Hi Jeremy,
I did consider DMI, but I want something more generic, i.e. could
cover embedded/DT systems also.
And I need to double check if DMI really has the info I require. Last
time I checked, it didn't for my dev board, but I know that some
fields are simply not filled in.
Well the info is probably there, but that doesn't mean it should be used
programmatically. As your board shows, its not that reliable. And
looking at the linked patch I see you mention that.
Right, I am trying to stay away from that.
But, quickly looking at the use case, I can't help but think you
don't really want any of the above, or the PPTT id. It seems the
mapping should actually be tied directly to the uncore PMU
definition, rather than a soc/machine/whatever identifier. Which
would imply keying off one of the ACPI object identifiers for the PMU
itself.
So a PMU device (/sys/bus/event_source/devices) does not have a link
to the ACPI object identifiers or uncore PMU platform device etc.
And even if it did, there is no clear link between that ACPI object
and the events it supports for that implementation.
Having a direct link isn't ideal either. It seems you do mention the pmu
naming conventions, which can be controlled based on ACPI object
identifiers.
Not necessarily.
Something like "uncore_dmc_hsi1" where the appended bits
could for example vary on _CID+_UID or DT name.
We already do include some naming from ACPI tables in naming (see
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/perf/hisilicon/hisi_uncore_ddrc_pmu.c?h=v5.5#n377),
but this is not good enough. I'll explain below.
Not sure that is a particularly good suggestion either, but I do think
its a better idea to tie the mapping to the pmu type/man/version concept
than the SOC it appears in. The sysfs-bus-event_source-devices-* ABI
docs are noticeably silent on the format of the pmu name (is that
somewhere else?).
I would say that there is a lack of PMU naming convention, which I did
note in my referenced patchset.
Apart from that, I think that this problem can be better explained with
the SMMUv3 PMCG example.
So this PMU has support for a number of IMP DEF events. The SMMUv3 PMCG
has no method to identify the implementation, so we cannot know which
IMP DEF events are supported for a specific implementation.
The PMCG PMU naming is fixed, and is in the form smmuv3_pmcg_XXXX - so
we cannot use some special naming. And the XXXX does not tell us
anything about the implementation to help know the IMP DEF events.
Now the perf tool has support to know which CPU+uncore events are
supported for a particular CPU through pmu-events feature - see
tools/perf/pmu-events/README
The perf tool includes a number of per-CPU event tables.
The matching of per-CPU event table the perf tool uses is based on
knowing the host CPUID - this is easy to retrieve this via some special
arch-specific CPU reg, etc. So once it knows the CPUID, "perf list"
command can show all the events for that CPU.
Now we can extend this idea for the PMCG PMU to support the IMP DEF
events. For this, we add support for a table of "system" PMU events per
SoC - similar to the CPU tables - containing the PMCG events. We cannot
use the CPUID to match the event table for SoC, as a CPUID is not always
specific to a SoC - that's definite for ARM world and definite for
SMMUv3 PMCG. So then perf tool needs to know some SoC identifier to
match the per-SoC events table. That's why I want the SoC id in readable
form in sysfs.
To add a final note on uncore PMUs, for ARM this is bit of grey area. So
currently we match uncore PMUs on CPUID. However I figure some SoC
implementer could take, for example, an A72, and add some uncore PMUs.
As such, we cannot always match on CPUID, so being able to match on a
SoC identifier would be better also.
Hope it explains.
Thanks,
John
