Re: [PATCH RESEND v7 00/21] drivers/perf: apple_m1: Add Apple A7-A11, T2 SoC support

[Nick Chan]

Will Deacon 於 2025/7/14 夜晚11:12 寫道:
> On Mon, Jun 16, 2025 at 09:31:49AM +0800, Nick Chan wrote:
>> This series adds support for the CPU PMU in the older Apple A7-A11, T2
>> SoCs. These PMUs may have a different event layout, less counters, or
>> deliver their interrupts via IRQ instead of a FIQ. Since some of those
>> older SoCs support 32-bit EL0, counting for 32-bit EL0 also need to
>> be enabled by the driver where applicable.
>>
>> Patch 1 adds the DT bindings.
>> Patch 2-7 prepares the driver to allow adding support for those
>> older SoCs.
> Modulo my nits, the patches look alright to this point...
>
>> Patch 8-12 adds support for the older SoCs.
> ... but I'm not sure if anybody actually cares about these older SoCs
> and, even if they do, what the state of the rest of Linux is on those
> parts. I recall horror stories about the OS being quietly migrated
> between CPUs with incompatible features, at which point I think we have
> to question whether we actually care about supporting this hardware.
The "horror" story you mentioned is about Apple A10/A10X/T2, which
has a big little switcher integrated into the cpufreq block, so when the
cpufreq driver switch between states in the same way as on other
SoCs, on these SoCs that would silently cause a CPU migration. There
is only one incompatible feature that I am aware of which is 32-bit EL0
support. However, since the CPUs in these SoCs does not support
4K pages anyways in practice this is not an issue for as long as
CONFIG_EXPERT is disabled.
>
> On the other hand, if it all works swimmingly and it's just the PMU
> driver that needs updating, then I could get on board with it.

As mentioned above, it does all work fine when CONFIG_EXPERT is not
enabled, and if it is enabled, then 32-bit process may crash with illegal
instruction but everything else will still works fine.

>
> Will
>
Nick Chan