[yocto] RFC: User configurable recipe features

[Philip Balister]

On 10/11/2011 07:51 PM, Khem Raj wrote:
> On Tue, Oct 11, 2011 at 3:15 PM, Richard Purdie
> <richard.purdie at linuxfoundation.org> wrote:
>> On Mon, 2011-10-10 at 11:41 -0700, Darren Hart wrote:
>>> As part of working on meta-tiny, I've come across a need (want?) to
>>> present users with the ability to select some set of features in a local
>>> configuration file that will impact the build of the image and a set of
>>> recipes.
>>>
>>> It is currently possible to affect which packages are installed in an
>>> image with variables like POKY_EXTRA_INSTALL. What I'm not finding a way
>>> to do is specify some set of features that will impact how a recipe is
>>> built.
>>>
>>> For example, a user may or may not want networking support or virtual
>>> terminal support in their image. This impacts both the kernel and
>>> busybox (at least). The linux-yocto infrastructure provides us with
>>> config fragment functionality, something similar will need to be added
>>> to busybox. Access to that is still bound to the machine config by means
>>> of the SRC_URI machine override mechanism, but it would be useful to be
>>> able to influence it from the image config or the user's local config.
>>>
>>> For example, when building a tiny image I may decide I do not want VT
>>> nor INET support. I might wish to specify this like this (by removing
>>> them from the default features):
>>>
>>> local.conf:
>>> #CORE_IMAGE_TINY_FEATURES = "VT INET MDEV"
>>> CORE_IMAGE_TINY_FEATURES = "MDEV"
>>>
>>> I would want this to affect linux-yocto-tiny by dropping the vt.cfg and
>>> inet.cfg fragments from the SRC_URI (or from the .scc descriptor files
>>> assembled by the linux-yocto meta indrastructure).
>>>
>>> Busybox would need a similar configuration mechanism, and would also
>>> need to add a "no-vt-support.patch" patch to the SRC_URI to avoid a
>>> bug/oversight in the busybox init routine.
>>>
>>> I'd appreciate some help determining the proper bitbake way of doing
>>> this. I want to avoid having to create a new machine.conf and/or recipes
>>> for every possible combination of features that a user may want to turn
>>> on or off.
>>
>> We have a few mechanisms around for this but its a difficult problem to
>> do totally generically since everyone has their own ideas about what
>> should/shouldn't happen.
>>
>> One tricky aspect is that some people care about package feeds and the
>> output into those needs to be deterministic. This is why DISTRO_FEATURES
>> exist which state things like "does x11 make sense"? This means dbus may
>> or may not be compiled with X but given a set of policy decisions by the
>> distro, the output is determined.
>>
>> Recently we've taken the idea of PACKAGECONFIG on board. This is recipe
>> level policy which can enable/disable features in a given recipe (e.g.
>> does gsteamer depend on and build flac or not?). Whilst we have a high
>> level setup for this for autotools recipes, this is probably something
>> we need to do a more custom implementation of for busybox and the
>> features you mention above would map well to this. It would be good to
>> have a standardised way of representing this (and we may also want to
>> look at moving the kernel feature control towards this variable name
>> too).
>>
>> What we need to be really really careful about is getting the
>> namespacing right and your CORE_IMAGE_TINY_FEATURES = "VT INET MDEV"
>> example above scares me as it mixes up several different things. My
>> worry is for example trying to build two different versions of busybox
>> in the same tmpdir depending on what image you build, for example what
>> does "bitbake core-image-tiny core-image-sato" do?
>>
>> Contrast this to some settings:
>>
>> PACKAGECONFIG_pn-linux-yocto = "vt inet"
>> PACKAGECONFIG_pn-busybox = "mdev"
>>
>> which then mean you have one set of configuration for these recipes and
>> its clear what the bitbake command above would result in.
>>
>> One of the bigger problems we're going to have with tiny is its
>> effectively a different set of distro settings to our normal builds. The
>> side effect of that is that you couldn't share a tmpdir with a "big"
>> build but I'm not sure that is an issue in practise, we just need to do
>> it in a way which doesn't give us the nasty configuration corner cases.
>>
>> So I guess what I'm saying is the end result of your work is likely a
>> "poky-tiny" distro setting which would take the "poky" distro but tweak
>> some pieces for really small images. It would need a separate tmpdir and
>> we should look in the PACKAGECONFIG variable direction for handling
>> recipe specific customisations...
>>
> 
> FWIW I agree it seems like a new distro to me. I think if we add more to mix
> it just will complicate the customizations and may even make it
> difficult to share
> things.

Along the same line of thought, does this overlap with the micro distro?

Philip