RE: [RESEND PATCH v3 1/2] perf: coresight_pmu: Add support for ARM CoreSight... | linux-arm-kernel

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Hi

-----Original Message-----
From: Suzuki K Poulose <suzuki.poulose@arm.com>
Sent: Tuesday, August 2, 2022 4:39 AM
To: Besar Wicaksono <redacted>; Mathieu Poirier
[off-list ref]
Cc: Robin Murphy <robin.murphy@arm.com>; catalin.marinas@arm.com;
will@kernel.org; mark.rutland@arm.com; linux-arm-
kernel@lists.infradead.org; linux-kernel@vger.kernel.org; linux-
tegra@vger.kernel.org; sudeep.holla@arm.com;
thanu.rangarajan@arm.com; Michael.Williams@arm.com; Thierry Reding
[off-list ref]; Jonathan Hunter [off-list ref]; Vikram
Sethi [off-list ref]; mike.leach@linaro.org; leo.yan@linaro.org
Subject: Re: [RESEND PATCH v3 1/2] perf: coresight_pmu: Add support for
ARM CoreSight PMU driver

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Hi

On 01/08/2022 23:27, Besar Wicaksono wrote:
Hi

-----Original Message-----
From: Mathieu Poirier <mathieu.poirier@linaro.org>
Sent: Thursday, July 21, 2022 10:36 AM
To: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Besar Wicaksono <redacted>; Robin Murphy
[off-list ref]; catalin.marinas@arm.com; will@kernel.org;
mark.rutland@arm.com; linux-arm-kernel@lists.infradead.org; linux-
kernel@vger.kernel.org; linux-tegra@vger.kernel.org;
sudeep.holla@arm.com; thanu.rangarajan@arm.com;
Michael.Williams@arm.com; Thierry Reding [off-list ref];
Jonathan
Hunter [off-list ref]; Vikram Sethi [off-list ref];
mike.leach@linaro.org; leo.yan@linaro.org
Subject: Re: [RESEND PATCH v3 1/2] perf: coresight_pmu: Add support for
ARM CoreSight PMU driver

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On Thu, 21 Jul 2022 at 03:19, Suzuki K Poulose [off-list ref]
wrote:
Hi

On 14/07/2022 05:47, Besar Wicaksono wrote:

-----Original Message-----
From: Robin Murphy <robin.murphy@arm.com>
Sent: Wednesday, July 13, 2022 3:13 AM
To: Mathieu Poirier <mathieu.poirier@linaro.org>; Besar Wicaksono
[off-list ref]
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>;
catalin.marinas@arm.com;
will@kernel.org; mark.rutland@arm.com; linux-arm-
kernel@lists.infradead.org; linux-kernel@vger.kernel.org; linux-
tegra@vger.kernel.org; sudeep.holla@arm.com;
thanu.rangarajan@arm.com; Michael.Williams@arm.com; Thierry
Reding
[off-list ref]; Jonathan Hunter [off-list ref];
Vikram
Sethi [off-list ref]; mike.leach@linaro.org;
leo.yan@linaro.org
Subject: Re: [RESEND PATCH v3 1/2] perf: coresight_pmu: Add
support
for
ARM CoreSight PMU driver

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On 2022-07-12 17:36, Mathieu Poirier wrote:
[...]
If we have decied to call this arm_system_pmu, (which I am
perfectly
happy with), could we please stick to that name for functions that
we
export ?

e.g,
s/coresight_pmu_sysfs_event_show/arm_system_pmu_event_show()/
Just want to confirm, is it just the public functions or do we need to
replace
all that has "coresight" naming ? Including the static functions,
structs,
filename.
I think all references to "coresight" should be changed to
"arm_system_pmu",
including filenames.  That way there is no doubt this IP block is not
related, and does not interoperate, with the any of the "coresight"
IP
blocks
already supported[1] in the kernel.

