linux-kernel - Re: [PATCH v2 2/3] dt-bindings: timer: Document arm, cortex-a7-timer in arch timer

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Date:   Sun, 20 Mar 2022 14:47:56 -0400
From:   Rob Herring <robh@...nel.org>
To:     Kuldeep Singh <singh.kuldeep87k@...il.com>
Cc:     Robin Murphy <robin.murphy@....com>, Marc Zyngier <maz@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/3] dt-bindings: timer: Document arm, cortex-a7-timer
 in arch timer

On Fri, Mar 18, 2022 at 02:55:08AM +0530, Kuldeep Singh wrote:
> On Thu, Mar 17, 2022 at 08:25:12PM +0000, Robin Murphy wrote:
> > On 2022-03-17 19:15, Kuldeep Singh wrote:
> > > Renesas RZ/N1D platform uses compatible "arm,cortex-a7-timer" in
> > > conjugation with "arm,armv7-timer". Since, initial entry is not
> > > documented, it start raising dtbs_check warnings.
> > > 
> > > ['arm,cortex-a7-timer', 'arm,armv7-timer'] is too long
> > > 'arm,cortex-a7-timer' is not one of ['arm,armv7-timer', 'arm,armv8-timer']
> > > 'arm,cortex-a7-timer' is not one of ['arm,cortex-a15-timer']
> > > 
> > > Document this compatible to address it. The motivation to add this
> > > change is taken from an already existing entry "arm,cortex-a15-timer".
> > > Please note, this will not hurt any arch timer users.
> > 
> > Eh, if it's never been documented or supported, I say just get rid of it.
> > The arch timer interface is by definition part of a CPU, and we can tell
> > what the CPU is by reading its ID registers. Indeed that's how the driver
> > handles the non-zero number of CPU-specific errata that already exist - we
> > don't need compatibles for that.
> > 
> > In some ways it might have been nice to have *SoC-specific* compatibles
> > given the difficulty some integrators seem to have had in wiring up a stable
> > count *to* the interface, but it's not like they could be magically added to
> > already-deployed DTs after a bug is discovered, and nor could we have
> > mandated them from day 1 just in case and subsequently maintained a binding
> > that is just an ever-growing list of every SoC. Oh well.
> 
> Robin, A similar discussion was already done on v1 thread. Please see
> below for details:
> https://lore.kernel.org/linux-devicetree/20220317065925.GA9158@9a2d8922b8f1/
> https://lore.kernel.org/linux-devicetree/726bde76-d792-febf-d364-6eedeb748c3b@canonical.com/
> 
> And final outcome of discussion turns out to add this compatible string.

I agree with Robin on dropping. More specific here is not useful. If 
we're going to add some cores, then we should add every core 
implementation.

If one has a big.LITTLE system with A15/A7 what would be the right 
compatible value?

> 
> I see people with different set of perspective in regard to whether keep
> compatible string or not. We should have some sort of evidences to
> support claims so that next time when similar situation arises, we'll be
> aware beforehand how to proceed.

Every situation tends to be different.

Rob