linux-kernel - Re: [PATCH v2 8/8] drivers/perf: Add Apple icestorm/firestorm CPU PMU driver

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Date:   Wed, 1 Dec 2021 12:56:26 -0500
From:   Alyssa Rosenzweig <alyssa@...enzweig.io>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Marc Zyngier <maz@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dougall <dougallj@...il.com>, kernel-team@...roid.com
Subject: Re: [PATCH v2 8/8] drivers/perf: Add Apple icestorm/firestorm CPU
 PMU driver

> > Add a new, weird and wonderful driver for the equally weird Apple
> > PMU HW. Although the PMU itself is functional, we don't know much
> > about the events yet, so this can be considered as yet another
> > random number generator...
> 
> It's really frustrating that Apple built this rather than the architected PMU,
> because we've generally pushed back on IMPLEMENTATION DEFINED junk in this
> area, and supporting this makes it harder to push back on other vendors going
> the same route, which I'm not keen on. That, and the usual state of IMP-DEF
> stuff making this stupidly painful to reason about.

Rules can be a bit stricter for vendors than for ragtag
reverse-engineers. The kernel community can push back on vendor's
choices because vendors have the power to choose otherwise.
But reverse engineers' hands are sometimes forced by bad vendor
decisions; rejecting the driver means mainline can never support the
hardware. I believe there's precedent for distinguishing these cases,
at least in the graphics subsystem.

I don't know if this applies to this driver. I only wish to offer a
rebuttal to a future vendor trying to mainline something questionable
with the defence "Asahi Linux / Nouveau / ... did it, so we can too".

(This will be relevant to the Apple M1 display controller driver, which
would be a hard NAK if submitted by Apple...)