[yocto] The role of "distro" and "image"

[Richard Purdie]

On Thu, 2011-12-15 at 10:54 -0700, Chris Larson wrote:
> On Thu, Dec 15, 2011 at 10:29 AM, Koen Kooi <koen at beagleboard.org> wrote:
> > Op 13 dec. 2011, om 22:45 heeft Chris Larson het volgende geschreven:
> >
> >> On Tue, Dec 13, 2011 at 2:36 PM, Richard Purdie
> >> <richard.purdie at linuxfoundation.org> wrote:
> >>> Not all images can be built with a given distro, our base config is
> >>> rather ride ranging though for obvious reasons. You could "enforce" this
> >>> by adding some anonymous python to the distro which does something like:
> >>>
> >>> if inherits("image") and PN != core-image-minimal:
> >>>    raise SkipPackage("Image PN not compatible with DISTRO=XXX")
> >>
> >> This is a case where one is best 'controlling' this via policy, not
> >> mechanism, in my opinion. This sort of arbitrary technical limitation
> >> tends to be foolish and often bites someone somewhere down the line.
> >> I'm sure you just wanted to note how it could be done, not recommend
> >> that it should be done, but I thought it should be made clear. I
> >> wouldn't recommend that anyone do this unless there is an extremely
> >> good reason for it.
> >
> > We had a similar problem at work, people were sprinkling
> COMPATIBLE_MACHINE left and right just to mark "I tested recipe X on
> machine Y". You can guess what happened when new machines needed to
> get added.
> 
> Haha, ouch. It's also worth taking a moment to emphasize that the way
> distro/machine/image is structured is by design, not accidental.
> Having these pieces be orthogonal buys us a great deal of flexibility
> and capability, which is why we did it this way. Now, whether a given
> distro/machine/image combination is *useful* is a different question,
> and one I think is best addressed via policy / documentation.

I understand the flexibility but I also think we need to consider
usability a little. We have people using the system to whom it might not
be immediately obvious why building a sato image with the tiny distro is
a bad idea.

If you know enough to be able to make that work, the warning is easily
bypassed and isn't going to bother you.

So I think there is a case for warning the user if the combination
they're selecting is not one that is likely to work and we can easily
predict it. There are many combinations we can't predict which is fine.

Like any mechanism, it can be abused of course and using
COMPATIBLE_MACHINE as a sign of recipe testing clearly isn't a good idea
or what it was designed for.

Cheers,

Richard