[PATCH] riscv: hwprobe: simplify has_fpu() to check D extension only

[PATCH] riscv: hwprobe: simplify has_fpu() to check D extension only

[Ivy Lopez]

The kernel never supports D without F, since D depends on F. The
D-extension flag is cleared during devicetree/ACPI parsing whenever
F is not present, so has_fpu() checking either extension with '||'
never actually produces a different result than checking D alone -
F without D cannot occur in practice, and there is no observable
impact on RISCV_HWPROBE_IMA_FD or userspace.

Simplify has_fpu() to check D only, matching the expectations set
elsewhere in the kernel for this dependency, rather than relying on
a redundant OR condition.

sys_hwprobe.c already calls has_fpu() and needs no changes.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=221874
Suggested-by: Conor Dooley <conor.dooley@microchip.com>
Suggested-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Ivy Lopez <skunkolee@gmail.com>
---
Changes in v4:
- Rewrite commit message: drop the "weakens semantics"/incorrect-report
  framing (Conor pointed out F-without-D cannot occur since the D flag
  is cleared during devicetree/ACPI parsing - confirmed in
  riscv_ext_f_validate(), arch/riscv/kernel/cpufeature.c - so there's
  no actual behavioral impact). Reframe as a simplification matching
  existing kernel conventions, not a correctness fix.
- Fix "F without D" -> "D without F" (Andreas).

Changes in v3:
- Drop the sys_hwprobe.c hunk entirely: it already calls has_fpu(),
  no change needed there since v1 was never merged.
- New thread per maintainer request, not a reply to v1/v2.
---
 arch/riscv/include/asm/switch_to.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 0e71eb82f920..8186cda88e17 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -60,8 +60,8 @@ static inline void __switch_to_fpu(struct task_struct *prev,
 
 static __always_inline bool has_fpu(void)
 {
-	return riscv_has_extension_likely(RISCV_ISA_EXT_f) ||
-		riscv_has_extension_likely(RISCV_ISA_EXT_d);
+	/* D extension depends on F, so checking D alone is sufficient. */
+	return riscv_has_extension_likely(RISCV_ISA_EXT_d);
 }
 #else
 static __always_inline bool has_fpu(void) { return false; }
-- 
2.55.0

Re: [PATCH] riscv: hwprobe: simplify has_fpu() to check D extension only

[Conor Dooley]

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On Mon, Aug 31, 2026 at 07:37:46PM -0600, Ivy Lopez wrote:
> The kernel never supports D without F, since D depends on F. The
> D-extension flag is cleared during devicetree/ACPI parsing whenever
> F is not present, so has_fpu() checking either extension with '||'
> never actually produces a different result than checking D alone -
> F without D cannot occur in practice, and there is no observable
> impact on RISCV_HWPROBE_IMA_FD or userspace.
> 
> Simplify has_fpu() to check D only, matching the expectations set
> elsewhere in the kernel for this dependency, rather than relying on
> a redundant OR condition.
> 
> sys_hwprobe.c already calls has_fpu() and needs no changes.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=221874
> Suggested-by: Conor Dooley <conor.dooley@microchip.com>
> Suggested-by: Andreas Schwab <schwab@suse.de>
> Signed-off-by: Ivy Lopez <skunkolee@gmail.com>

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>

Thanks,
Conor.

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Re: [PATCH] riscv: hwprobe: simplify has_fpu() to check D extension only

[Paul Walmsley]

On Mon, 31 Aug 2026, Ivy Lopez wrote:

> The kernel never supports D without F, since D depends on F. The
> D-extension flag is cleared during devicetree/ACPI parsing whenever
> F is not present, so has_fpu() checking either extension with '||'
> never actually produces a different result than checking D alone -
> F without D cannot occur in practice, and there is no observable
> impact on RISCV_HWPROBE_IMA_FD or userspace.
> 
> Simplify has_fpu() to check D only, matching the expectations set
> elsewhere in the kernel for this dependency, rather than relying on
> a redundant OR condition.
> 
> sys_hwprobe.c already calls has_fpu() and needs no changes.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=221874
> Suggested-by: Conor Dooley <conor.dooley@microchip.com>
> Suggested-by: Andreas Schwab <schwab@suse.de>
> Signed-off-by: Ivy Lopez <skunkolee@gmail.com>

Thanks, updated to apply and queued for v7.3-rc.


- Paul