Re: [PATCH RFC 1/4] driver/perf: Add identifier sysfs file for CMN

On 29/03/2023 18:47, Robin Murphy wrote:

quoted

Do Illka and Robin know that there is such a register that can 
identify different CMN versions? Looking forward to your suggestions.

In principle the "part number" fields from CFGM_PERIPH_ID_0/1 are 
supposed to identify the model, but for various reasons I'm suspicious 
of that being unreliable (not least that no actual values are 
documented, only "configuration-dependent"). That's why I went down the 
route of making sure we have explicit ACPI/DT identifiers for every model.

However, the model alone seems either too specific or not specific 
enough for a jevents identifier. The defined metrics are pretty trivial 
and should have no real reason not to be common to *any* CMN PMU where 
the underlying events are present. On the other hand, if we want to get 
down to the level of specific events in JSON then we'd need to consider 
the revision as well, since there are several events which only exist on 
certain revisions of a given model (but often are also common to later 
models).

This actually foreshadows a question I was planning to bring up in the 
context of another driver I'm working on - for this one I would rather 
like to try using jevents rather than have to maintain another sprawl of 
event tables in a driver, but it's still going to have the same thing of 
wanting model/revision matching along the lines of what 
arm_cmn_event_attr_is_visible() is doing for CMN events. AFAICS this 
would need jevents to grow a rather more flexible way of encoding and 
matching identifiers, since having dozens of almost-identical copies of 
event definitions for every exact identifier value is clearly 
unworkable.

This sort of problem has not occurred yet as perf tool only supports 
"system" events for a handful of SoCs so far :)

Does anyone happen to have any thoughts or preferences 
around how that might be approached?

Currently the perf tool will just match system events based on the exact 
HW identifier and PMU name.

However, if you consider PMCG PMU support as an example of possible area 
of improvement, it has a number of fixed events and a number of IMPDEF 
events. There should be no reason to need to describe in a separate JSON 
those fixed events for every instance of that PMU.

So a simple change would be to teach perf tool that for certain fixed 
events we only need to match based on the PMU name. For others we need 
to match based on some identifier.

If matching based on an identifier still leads to unwieldy amounts of 
tables, then maybe HW identifier wildcard matching may suit, like what 
is done for CPU events.

Thanks,
John


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