Re: [PATCH 1/6] lib: include crc32.h conditionally on CONFIG_CRC32

["Arnd Bergmann" <arnd@arndb.de>]

On Thu, Apr 30, 2026, at 23:13, Yury Norov wrote:
> Currently, bitreverse API is either declared based on
> CONFIG_HAVE_ARCH_BITREVERSE, wired to arch implementation, or if the
> arch has no bitreverse, based on generic implementation.
>
> So, regardless of CONFIG_BITREVERSE=n, the corresponding API is always
> declared. If that happens, the functions become declared but not
> implemented, which is an error.

I'm not following that description. Why is it an error to declare
a funtion that is not implemented? Isn't that how optional interfaces
tend to work in general?

> The only header requiring the crc32 and bitreverse prototypes is
> include/linux/etherdevice.h. Thus, protect inclusion of corresponding
> headers in the etherdevice with CONFIG_CRC32, together with the only
> function depending on it.
...
>  #include <linux/if_ether.h>
>  #include <linux/netdevice.h>
>  #include <linux/random.h>
> +#ifdef CONFIG_CRC32
>  #include <linux/crc32.h>
> +#endif
>  #include <linux/unaligned.h>
>  #include <asm/bitsperlong.h>

Don't add #ifdef blocks around headers. If the header cannot
be included without side-effects, change the linux/crc32.h
file instead of its users.

It looks like the problem is the check for CONFIG_GENERIC_BITREVERSE
in include/asm-generic/bitops/__bitrev.h, which ends up
hinding the generic___bitrev32() helper without need.

Simply removing the #ifdef there should avoid the build failure.

> +#ifdef CONFIG_CRC32
>  /**
>   * eth_hw_addr_crc - Calculate CRC from netdev_hw_addr
>   * @ha: pointer to hardware address
> @@ -291,6 +294,7 @@ static inline u32 eth_hw_addr_crc(struct netdev_hw_addr *ha)
>  {
>  	return ether_crc(ETH_ALEN, ha->addr);
>  }
> +#endif

I see there are only user users of this function, neither of
them are performance critical. So the other options would
be to either open-code this function in the two callers
and remove it entirely, or move it into net/ethernet/eth.c.

      Arnd