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

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Date:   Thu, 21 May 2020 15:00:13 +0100
From:   John Garry <john.garry@...wei.com>
To:     Will Deacon <will@...nel.org>, Rob Herring <robh@...nel.org>
CC:     Joakim Zhang <qiangqing.zhang@....com>,
        Mark Rutland <mark.rutland@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        NXP Linux Team <linux-imx@....com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V1 RESEND 1/3] perf/imx_ddr: Add system PMU identifier for
 userspace

On 21/05/2020 14:04, Will Deacon wrote:
> On Wed, May 20, 2020 at 09:23:41AM -0600, Rob Herring wrote:
>> On Wed, May 20, 2020 at 1:33 AM Will Deacon <will@...nel.org> wrote:
>>>
>>> On Tue, May 19, 2020 at 12:51:25PM -0600, Rob Herring wrote:
>>>> On Tue, May 12, 2020 at 03:31:13PM +0800, Joakim Zhang wrote:
>>>>> +static ssize_t ddr_perf_identifier_show(struct device *dev,
>>>>> +                                   struct device_attribute *attr,
>>>>> +                                   char *page)
>>>>> +{
>>>>> +   struct ddr_pmu *pmu = dev_get_drvdata(dev);
>>>>> +
>>>>> +   return sprintf(page, "%s\n", pmu->devtype_data->identifier);
>>>>
>>>> Why do we need yet another way to identify the SoC from 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.

Thanks,
John