[yocto] linux-yocto: ktypes/tiny and some questions along the way

[Bruce Ashfield]

On 11-12-09 5:52 PM, Darren Hart wrote:
> Bruce,
>
> I'm looking at introducing a new kernel type, ktypes/tiny.
>
> Tiny will define a core set of kernel policy options, such as proc, sys,
> devtmpfs, futex, epoll, elf bin format, etc. It will not enable any
> drivers, filesystems, debug options, or networking options by default.
> This would be the responsibility of the BSP to add a named feature
> implementing this.
>
> Taking a look at the ktypes we have now resulted in some questions:
>
> dvhart at envy:~/source/linux/linux-yocto-3.0/meta/cfg/kernel-cache/ktypes
> $ tree
> .
> ├── base
> │   ├── base.cfg
> │   ├── base.scc
> │   ├── hardware.cfg
> │   ├── hardware.kcf
> │   ├── non-hardware.cfg
> │   └── non-hardware.kcf
> ├── preempt-rt
> │   ├── preempt-rt.cfg
> │   └── preempt-rt.scc
> ├── standard
> │   ├── perf-force-include-of-stdbool.h.patch
> │   ├── perf-hard-code-NO_LIBPERL-NO_LIBPYTHON.patch
> │   ├── standard-patches.scc
> │   ├── standard.cfg
> │   ├── standard.scc
> │   └── x86-add-TIF_32BIT-compatibility-define.patch
>
> These form a hiearchy, each inheriting from the layer beneath like so:
>
> base/standard/preempt-rt
>
> As I dig into this I see that some policy is infact laid down by base,
> including things like:
>
> CONFIG_MODULES=y
> CONFIG_INET=y
> CONFIG_PREEMPT=y
> CONFIG_NFS_FS=y
> CONFIG_MSDOS_PARTITION=y
>
> These pull in the IP stack, the block layer, etc. Because of this, I
> can't really inherit from base for ktypes/tiny. I would like to inherit
> the hardware and non-hardware kcf files though, as well as any patches
> that might make their way into base. Which leads me to standard. I would
> like to see a much smaller set of config policy options set in base.
> Most likely these should be exactly what we agree on for tiny, and tiny
> wouldn't add any additional policy as it implements only what is
> required for a Yocto Project built kernel.
>
> NOTE: kernel version is 3.0+
> $ cat base/base.scc | head -n 1
> set_kernel_version 2.6.37

FYI: nothing uses this at the moment. But I fixed that here.

>
> There are three patches in standard, only two of which (the perf ones)
> are listed in the standard.scc. As I believe any kernel we build should
> have these, I would like to see any global patches applied to base,
> leaving standard to define policy, and include named features.

standard only includes 3 patches because they were a bit hard to
classify elsewhere. In a more fully populated kernel, it does include
quite a bit more directly.

>
> With these changes, I could add ktypes/tiny as follows:
> $ cat tiny/tiny.scc
> include ktypes/base
> branch tiny
> include features/xyz/xyz.scc
> include cfg/abc.scc
>
> Another point of interest is preempt-rt, as I can see people wanting to
> build tiny preempt-rt. I think the best approach here is to create
> ktypes/tiny/preempt-rt-tiny/preempt-rt-tiny.scc.
>
> Note: I believe this fails with .patch.patch
> $ cat features/rt/rt.scc | grep patch
> patch rt-apply-patch-3.0.10-rt27.patch.patch

Love my "patch.patch". Script went wild on that one, luckily the content
in the branch is king :) I fixed that here when I was auditing 3.0.12+rt
so that looks more sane now.

>
> I'm working on a patch series that implements the suggestions I've made
> above. Bruce, do you have any issues with this approach?

Allow me to ramble a bit below ...

No big issues. At some level(s) it is just a shell game. We can move
configs and patches around to the appropriate level to get the building
blocks that we need and get common functionality into a common location.

base was (is) intended to document the branch point from the mainline
kernel, and contain largely configuration and very few patches. Any
important or larger size functional additions go into the standard
kernel.

So branching from base for a new kernel type is something we can do, but
it will risk missing additions (say for example tracing fixes, or the
next super-duper debug via kprobes patch series), since they'll go into
the standard kernel. If I just slide all the patches down into base,
why not just call 'base' 'standard' or just make standard have the
configuration you are working on for tiny.

The point I'm attempting to make, is that the base/common point can be
whatever we decide it needs to be, "standard" can be renamed, content
moved up and down, etc. i.e. standard could have it's config changed,
a new branch created off standard ( and options moved there), or
standard  could be streamlined and the boards include more common
config options manually vs inheriting them, etc.

I need a bit more time to think about that myself.

Another option is to let tiny inherit from standard, but force a
new baseline for configuration options. i.e. your allnoconfig
technique buried in the middle of inheritance tree. With the
optional/required designations I'm finishing up for 1.2 you won't get
hit with reams of redefined configuration warnings. With that set, you
can then go about adding kernel type specific options/features/patches
as we see fit .. and you'll pickup those features that I was talking
about above. If you bear with me by doing some of this manually for
now (setting the baseline), it is easy to add a construct to do that
reset automagically.

It's the features and patch balancing that's the hard part, not getting
the configs how we want them. I've lived the madness of the other!

What we don't want to get into (I've been there, with an old "small"
kernel type) is having a different stack of patches and fixes on
multiple kernel types. The patches start failing, and you end up always
merging a fix in two different ways due to changes in the feature mix
on each kernel type. (This comment more applies to the tiny and -rt
mix). Better mileage might be gained by doing.

   yocto/standard/preempt-rt/tiny

versus moving the big patch onto yocto/tiny/preempt-rt, but that implies
that tiny is providing a set of configuration values that clobber what
has been set by both standard and -rt .. something that trades patch 
maintenance with config maintenance.

Working through all that, what I'm saying is that I'd prefer to make
standard do what you want, and keep the base similar to what it is.
We can shuffle undesirable standard configuration items up versus
shuffling all patches and functionality down to yocto/base. But the
approach is fundamentally the same.

i.e. I see no reason why the tiny config wouldn't be fine as
the standard config or as something that makes significant changes
to standard's configuration. But definitely something that wants the
patches and features in standard.

Cheers,

Bruce






>