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

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Date:   Tue, 24 Nov 2020 10:52:15 +0530
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Marc Zyngier <maz@...nel.org>, sudeep.holla@....com
Cc:     suzuki.poulose@....com, ionela.voinescu@....com,
        MSM <linux-arm-msm@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, catalin.marinas@....com,
        Will Deacon <will@...nel.org>, valentin.schneider@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: AMU extension v1 support for cortex A76, A77, A78 CPUs

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.
> 

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