Re: [RESEND PATCH v3 1/2] perf: coresight_pmu: Add support for ARM CoreSight PMU driver
From: Suzuki K Poulose
Date: Thu Jul 21 2022 - 05:19:47 EST
Hi
On 14/07/2022 05:47, Besar Wicaksono wrote:
-----Original Message-----
From: Robin Murphy <robin.murphy@xxxxxxx>
Sent: Wednesday, July 13, 2022 3:13 AM
To: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>; Besar Wicaksono
<bwicaksono@xxxxxxxxxx>
Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>; catalin.marinas@xxxxxxx;
will@xxxxxxxxxx; mark.rutland@xxxxxxx; linux-arm-
kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
tegra@xxxxxxxxxxxxxxx; sudeep.holla@xxxxxxx;
thanu.rangarajan@xxxxxxx; Michael.Williams@xxxxxxx; Thierry Reding
<treding@xxxxxxxxxx>; Jonathan Hunter <jonathanh@xxxxxxxxxx>; Vikram
Sethi <vsethi@xxxxxxxxxx>; mike.leach@xxxxxxxxxx; leo.yan@xxxxxxxxxx
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.
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") ?
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