[OE-core] a few questions about "COMPATIBLE_MACHINE" variable

[Christopher Larson]

On Mon, Dec 19, 2016 at 10:56 AM, Robert P. J. Day <rpjday at crashcourse.ca>
wrote:

> i suspected as much. still, would consistency dictate using
> parentheses or not? just a style thing.
>

I think it’s largely personal preference. Personally, using a capture when
we don’t need one is ugly to me, so I’d rather see it only used when
needed, but that’s just my opinion. The upside to always using a capture is
it makes it clearer, at a glance, to a new user that it’s not just a word
or list of words, but a regular expression.


> >       next, if the possibilities in a list are REs, what is the
> >       point of explicitly listing, say, "qemuarm|qemuarm64"? would
> >       not the RE "qemuarm" subsume the more explicit "qemuarm64"?
> >       same for the other architectures. (one could suggest that that
> >       entire line could be shortened to "... = (^qemu)".)
> >
> > Just qemu would potentially match future qemu machines which aren’t
> > actually supported, so I don’t think that would be appropriate. It’d
> > match too much.
>
>   i realized that, i was just being technical. :-) in any event, as it
> is, it would *still* match too much, anything starting with
> "qemuarm", for example.
>

Good point, yes.

>         the above seems pretty clear since the following lines would
> appear
> >       to say that *only* the qemux86 is compatible:
> >
> >         linux-yocto-tiny_4.1.bb:COMPATIBLE_MACHINE = "(qemux86$)"
> >         linux-yocto-tiny_4.4.bb:COMPATIBLE_MACHINE = "(qemux86$)"
> >         linux-yocto-tiny_4.8.bb:COMPATIBLE_MACHINE = "(qemux86$)"
> >
> >       which suggests the following passage from the YP kernel dev manual
> is
> >       a bit misleading:
> >
> >         "You must change it to match a list of the machines that your new
> >         recipe supports. For example, to support the qemux86 and
> qemux86-64
> >         machines, use the following form:
> >
> >              COMPATIBLE_MACHINE = "qemux86|qemux86-64"
> >
> >       and if all this is true, then if you're introducing a new machine,
> to
> >       be magnificently pedantic, one should not use:
> >
> >         COMPATIBLE_MACHINE_machinename = "machinename"
> >
> >       but
> >
> >         COMPATIBLE_MACHINE_machinename = "^machinename$"
> >
> >       just to play it safe. am i reading all this correctly?
> >
> >
> > Yes, that’s correct, though we need the trailing $ but not the
> > leading ^, as it’s using re.match, not re.search — meaning it only
> > matches at the beginning of the string, it doesn’t search the string
> > to find a match.
>
>   ah, quite right, i had forgotten that. again, in any event, both the
> code and documentation could probably be tweaked to make all of this
> more obvious.
>

Absolutely agreed. For this regex specifically, an argument could be made
that a lack of a trailing $ is almost certainly a bug. I could argue that
it should either be implicit, though that would be limiting, or failing to
include it could trigger a warning unless you explicitly state you know
what you’re doing :) Hmm..
-- 
Christopher Larson
clarson at kergoth dot com
Founder - BitBake, OpenEmbedded, OpenZaurus
Maintainer - Tslib
Senior Software Engineer, Mentor Graphics
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openembedded.org/pipermail/openembedded-core/attachments/20161219/5fbc0fd4/attachment-0002.html>