Re: [PATCH v4 00/31] Introduce SCMI Telemetry FS support

["David Hildenbrand (Arm)" <david@kernel.org>]

Hi,

asking some clarifying questions that I assume also Christian might want to know.

>>> In a nutshell, the SCMI Telemetry protocol allows an agent to discover at
>>> runtime the set of Telemetry Data Events (DEs) available on a specific
>>> platform and provides the means to configure the set of DEs that a user is

Is the configuration aspect limited to enabling selected events, or is there
more that can be configured?

>>> interested into, while reading them back using the collection method that
>>> is deeemed more suitable for the usecase at hand. (...amongst the various
>>> possible collection methods allowed by SCMI specification)
>>>
>>> Without delving into the gory details of the whole SCMI Telemetry protocol
>>> let's just say that the SCMI platform/server firmware advertises a number
>>> of Telemetry Data Events, each one identified by a 32bit unique ID, and an
>>> SCMI agent/client, like Linux, can discover them and read back at will the
>>> associated data value in a number of ways.
>>> Data collection is mainly intended to happen on demand via shared memory
>>> areas exposed by the platform firmware, discovered dynamically via SCMI
>>> Telemetry and accessed by Linux on-demand, but some DE can also be reported
>>> via SCMI Notifications asynchronous messages or via direct dedicated
>>> FastChannels (another kind of SCMI memory based access): all of this
>>> underlying mechanism is anyway hidden to the user since it is mediated by
>>> the kernel driver which will return the proper data value when queried.
>>>
>>> Anyway, the set of well-known architected DE IDs defined by the spec is
>>> limited to a dozen IDs, which means that the vast majority of DE IDs are
>>> customizable per-platform: as a consequence, though, the same ID, say
>>> '0x1234', could represent completely different things on different systems.
>>>
>>> Precise definitions and semantic of such custom Data Event IDs are out of
>>> the scope of the SCMI Telemetry specification and of this implementation:
>>> they are supposed to be provided using some kind of JSON-like description
>>> file that will have to be consumed by a userspace tool which would be
>>> finally in charge of making sense of the set of available DEs.

You mention json here ... but I assume the data we are getting fed by the
protocol is not in some default format? (e.g., json)

>>>
>>> IOW, in turn, this means that even though the DEs enumerated via SCMI come
>>> with some sort of topological and qualitative description provided by the
>>> protocol (like unit of measurements, name, topology info etc), kernel-wise
>>> we CANNOT be completely sure of "what is what" without being fed-back some
>>> sort of information about the DEs by the afore mentioned userspace tool.

Maybe you have it in some of the patches here, but what does the typical
directory + file structure look like in the current implementation?

Do you have an example?

Also, is everything in that filesystem read-only, or are there some writable
file (IOW, how is stuff configured?).

>>>
>>> For these reasons, currently this series does NOT attempt to register any
>>> of these DEs with any of the usual in-kernel subsystems (like HWMON, IIO,
>>> PERF etc), simply because we cannot be sure which DE is suitable, or even
>>> desirable, for a given subsystem. This also means there are NO in-kernel
>>> users of these Telemetry data events as of now.

Okay, so you really only feed this data to user space, exposing all the data you
have easily available as part of the protocol.

>>>
>>> So, while we do not exclude, for the future, to feed/register some of the
>>> discovered DEs to/with some of the above mentioned Kernel subsystems, as
>>> of now we have ONLY modeled a custom userspace API to make SCMI Telemetry
>>> available to userspace tools.

It's a good question how that could be done, if you need more information about
these events from user space.

>>>
>>> In deciding which kind of interface to expose SCMI Telemetry data to a
>>> user, this new SCMI Telemetry driver aims at satisfying 2 main reqs:
>>>
>>>  - exposing an FS-based human-readable interface that can be used to
>>>    discover, configure and access our Telemetry data directly also from
>>>    the shell without special tools
>>>
>>>  - exposing alternative machine-friendly, more-performant, binary
>>>    interfaces that can be used to avoid the overhead of multiple accesses
>>>    to the VFS and that can be more suitable to access with custom tools
[...]

>>>
>>> Due to the above reasoning, since V1 we opted for a new approach with the
>>> proposed interfaces now based on a full fledged, unified, virtual pseudo
>>> filesystem implemented from scratch, so that we can:
>>>
>>>  - expose all the DEs property we like as before with SysFS, but without
>>>    any of the constraint imposed by the usage of SysFs or kernfs.
>>>
>>>  - easily expose additional alternative views of the same set of DEs
>>>    using symlinking capabilities (e.g. alternative topological view)

That sounds reasonable.

[...]

> ...I would not say that this was the kind of feedback I was hoping for,
> but I am NOT gonna argue, given that you shot down already what I thought
> were all my best selling points :P
> 
> At this point my understanding is that the way forward must be to use
> a custom tool to configure/extract/translate the raw Telemetry data and
> move up into userspace the whole human readable FS layer via FUSE, if
> really needed.
> 
> I suppose that the new kernel/user interface has to be some dedicated char
> device implementing proper fops. (like I did previously in early versions
> of this series and then abandoned...)
> 
> Is this you have in mind ? Dedicated character device(s) with enough fops
> to be able to configure/extract Telemetry data with a custom tool ?

I cannot speak for Christian, but I guess you could have some kind of libscmi in
user space that can obtain the information (as you say, probably char device,
not sure which alternatives we have), to expose the data through a nice ABI, to
then either make tools build upon that directly, or have a fuse server in user
space that mimics what you currently do with the file system.

One thing that is not clear to me yet is how stuff would be configured, and how
possibly multiple users of libscmi would possibly interact.

> 
> Should/could such a tool live in the kernel tree (tools/) at least for
> ease of development/deployment ?

I think OOT.

-- 
Cheers,

David