3
>   CRn: 13
>   CRm: 2
>   Op2: 4
>
> ... so that's a different register with the same name, which is
> confusing and unfortunate.
>
> The encodings are different (and I haven't checked whether the fields /
> semantics are the same), so it's not just a matter of wiring up new
> detection code. There are also IMP-DEF traps in ACTLR_EL3 and ACTLR_EL2
> which we can't be certain of the configuration of, and as the registers
> are in the IMP-DEF encoding space they'll be trapped by HCR_EL2.TIDCP
> and emulated as UNDEFINED by a hypervisor. All of that means that going
> by the MIDR alone is not sufficient to know we can safely access the
> registers.
>
> So as usual for IMP-DEF stuff I don't think we can or should make use of
> this.
>
> Thanks,
> Mark.

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation