2024-02-23 09:06:36

by Johannes Berg

Subject: [PATCH] bitfield: suppress "dubious: x & !y" sparse warning

From: Johannes Berg <[email protected]>

There's a somewhat common pattern of using FIELD_PREP()
even for single bits, e.g.

cmd->info1 |= FIELD_PREP(HTT_SRNG_SETUP_CMD_INFO1_RING_FLAGS_MSI_SWAP,
!!(params.flags & HAL_SRNG_FLAGS_MSI_SWAP));

which might as well be written as

if (params.flags & HAL_SRNG_FLAGS_MSI_SWAP)
cmd->info1 |= HTT_SRNG_SETUP_CMD_INFO1_RING_FLAGS_MSI_SWAP;

(since info1 is fully initialized to start with), but in
a long chain of FIELD_PREP() this really seems fine.

However, it triggers a sparse warning, in the check in
the macro for whether a constant value fits into the mask,
as this contains a "& (_val)". In this case, this really
is always intentional, so just suppress the warning by
adding "0+" to the expression, indicating explicitly that
this is correct.

Signed-off-by: Johannes Berg <[email protected]>
---
include/linux/bitfield.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index ebfa12f69501..63928f173223 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -66,7 +66,8 @@
_pfx "mask is not constant"); \
BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero"); \
BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
- ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
+ ~((_mask) >> __bf_shf(_mask)) & \
+ (0 + (_val)) : 0, \
_pfx "value too large for the field"); \
BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) > \
__bf_cast_unsigned(_reg, ~0ull), \
--
2.43.2

2024-02-28 11:59:33

by Kalle Valo

Subject: Re: [PATCH] bitfield: suppress "dubious: x & !y" sparse warning

Johannes Berg <[email protected]> wrote:

> From: Johannes Berg <[email protected]>
>
> There's a somewhat common pattern of using FIELD_PREP()
> even for single bits, e.g.
>
> cmd->info1 |= FIELD_PREP(HTT_SRNG_SETUP_CMD_INFO1_RING_FLAGS_MSI_SWAP,
> !!(params.flags & HAL_SRNG_FLAGS_MSI_SWAP));
>
> which might as well be written as
>
> if (params.flags & HAL_SRNG_FLAGS_MSI_SWAP)
> cmd->info1 |= HTT_SRNG_SETUP_CMD_INFO1_RING_FLAGS_MSI_SWAP;
>
> (since info1 is fully initialized to start with), but in
> a long chain of FIELD_PREP() this really seems fine.
>
> However, it triggers a sparse warning, in the check in
> the macro for whether a constant value fits into the mask,
> as this contains a "& (_val)". In this case, this really
> is always intentional, so just suppress the warning by
> adding "0+" to the expression, indicating explicitly that
> this is correct.
>
> Signed-off-by: Johannes Berg <[email protected]>

Patch applied to wireless-next.git, thanks.

416eb60317c6 bitfield: suppress "dubious: x & !y" sparse warning

--
https://patchwork.kernel.org/project/linux-wireless/patch/20240223100146.d243b6b1a9a1.I033828b1187c6bccf086e31400f7e933bb8373e7@changeid/

https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches