[Buildroot] [RFC 3/6] system: add mdev-only /dev management (without devtmpfs)

[Peter Korsgaard <peter@korsgaard.com>]

>>>>> "Thomas" == Thomas Petazzoni <thomas.petazzoni@free-electrons.com> writes:

Hi,

>> Sure, I get that. I just question the sensibility of combining a 6+ year
 >> old kernel with modern user space.

 > Well, it is quite understandable that Luca would like to use a recent
 > and modern Buildroot for his project. But since Buildroot is a bundle
 > of a build infrastructure *and* package recipes, you can't get a recent
 > and modern Buildroot without getting a recent and modern user-space.

Yes, and these new packages may or may not build/work with the old
kernel (and presumably toolchain).

 >> I do, it is yet another slightly different variant of the /dev
 >> handling to confuse new users and complicate the build logic.

 > Well, the /dev handling already has a number of different options, and
 > the user must anyway understand a little bit how a Linux system is
 > working to make a sensible choice between the different options. The
 > default is sane, and will remain the same, so that clueless users
 > continue to get the same experience/result.

Yes, but adding one more option that is only a slight variant of one of
them doesn't help.

When would we remove this option again? When 2.6.32 comes 10 years old?
15?


 >> I agree that it isn't a _LOT_ of complexity, but it isn't non trivial
 >> either.

 > If it "isn't non trivial", then it is trivial, right ? :-)

Argh, the 'non' shouldn't have been there ;)


 >> What I meant is actually if this option is only useful for the (few?)
 >> Buildroot users with <2.6.32 kernels that want mdev support, or also for
 >> others?

 > Yes, it is probably only for people using kernels older than 2.6.32 who
 > need mdev for automatic module loading, or firmware loading.

And we expect that to be a significant amount of people?


 > There might be more users of such a thing that there are users of some
 > of our weird packages. Admitted, a package is just a package and
 > doesn't add any complexity to the core. But the additional complexity
 > brought by Luca's work is relatively small. And his work has in fact
 > uncovered a few issues in our existing code (the fact that we redirect
 > the output of all inittab commands to /dev/null, and that some of them
 > are in fact failing without us noticing).

Packages indeed very rarely add complexity elsewhere. And yes, reviewing
the finer inittab details is indeed good.


 >> Take a look at 2b2af54a5bb6f7e80ccf78f20084b93c398c3a8b in the
 >> kernel. To me it looks quite self contained, so backporting it to
 >> something close to 2.6.32 doesn't look too bad.

 > Indeed. I wonder how many dependencies on other patches it has, though.
 > Luca, can you give it a try and see if it would work for you?

From a quick look (depending on how old the kernel is) I think it is
basically just device_get_nodename():

commit 6fcf53acccf85b4b0d0260e66c692a341760f464
Author: Kay Sievers <kay.sievers@vrfy.org>
Date:   Thu Apr 30 15:23:42 2009 +0200

    Driver Core: add nodename callbacks
    
    This adds the nodename callback for struct class, struct device_type and
    struct device, to allow drivers to send userspace hints on the device
    name and subdirectory that should be used for it.
    
    Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
    Signed-off-by: Jan Blunck <jblunck@suse.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>


 >> I'm not sure I'm really interested in seeing a bunch of patches adding
 >> 'depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_2_6_<old>' / comments to
 >> packages.

 > Right, this I understand. We could keep the current state of things:
 > dependencies are only for >= 3.x kernels. So if something builds with
 > 2.6.38 but not 2.6.32, then we don't care, and we make it depend on >=
 > 3.x. Though I agree some people using 2.6.35 might complain, as they
 > can technically build that thing.

So in other words, <3.x isn't really supported.


 >> But I still need to be convinced that adding this other mdev variant is
 >> the best tradeoff between:
 >> 
 >> - Adding the patches
 >> 
 >> - Ask people to backport devtmpfs to their kernel
 >> 
 >> - Ask people to handle it in a post-build script
 >> 
 >> - Ask people to add a initramfs mounting a tmpfs on /dev + mknod before
 >> executing init
 >> 
 >> - Asking people to stick with user space of the same "maturity" ;) as
 >> their kernel

Or alternatively, extend the S10mdev logic to detect that it need to
create a tmpfs on /dev (by looking for devtmpfs in /proc/filesystems or
detecting that /dev is RO) and have all the fixup there. That way it
atleast doesn't add any more complications to the rest of Buildroot /
users.


>> If there's other people interested in this feature, then it would be
 >> nice to hear from you.

 > I agree getting more feedback from other potential users would be good.
 > However, I fear that many of the people who are stuck with old crappy
 > kernels are also in companies that are not necessarily very open, and
 > we may simply never hear from them.

 > So, as suggested above, Luca, can you try to backport devtmpfs? If that
 > solves the problem, then it's probably a good enough solution.

Or alternatively, one of the other options suggested above.

-- 
Venlig hilsen,
Peter Korsgaard