kernel-hwcap.h change causing full kernel rebuilds

kernel-hwcap.h change causing full kernel rebuilds

[Jens Axboe]

Hi,

Maybe this has already been reported, but a quick search on lore did
not find anything. So here goes... After commit:

commit abed23c3c44f565dc812563ac015be70dd61e97b
Author: Mark Brown <broonie@kernel.org>
Date:   Mon Mar 2 22:53:16 2026 +0000

    arm64/hwcap: Generate the KERNEL_HWCAP_ definitions for the hwcaps

arch/arm64/include/generated/asm/kernel-hwcap.h is generated for
_every build_, which means that you can literally run make back to back
without _any_ source changes and you are rewarded with a full kernel
rebuild. Not sure how this ever got past any kind of testing, because
it's very noticeable.

For now I've reverted this change locally as any kind of testing now
kicks off a full build, when a single file rebuild is all that is
needed.

-- 
Jens Axboe

Re: kernel-hwcap.h change causing full kernel rebuilds

[Joey Gouly]

Hi!

On Tue, Apr 21, 2026 at 06:52:55AM -0600, Jens Axboe wrote:
> Hi,
> 
> Maybe this has already been reported, but a quick search on lore did
> not find anything. So here goes... After commit:

Here you go!

https://lore.kernel.org/linux-arm-kernel/20260413-arm64-hwcap-gen-fix-v1-1-26c56aed6908@kernel.org/

and on it's in rc1 now:

https://lore.kernel.org/linux-arm-kernel/aeaDFH0-2zhOQTxy@arm.com/


Thanks,
Joey

> 
> commit abed23c3c44f565dc812563ac015be70dd61e97b
> Author: Mark Brown <broonie@kernel.org>
> Date:   Mon Mar 2 22:53:16 2026 +0000
> 
>     arm64/hwcap: Generate the KERNEL_HWCAP_ definitions for the hwcaps
> 
> arch/arm64/include/generated/asm/kernel-hwcap.h is generated for
> _every build_, which means that you can literally run make back to back
> without _any_ source changes and you are rewarded with a full kernel
> rebuild. Not sure how this ever got past any kind of testing, because
> it's very noticeable.
> 
> For now I've reverted this change locally as any kind of testing now
> kicks off a full build, when a single file rebuild is all that is
> needed.
> 
> -- 
> Jens Axboe
> 
> 
> 

Re: kernel-hwcap.h change causing full kernel rebuilds

[Jens Axboe]

On 4/21/26 6:56 AM, Joey Gouly wrote:
> Hi!
> 
> On Tue, Apr 21, 2026 at 06:52:55AM -0600, Jens Axboe wrote:
>> Hi,
>>
>> Maybe this has already been reported, but a quick search on lore did
>> not find anything. So here goes... After commit:
> 
> Here you go!
> 
> https://lore.kernel.org/linux-arm-kernel/20260413-arm64-hwcap-gen-fix-v1-1-26c56aed6908@kernel.org/

Thanks, glad it's fixed, because that was pretty terrible yesterday
until I started pondering wtf any kind of testing was taking literally
forever. Guess I'll rebase my 7.1 based branches after -rc1, anything
post abed23c3c44f565dc812563ac015be70dd61e97b and pre that fix is
basically unusable on arm64 for any kind of development.

-- 
Jens Axboe