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From:   John Garry <john.garry@huawei.com>
Subject: Re: [PATCH V1 RESEND 1/3] perf/imx_ddr: Add system PMU identifier for
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To:     Will Deacon <will@kernel.org>, Rob Herring <robh@kernel.org>
CC:     Mark Rutland <mark.rutland@arm.com>, <devicetree@vger.kernel.org>,
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Date:   Wed, 27 May 2020 15:34:48 +0100
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>>>>
>>>> I also really dislike this. What's the preferred way to identify the 
>>>> SoC
>>>> from userspace?
>>>
>>> /proc/cpuinfo? ;)
>>
>> The *SoC*!
>>
>>> For an non-firmware specific case, I'd say soc_device should be. I'd
>>> guess ACPI systems don't use it and for them it's dmidecode typically.
>>> The other problem I have with soc_device is it is optional.
>>
> 
> Hi Will,
> 
>> John -- what do you think about using soc_device to expose this 
>> information,
>> with ACPI systems using DMI data instead?
> 
> Generally I don't think that DMI is reliable, and I saw this as the 
> least preferred choice. I'm looking at the sysfs DMI info for my dev 
> board, and I don't even see anything like a SoC identifier.
> 
> As for the event_source device sysfs identifier file, it would not 
> always contain effectively the same as the SoC ID.
> 
> Certain PMUs which I'm interested in plan to have probe-able 
> identification info available in future.
> 

BTW, Shaokun now tells me that the HiSi uncore PMU HW have such 
registers to identify the implementation. I didn't know.

So we could add that identifier file for those PMUs as proof-of-concept, 
exposing that register.

As for other PMUs which I'm interested in, again, future versions should 
have such registers to self-identify.

So using something derived from the DT compat string would hopefully be 
the uncommon case.

Cheers,
John