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

[Bruce Ashfield]

On 11-12-12 06:17 PM, Darren Hart wrote:
> On 12/11/2011 09:14 PM, Bruce Ashfield wrote:
>> 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.

Sorry for the slow reply, I got bogged down trying to write some
python code yesterday.

>
> directly = git repository commits?


Sort of. Or what I meant by directly was the "patch <foo>" right
in standard.scc. As you well know, for the most part, standard pulls in
patches by including other features, and those are just as direct
as a patch/commit listed right in the standard kernel .scc file.

The yocto kernels have thus far had relatively few additional features,
but if that changes, the standard kernel will have more direct changes
being applied to the branches.

>
>>>
>>> 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.
>
> What is the value of having base and standard as separate branches?
> Isn't base easily derivable from the standard git history? Still if it
> isn't used anywhere, then the branch is a no-op and adds no complexity,
> so that's fine.

base can definitely be derived from git history, or via git merge-base,
but the branch is a very clear delineation of the jumping point
and contains things like the initial commits to .gitignore. It also
provides a common branch point for the meta branch and standard, so
really basic things can be shared. It has also been used in the past
for a simple branch to build the stock korg kernel from where we jump
into added content.

It also reduces complexity in the tools by having a defined/known
branch name that can be used to detect the type of a repo.

So nothing earth shattering, but a series of smaller reasons.

>
>>
>> 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.
>
> OK, that's no good. My moving patches down to base was intended to avoid
> just that problem. And given the above, leaving them in standard is
> preferred. OK, that poses some issues.... discussed below.
>
>> 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.
>
> The above is fine. Or rather, any of the above can work. Let's discuss
> which one :-)

Indeed. It is just a matter of shifting things around to where
we want them.

>
>>
>> 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.
>
> I really want to avoid setting a base config, and then backtracking to
> reset it to something else in a derived branch. Chasing those overrides,
> etc can be a massive pain. Much better to get the main SOURCES and
> essential policy into a single branch, then let standard and tiny branch
> from there. Intuitively, Standard and Tiny should be siblings, not
> ancestors of eachother. Keeping this relationship apparent in the
> branching and source and config inheritance should help keep things
> clear and maintainable.

Agreed, and that's the trick, we have source changes and configuration
changes. We want to inherit (not cherry pick) both where possible, but
in the end, we sometimes have to chose.

BSPs are an example of this. We try and drive the patches for boards
down as far as possible until they are common. But sometimes, they are
so evil that they have to locked into a room by themselves. To share
those BSP changes between kernel types, they are essentially cherry
picks, as are their config values.

    yocto/standard/my-bsp
    yocto/standard/preempt-rt/my-bsp

Have the same patches (as can be checked by git patch-id), but they don't
have the same git hash.

The good news is of course that these changes are on the end of branches,
are largely orthogonal to the rest of the kernel (and hence don't
conflict, or need to be shared) and they are automatically done when
updating the kernel. Also to deal with this, the BSPs are split into
kernel type specific options and common options. That way we only
maintain one common set of configuration values for a board and share
it via scc includes.

So I see this as a similar situation/choice, do we go with:

a)
    yocto/standard/tiny

b)
    yocto/standard
    yocto/tiny

c)
    yocto/standard/standard-config-items/
    yocto/standard/tiny

In all options, we'd drive a decent/common configuration into the base
and both standard and tiny would inherit that, so consider that a
constant.

The rest of the issues/properties of the options are:

a) tiny would inherit standard, patches and config. We'd have to
    back off (or exclude (we can do that!)) standard's config values.
    But we get the patch inheritance that we want.

b) tiny and standard are peers, but tiny needs individual merges
    of all features and there aren't any common commits between the
    two outside of base (or korg). So this is the 'source complexity'
    issue.

c) tiny inherits standard, but we move out standard's config values
    into another branch (they are cheap after all). That way
    tiny gets the patches and features, but not the config of standard.
    In this example, we might want to kill "base", since we are adding
    another layering of branches, although one that would be trivial/small.

It may not be obvious, but we will need to have BSPs hanging off
the new tiny kernel type as well.

    yocto/standard/qemuppc
    yocto/tiny/qemppc

So it comes down to this:

   - Is tiny a full kernel type, and takes the "responsibilities" that
     this entails, or is it a refinement and streamlining of existing
     BSPs and kernel types ... i.e. is it a last minute overlay.

I've made both choices in the past and depending on the choice, we
arrange the tree differently.

>
>>
>> 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!
>
> I agree the source-work should not be duplicated.
>
>
>> 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.
>
> Agreed
>
>> (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.
>
> I intend to keep Tiny as config options as much as possible (but I can
> see some new Kconfig options from linux-tiny (or tiny-linux, whatever it
> is) creeping their way in. Still, I prefer tiny/preempt-rt for the
> reasons stated above.

We may need to normalize the names to keep things straight.
There are currently two kernel types in yocto: "standard" and
"preempt-rt". (Other instances of the tooling have more kernel
types).

In this discussion we are talking about adding another kernel
type "tiny". So it would be a peer to preempt-rt, and thus
we wouldn't want to re-use the term "preempt-rt".

Internally in the kernel-cache directories preempt-rt is the "kernel
type" and "rt" is the feature.

So we'd want something more like:

    yocto/standard
    yocto/standard/tiny
    yocto/standard/tiny/rt
    yocto/standard/preempt-rt

(That's just a suggestion, and yes, I admit that can also be
  confusing to someone simply reading the branches).

No matter how you slice it, there will be some duplication between
tiny/rt and preempt-rt, since you'll want both the kernel configs
and the patches from the rt feature (which is most of the preempt-rt
kernel type). So they will have to be merged twice (automatically
during tree generation of course).

.. and BTW. The names (even "standard", are not hardcoded anywhere),
so if adding new options, kernels, etc, makes some name just seem
wrong, we can change it (in a new release). i.e. standard could become
the (over used) "core" branch .. if it makes things read better.

That begs the question. If a patch is good enough for tiny, and it
is properly #ifdef'd, it can be merged to the standard branch, and
once that is done, we'll have a much easier time maintaining tiny +
stacked features, since those features will already have adapted
to the tiny imported features/source changes.

>
>>
>> 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.
>
> Agreed.

This is the way forward. Some variant of the options that I've listed
above.

I've been through a lot of this in the past, so I can make suggestions
and having a discussion like this is very helpful if anyone peering
in wonders what's going on behind the covers, since this is largely
unseen effort.

Cheers,

Bruce

>
> I'll re-work accordingly.
>