linux-kernel - Re: [PATCH V1 RESEND 1/3] perf/imx_ddr: Add system PMU identifier for userspace

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Message-ID: <51aa7cbc-0ce2-b96d-b056-fcc6013ccecf@huawei.com>
Date:   Wed, 27 May 2020 15:34:48 +0100
From:   John Garry <john.garry@...wei.com>
To:     Will Deacon <will@...nel.org>, Rob Herring <robh@...nel.org>
CC:     Mark Rutland <mark.rutland@....com>, <devicetree@...r.kernel.org>,
        Joakim Zhang <qiangqing.zhang@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        NXP Linux Team <linux-imx@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Zhangshaokun <zhangshaokun@...ilicon.com>
Subject: Re: [PATCH V1 RESEND 1/3] perf/imx_ddr: Add system PMU identifier for
 userspace

>>>>
>>>> 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