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Re: Maintaining a custom out-of-tree patched Debian kernel for specific hardware

On Tue, 2018-01-23 at 18:20 +0530, Kumar Appaiah wrote:
> On Tue, Jan 23, 2018 at 10:27:06AM -0200, Antonio Terceiro wrote:
> > On Mon, Jan 22, 2018 at 07:38:41PM +0530, Kumar Appaiah wrote:
> > > Dear Debian Developers,
> > > 
> > > I am part of a team working on getting Debian on low cost laptops
> > > (see
> > > http://www.rdp.in for details) so that they can be sold with
> > > Debian
> > > preinstalled. While vanilla Debian largely works, unfortunately,
> > > making Bluetooth and sound work require kernel rebuilding. The
> > > patches
> > > and config changes are not likely to be upstreamed any time soon,
> > > so
> > > we would have to ship the laptop with a patched (non-Debian
> > > kernel). Our team is, thus, taking up the responsibility of
> > > ensuring
> > > that up-to-date kernel (with security fixes) are made available
> > > to the
> > > users of our laptop. The patches and configs are adapted from
> > > here:
> > > https://github.com/sundarnagarajan/kernel_build
> > 
> > Are the the patches you need are really just those in patches/? If
> > yes,
> > they are small enough that if I were in your place I would talk to
> > the
> > Debian kernel team to see if they can be included in the official
> > Debian
> > kernel.
> > 
> > Also, the patches being that small, what's stopping them from
> > being upstreamed?
> This is something beyond my understanding. Other distributions, such
> as Linux Mint, Ubuntu etc. also do not possess those patches and
> config changes. My hunch is that the Cherry Trail processors may not
> be considered popular enough for inclusion in mainline at least
> yet. So, we are going with this approach for now. Naturally, when
> that's done, we'll offer a migration path to the stock Debian kernel
> and the need for this hack will be gone.

The patches appear to be one new ACPI match[0] and some changes to some
debug messages[1] (marked with a ".optional" suffix, so probably not
really needed). Forking the kernel packages for those trivial changes
is _way_ overkill for a first response. If those patches had been
submitted them upstream then I would expect them to be quickly and
easily merged into the relevant maintainer tree and thus be elligible
for application to the Debian kernels.

I don't know if the kernel config changes/requirements in [2] are
complete but if they are then they seem equally trivial and certainly
worth of a discussion with the Debian kernel maintainers before
deciding to fork.

Even simple forks are deceptively hard to back away from so avoiding
forking for trival reasons is usually a good default, or else you could
easily find yourself stuck with it for an extended period of time.

Googling around suggests that Cherry Trail is supported in mainline
Linux today, with some last issues having been resolved in 4.11 (see
e.g. [3]).


[0] https://github.com/sundarnagarajan/kernel_build/blob/master/patches/001_rfkill_bluetooth_rtl8723bs_bt.patch
[1] https://github.com/sundarnagarajan/kernel_build/blob/master/patches/002_rtl8723bs_nolinked_power_save_enter.patch.optional
[2] https://github.com/sundarnagarajan/kernel_build/blob/master/config.prefs
[3] https://liliputing.com/2017/03/linux-4-11-brings-improvements-intel-atom-pcs-bay-trail-cherry-trail.html