linux-kernel - Re: [PATCH] kbuild: give SUBLEVEL more room in KERNEL

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Message-ID: <20210118133821.GY4035784@sasha-vm>
Date:   Mon, 18 Jan 2021 08:38:21 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     masahiroy@...nel.org, michal.lkml@...kovi.net,
        torvalds@...ux-foundation.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH] kbuild: give SUBLEVEL more room in KERNEL_VERSION

On Mon, Jan 18, 2021 at 10:21:16AM +0100, Greg KH wrote:
>On Sun, Jan 17, 2021 at 08:49:51PM -0500, Sasha Levin wrote:
>> SUBLEVEL only has 8 bits of space, which means that we'll overflow it
>> once it reaches 256.
>>
>> Few of the stable branches will imminently overflow SUBLEVEL while
>> there's no risk of overflowing VERSION.
>>
>> Thus, give SUBLEVEL 8 more bits which will be stolen from VERSION, this
>> should create a better balance between the different version numbers we
>> use.
>>
>> The downside here is that Linus will have 8 bits less to play with, but
>> given our current release cadence (~10 weeks), the number of Linus's
>> fingers & toes (20), and the current VERSION (5) we can calculate that
>> VERSION will overflow in just over 1,000 years, so I'm kicking this can
>> down the road.
>>
>> Cc: stable@...nel.org
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>> ---
>>  Makefile | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/Makefile b/Makefile
>> index 9e73f82e0d863..dc2bad7a440d8 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -1252,8 +1252,8 @@ endef
>>
>>  define filechk_version.h
>>  	echo \#define LINUX_VERSION_CODE $(shell                         \
>> -	expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 0$(SUBLEVEL)); \
>> -	echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))'
>> +	expr $(VERSION) \* 16777216 + 0$(PATCHLEVEL) \* 65536 + 0$(SUBLEVEL)); \
>> +	echo '#define KERNEL_VERSION(a,b,c) (((a) << 24) + ((b) << 16) + (c))'
>
>As much as I agree, this will break in-tree users of LINUX_VERSION_CODE
>that try to suck out the version/patchlevel number of the kernel release
>into their own fields.  Things like USB host controller strings, v4l
>ioctl reports, scsi driver ioctls, and other places do fun bit-movements
>to try to unreverse this bit packing.
>
>So how about we just provide a "real" version/subversion/revision
>#define as well, and clean up all in-kernel users, so we can get this to
>work, and we can change it in the future more easily.

Uh, yes, I see what you mean. I'll fix those up and resend.

-- 
Thanks,
Sasha