Re: [PATCH v3] riscv: hwprobe: fix has_fpu() to require D extension only

[Conor Dooley <conor@kernel.org>]

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On Wed, Aug 26, 2026 at 07:53:05AM +0200, Andreas Schwab wrote:
> On Aug 25 2026, Ivy Lopez wrote:
> 
> > The kernel never supports F without D, since D depends on F. As such,
>                             D without F

|The kernel never supports F without D, since D depends on F. As such,
|has_fpu() checking either extension with '||' is incorrect: it
|reports FPU support when only F is present, which is not sufficient
|for D-dependent state save/restore, and weakens
|RISCV_HWPROBE_IMA_FD semantics to "F or D" instead of "F and D".

Thinking about it, this whole thing really is not right.

> The kernel never supports F without D

This is actually correct, the kernel itself doesn't support doing this.

> since D depends on F

But this is not the rationale, the rationale is simplicity of
implementation etc.

> As such,
> has_fpu() checking either extension with '||' is incorrect: it
> reports FPU support when only F is present, which is not sufficient

And this cannot happen, because the kernel will not enable F support if D
is not present, it clears the relevant flag during devicetree/acpi
parsing.

> for D-dependent state save/restore, and weakens
> RISCV_HWPROBE_IMA_FD semantics to "F or D" instead of "F and D".

The reason I would cite for changing the is matching the expectations
set elsewhere in the kernel, rather than impact on userspace or
behaviour, since F without D is not possible so there's no potential
impact here at all.

Cheers,
Conor.

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