linux-kernel - Re: [PATCH v2 1/5] driver/perf: Add identifier sysfs file for CMN

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Date:   Thu, 4 May 2023 12:02:48 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     Robin Murphy <robin.murphy@....com>,
        Jing Zhang <renyu.zj@...ux.alibaba.com>,
        Ian Rogers <irogers@...gle.com>, Will Deacon <will@...nel.org>,
        Shuai Xue <xueshuai@...ux.alibaba.com>
Cc:     James Clark <james.clark@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Ilkka Koskinen <ilkka@...amperecomputing.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org,
        Zhuo Song <zhuo.song@...ux.alibaba.com>
Subject: Re: [PATCH v2 1/5] driver/perf: Add identifier sysfs file for CMN

On 04/05/2023 10:47, Robin Murphy wrote:
>> nit: generally if (!val) is preferred

Hi Robin,

> 
> Although either way it can only be NULL in cases of memory corruption or 
> developers making broken changes to the driver, neither of which are 
> worth pretending to defend against.

If there was some broken code for setting this identifier, then we would 
just not show the identifier file, rather than show it containing "NULL" 
- that seems better. However, there may be other implications from that 
same broken code, so you maintainers and contributors please decide.

> 
>>> +    return attr->mode;
>>> +};
>>> +
>>> +static struct device_attribute arm_cmn_identifier_attr =
>>> +__ATTR(identifier, 0444, arm_cmn_identifier_show, NULL);
>>> +
>>> +static struct attribute *arm_cmn_identifier_attrs[] = {
>>> +    &arm_cmn_identifier_attr.attr,
>>> +    NULL
>>> +};
>>> +
>>> +static struct attribute_group arm_cmn_identifier_attr_group = {
>>> +    .attrs = arm_cmn_identifier_attrs,
>>> +    .is_visible = arm_cmn_identifier_attr_visible
>>> +};
>>> +
>>>   static const struct attribute_group *arm_cmn_attr_groups[] = {
>>>       &arm_cmn_event_attrs_group,
>>>       &arm_cmn_format_attrs_group,
>>>       &arm_cmn_cpumask_attr_group,
>>> +    &arm_cmn_identifier_attr_group,
>>>       NULL
>>>   };
>>> @@ -2241,6 +2273,22 @@ static int arm_cmn600_of_probe(struct 
>>> device_node *np)
>>>       return of_property_read_u32(np, "arm,root-node", &rootnode) ?: 
>>> rootnode;
>>>   }
>>> +const char *arm_cmn_identifier(unsigned long model)
>>> +{
>>> +    switch (model) {
>>> +    case CMN600:
>>> +        return "cmn600";
>>> +    case CMN650:
>>> +        return "cmn650";
>>> +    case CMN700:
>>> +        return "cmn700";
>>> +    case CI700:
>>> +        return "ci700";
>>> +    default:
>>> +        return NULL;
>>> +    }
>>
>> nit: I think that it would be nicer to have this per-model string 
>> stored statically in arm_cmn_acpi_match[].driver_data and 
>> arm_cmn_of_match[].data, so we have a straight lookup
> 
> Again, I'm not really convinced how useful this coarse per-model scheme 
> is - for instance, in terms of many events, CMN-600 r3 is closer to 
> CMN-650 than it is to CMN-600 r1, so what exactly would "CMN-600" mean 
> to the user?

ok, I see, that's what I was asking about in the cmn-700 JSON review; 
and from what you say, it is not the case that we always have the same 
events for every revision. So we need a more fine-grained identifier.

For DT support, I suppose per-revision compat strings could be added, 
but I would not be sure what to do about ACPI.

BTW, My comment was more about coding style of case a, case b, case c, 
... case z, does not scale well.

Thanks,
John