[PATCH 1/3] RISC-V: Improve use of isa2hwcap[]

[Andrew Jones]

On Sun, Oct 23, 2022 at 08:28:20PM +0100, Conor Dooley wrote:
> On Fri, Oct 21, 2022 at 12:59:03PM +0200, Andrew Jones wrote:
> > Improve isa2hwcap[] by removing it from static storage, as
> > riscv_fill_hwcap() is only called once, and by reducing its size
> > from 256 bytes to 26. The latter improvement is possible because
> > isa2hwcap[] will never be indexed with capital letters and we can
> > precompute the offsets from 'a'.
> 
> Hey Drew, couple questions for you - mostly due to naivety I think..
> 
> How do we know that isa2hwcap will never interact with capital letters?
> It pulls the isa string from dt and the no-capitals enforcement comes
> from there since one with capitals is invalid? I didn't dig particularly
> deeply into the code, but is there a risk that we regress some user that
> has a dt with capitals in the isa string? Or is that a "your dt was wrong
> and you're out-of-tree so that's your problem" situation?

Our ISA string parser here in riscv_fill_hwcap() ignores non-capital
letters, see arch/riscv/kernel/cpufeature.c lines 141, 165 and 196.
(Maybe we should be noisier about that in case there are DTs out there
with capitals which aren't realizing they're being ignored.)

> 
> Secondly, in the UAPI header, the COMPAT_HWCAP_ISA_FOO defines are
> computed as I - A rather than i - a. Should those be changed too for the
> sake of consistently using the lowercase everywhere, or do you think
> that doesn't really matter?

I think I'd leave it since it doesn't change the math and I'm reluctant
to churn UAPI.

Thanks,
drew