Re: [PATCH RFC] arm64: improve display about CPU architecture in cpuinfo

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>, Marc Zyngier <maz@kernel.org>,
	Rongwei Wang <rongwei.wang@linux.alibaba.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	joey.gouly@arm.com, Mark Rutland <mark.rutland@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] arm64: improve display about CPU architecture in cpuinfo
Date: Tue, 8 Mar 2022 17:57:46 +0000	[thread overview]
Message-ID: <YieZGoV+XR2kdHix@shell.armlinux.org.uk> (raw)
In-Reply-To: <9296f97c-f894-001c-53e6-41bbfe36ce71@arm.com>

On Mon, Mar 07, 2022 at 08:05:06PM +0000, Robin Murphy wrote:
> On 2022-03-07 19:30, Arnd Bergmann wrote:
> > On Mon, Mar 7, 2022 at 5:48 PM Robin Murphy <robin.murphy@arm.com> wrote:
> > 
> > > And arguably it's not even too late, because 10 years ago this *did* say
> > > "AArch64". I don't remember all the exact details behind commit
> > > 44b82b7700d0 ("arm64: Fix up /proc/cpuinfo") - this just tickled enough
> > > of a memory to go and look up the git history - but I don't think we
> > > changed any of those fields without a real reason.
> > > 
> > 
> > The patch description does state that this was done for compatibility with
> > 32-bit architectures, which does make some sense. I suppose for similar
> > reasons, the arch/arm/ version of /proc/cpuinfo is now stuck at
> > 'CPU architecture: 7', even for ARMv8 or higher in aarch32 mode.
> > 
> > The part that I find more annoying is how we leave out the one bit
> > of information that people are generally looking for in /proc/cpuinfo:
> > the name of the processor. Even though we already know the
> > exact processor type in order to handle the CPU errata, this is
> > always "model name\t: ARMv7 Processor rev %d (v7l)" on 32-bit,
> > and "model name\t: ARMv8 Processor rev %d (%s)" on 64-bit,
> > with the revision being the least important bit of information here...
> 
> Eh, it's hardly impossible to recompose a MIDR value from the implementer,
> part, variant and revision fields if one actually needs to. Maybe we could
> null-terminate the raw MIDR value and print it as a string of
> largely-unprintable characters in the "model name" field... I guess that
> might satisfy the crowd who want parity* with x86 CPUID, at least :)

Actually, it is impossible to do it reliably. I won't expand on this,
except what I said in my other reply - there are cases where the MIDR
value is not unique.

-- 
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