Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: Kernel version numbers after 4.9.255 and 4.4.255
To:     Jiri Slaby <jirislaby@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
References: <a85b7749-38b2-8ce9-c15a-8acb9a54c5b5@kernel.org>
From:   Tony Battersby <tonyb@cybernetics.com>
Cc:     linux-kernel@vger.kernel.org,
        Jari Ruusu <jariruusu@protonmail.com>,
        David Laight <David.Laight@ACULAB.COM>,
        Christoph Biedl <linux-kernel.bfrz@manchmal.in-ulm.de>
Message-ID: <0b12bac9-1b4e-ec4a-8a45-5eb3f1dbbeca@cybernetics.com>
Date:   Fri, 5 Feb 2021 12:31:05 -0500
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <a85b7749-38b2-8ce9-c15a-8acb9a54c5b5@kernel.org>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Precedence: bulk

On 2/4/21 6:00 AM, Jiri Slaby wrote:
> Agreed. But currently, sublevel won't "wrap", it will "overflow" to 
> patchlevel. And that might be a problem. So we might need to update the 
> header generation using e.g. "sublevel & 0xff" (wrap around) or 
> "sublevel > 255 : 255 : sublevel" (be monotonic and get stuck at 255).
>
> In both LINUX_VERSION_CODE generation and KERNEL_VERSION proper.

My preference would be to be monotonic and get stuck at 255 to avoid
breaking out-of-tree modules.  If needed, add another macro that
increases the number of bits that can be used to check for sublevels >
255, while keeping the old macros for compatibility reasons.  Since
sublevels > 255 have never existed before, any such checks must be
newly-added, so they can be required to use the new macros.

I do not run the 4.4/4.9 kernels usually, but I do sometimes test a wide
range of kernels from 3.18 (gasp!) up to the latest when bisecting,
benchmarking, or debugging problems.  And I use a number of out-of-tree
modules that rely on the KERNEL_VERSION to make everything work.  Some
out-of-tree modules like an updated igb network driver might be needed
to make it possible to test the old kernel on particular hardware.

In the worst case, I can patch LINUX_VERSION_CODE and KERNEL_VERSION
locally to make out-of-tree modules work.  Or else just not test kernels
with sublevel > 255.

Tony Battersby
Cybernetics