linux-kernel - Re: [PATCH 2/2] perf/arm-cmn: Add sysfs identifier

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Message-ID: <5efa7af9-6347-a409-370c-b8b78b3d53aa@oracle.com>
Date:   Wed, 14 Jun 2023 15:55:42 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     Robin Murphy <robin.murphy@....com>, will@...nel.org
Cc:     mark.rutland@....com, ilkka@...amperecomputing.com,
        renyu.zj@...ux.alibaba.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] perf/arm-cmn: Add sysfs identifier

On 12/06/2023 18:16, Robin Murphy wrote:
> Expose a sysfs identifier encapsulating the CMN part number and revision
> so that jevents can narrow down a fundamental set of possible events for
> calculating metrics. Configuration-dependent aspects - such as whether a
> given node type is present, and/or a given node ID is valid - are still
> not covered, and in general it's hard to see how userspace could handle
> them, so we won't be removing any data or validation logic from the
> driver any time soon, but at least it's a step in a useful direction.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>

FWIW, Reviewed-by:
John Garry <john.g.garry@...cle.com>

However a comment, below.

> ---
>   drivers/perf/arm-cmn.c | 20 ++++++++++++++++----
>   1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> index 8cf4ed932950..088dc5c690a4 100644
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c
> @@ -1199,19 +1199,31 @@ static ssize_t arm_cmn_cpumask_show(struct device *dev,
>   static struct device_attribute arm_cmn_cpumask_attr =
>   		__ATTR(cpumask, 0444, arm_cmn_cpumask_show, NULL);
>   
> -static struct attribute *arm_cmn_cpumask_attrs[] = {
> +static ssize_t arm_cmn_identifier_show(struct device *dev,
> +				       struct device_attribute *attr, char *buf)
> +{
> +	struct arm_cmn *cmn = to_cmn(dev_get_drvdata(dev));
> +
> +	return sysfs_emit(buf, "%03x%02x\n", cmn->part, cmn->rev);

I don't think that this gives the "0x" prefix, right? It is just an 
encoded string of values, so not strictly necessary or even appropriate, 
I suppose.

However, if userspace wants to improve scalability by, say, matching an 
event for all revs of a model, userspace (perf tool) needs to be 
programmed in the JSONs somehow since we have no delimiter.

Thanks,
John

> +}
> +
> +static struct device_attribute arm_cmn_identifier_attr =
> +		__ATTR(identifier, 0444, arm_cmn_identifier_show, NULL);
> +
> +static struct attribute *arm_cmn_other_attrs[] = {
>   	&arm_cmn_cpumask_attr.attr,
> +	&arm_cmn_identifier_attr.attr,
>   	NULL,
>   };
>   
> -static const struct attribute_group arm_cmn_cpumask_attr_group = {
> -	.attrs = arm_cmn_cpumask_attrs,
> +static const struct attribute_group arm_cmn_other_attrs_group = {
> +	.attrs = arm_cmn_other_attrs,
>   };
>   
>   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_other_attrs_group,
>   	NULL
>   };
>