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

[Robin Murphy <robin.murphy@arm.com>]

On 2022-03-08 17:18, Catalin Marinas wrote:
> 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 :)
> 
> You can get the MIDR from
> /sys/devices/system/cpu/cpu*/regs/identification/midr_el1.
> 
> As for printing the actual names, we thought we'd leave it to tools like
> lscpu. I'm not keen on maintaining a dictionary of MIDR to CPU marketing
> names in the kernel, deal with rebranding and so on. For x86 you can get
> the name from the CPU itself IIUC, that's not the case for arm.

Yes, CPUID reads a string out of the hardware which is the name for the 
*physical product*, which among other things is allegedly useful to 
dissuade unscrupulous people from grinding the markings off and 
re-etching them to fake a higher-spec chip. We obviously can't maintain 
a whole database of SoC names in Linux.

In fact on my x86 box, even lscpu still doesn't tell me what the actual 
CPU cores are other than family 6 model 79, so in that respect Arm 
support is already ahead! :D

Robin.