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

Arnd Bergmann
2026-05-04T19:05:54Z

On Mon, May 4, 2026, at 20:32, Yury Norov wrote:
> On Mon, May 04, 2026 at 07:18:49PM +0200, Arnd Bergmann wrote:
>> On Mon, May 4, 2026, at 18:46, Yury Norov wrote:
>> > Never heard about such a thing like "optional interface". And git grep
>> > tends to second that...
>>
>> I meant any library interface that can be turned on or off
>
> So? If I disable CRC32, can I use the either_crc()? In case of that
> networking header, the answer is yes. In some other piece of code
> the answer is no. Is that correct?

Since it's a macro defiend in terms of both bitref32 and
crc32_le, you can only call it from dead code, such as an
inline function that is not itself used, or from inside of
a block that is protected with IS_ENABLED(CONFIG_CRC32) etc.

>> >>
>> >> 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.
>> >
>> > linux/acpi.h does that like many othes. What exactly is wrong with
>> > protecting headers inclusion?
>>
>> There is no "protecting" here, you just add complexity to the
>> build when headers are sometimes included indirectly and but
>> other times are not, depending on kernel configuration.
>
> Sorry, don't understand... I use the 'protecting' term with the meaning:
> the functionality that is explicitly disabled should be never used.
> Otherwise, what for we disable it?

Arguably, both configuration symbols are at the point of not actually
saving enough object code size to actually be worth the Kconfig
dependencies.

As long as we have CONFIG_CRC32 and CONFIG_BITREVERSE, the
point of having the Kconfig symbols is to let drivers request
the inclusion of the library helpers.

>> It's unlikely to cause problems for the crc32.h header, but
>> the acpi example definitely risks running into circular
>> inclusions when you end up with some other header that depending
>> on configuration ends up including linux/acpi.h while also
>> bring included indirectly from that one.
>>
>> >> 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.
>> >
>> > OK, it seems like this is what I don't understand.
>> >
>> > We've got an optional feature, like CRC32, which is enabled by
>> > CONFIG_CRC32. The most conservative way is to declare everything
>> > CRC32-related in the corresponding header, and then protect the header
>> > with IS_ENABLED(CONFIG_CRC32).
>> >
>> > I understand that from practical perspective, we can declare some simple
>> > macros, like header size, unprotected. But what we've got now is a sort
>> > of mess: all CRC32-related functions are declared unprotected, and
>> > generic headers are good to use them. Compiler is happy while those
>> > functions are actually unused. Next, CRC32 depends on BITREVERSE, which
>> > is again unprotected, and it may optionally have an arch implementation.
>> >
>> > So if arch bitrev() is implemented, you can use part of bitreverse and
>> > crc32 APIs despite that they are explicitly disabled - just because they
>> > are implemented as macros in unprotected headers. And you cannot use some
>> > others - because they are implemented differently, as a real functions.
>>
>> I think you trying to solve a non-problem here.
>
> This was reported by Nathan for tinyconfig. At least x86 and s390 are
> affected.
>
> https://lore.kernel.org/all/20260429202922.GA3575295@ax162/
>
> Is tinyconfig important?

Nathan reported a build regression caused by a small mistake
in 596a9ea9015b ("bitops: Define generic __bitrev8/16/32 for reuse"),
which is of course needs to be fixed.

What I meant is that there is no reason to not use the obvious
fix and do

--- a/include/asm-generic/bitops/__bitrev.h
+++ b/include/asm-generic/bitops/__bitrev.h
@@ -2,7 +2,6 @@
#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
#define _ASM_GENERIC_BITOPS___BITREV_H_

-#ifdef CONFIG_GENERIC_BITREVERSE
#include <asm/types.h>

extern u8 const byte_rev_table[256];
@@ -20,6 +19,5 @@ static __always_inline __attribute_const__ u32 generic___bitrev32(u32 x)
{
return (generic___bitrev16(x & 0xffff) << 16) | generic___bitrev16(x >> 16);
}
-#endif /* CONFIG_GENERIC_BITREVERSE */

#endif /* _ASM_GENERIC_BITOPS___BITREV_H_ */

> Right now half CRC32 is available if CONFIG_CRC32 is on, and half is
> not available. The bitreverse is the same. If HAVE_ARCH_BITREVERSE is
> enabled, one can use the API, bypassing the BITREVERSE. This doesn't
> sound right to me long-term.
>
> Whatever this ends up, let's figure out a consistent solution please?

I really don't think we need any sort of solution here, aside from
the trivial regression fix that returns it to the previous working
state.

Arnd