I have looked at the documentation[2] in the cover letter and I agree
with an earlier comment from Sudeep that this IP has very little to do
with
any
of the other CoreSight IP blocks found in the CoreSight
framework[1].
Using the
"coresight" naming convention in this driver would be _extremely_
confusing,
especially when it comes to exported functions.
But conversely, how is it not confusing to make up completely
different
names for things than what they're actually called? The CoreSight
Performance Monitoring Unit is a part of the Arm CoreSight
architecture,
it says it right there on page 1. What if I instinctively associate the
name Mathieu with someone more familiar to me, so to avoid
confusion
I'd
prefer to call you Steve? Is that OK?
What is the naming convention for modules under drivers/perf ?
In my observation, the names there correspond to the part monitored
by
the PMU. The confusion on using "coresight_pmu" naming could be
that
people may think the PMU monitors coresight system, i.e the trace
system under hwtracing.
However, the driver in this patch is for a new PMU standard that
monitors uncore
parts. Uncore was considered as terminology from Intel, so "system"
was
picked instead.
Please see this thread for reference:
    https://lore.kernel.org/linux-arm-
kernel/20220510111318.GD27557@willie-the-truck/
I think we all understand the state of affairs.

- We have an architecutre specification for PMUs, Arm CoreSight PMU
Architecutre, which has absolutely no relationship with :

     either CoreSight Self-Hosted Tracing (handled by "coresight"
subsystem in the kernel under drivers/hwtracing/coresight/, with a user
visible pmu as "cs_etm")

     or the CoreSight Architecture (except for the name). This is of less
significance in general. But has a significant impact on the "name"
users might expect for the driver/Kconfig etc.

- We want to be able to make it easier for the users/developers to
choose what they want without causing confusion.

For an end-user: Having the PMU instance named after the "System IP"
(as implememented in the driver solves the problem and falling back to
arm_system_pmu is a good enough choice. So let us stick with that)

Kconfig: May be we can choose
CONFIG_ARM_CORESIGHT_PMU_ARCH_PMU
or even
CONFIG_ARM_CORESIGHT_PMU_ARCH_SYSTEM_PMU

with appropriate help text to ensure there is enough stress about what
this is and what this is not would be sufficient.
CONFIG_ARM_CORESIGHT_PMU_ARCH_SYSTEM_PMU sounds good to
me.

Now the remaining contention is about the name of the "subsystem"
and
also the dir/files.  This may sound insignificant. But it is also
important to get this right. e.g., helps the reviewers unambiguously
identify the change or maintainers accepting pull requests (remember
these two PMUs (cs_etm and this one) go via different trees.). Not
everyone who deals with this in the community may be aware of how
these are different.

We could choose arm_cspmu_ or simply cspmu. Given that only the
"normal" users care about the "association" with the "architecture"
and more advanced users (e.g, developers) can easily map "Kconfig"
to driver files, may be we could even stick to the "arm_syspmu"
(from "arm system pmu") ?
+1 on "arm_syspmu"
I am fine too with arm_syspmu.

If there is no objection, I am going to post new update by end of this week
or early next week.
Unfortunately, I have been told that we have a potential problem with
"arm_syspmu" and even choosing "arm-system-pmu" for the name as it may
conflict with something that is coming soon. So we may have to go back
to something else, to avoid this exact same conversation in the near
future. Apologies for that.

Could we use "arm_cspmu" for the code/subsystem and may be
"arm-csarch-pmu" / "arm-cs-arch-pmu" for the device name ?
In that case, including "Coresight Architecture" in the name makes more sense.
I am fine with "arm_cspmu" and "arm-cs-arch-pmu".

Thanks,
Besar

Other suggestions ?

Suzuki

Thanks,
Besar

Suzuki

As it happens, Steve, I do actually agree with you that "coresight_" is
a bad prefix here, but only for the reason that it's too general. TBH I
think that's true of the existing Linux subsystem too, but that damage
is already done, and I'd concur that there's little value in trying to
unpick that now, despite the clear existence of products like
CoreSight
DAP and CoreSight ELA which don't have all that much to do with
program
trace either.

However, hindsight and inertia are hardly good reasons to double
down
on
poor decisions, so if I was going to vote for anything here it would be
"cspmu_", which is about as
obviously-related-to-the-thing-it-actually-is as we can get while also
being pleasantly concise.

[ And no, this isn't bikeshedding. Naming things right is *important* ]
I agree having the correct name is important, especially at this early
stage.
A direction of what the naming should describe would be very helpful
here.

Cheers,
Robin.

Thanks,
Steve

[1]. drivers/hwtracing/coresight/
[2]. https://developer.arm.com/documentation/ihi0091/latest
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