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

[Robert P. J. Day]

On Mon, 19 Dec 2016, Christopher Larson wrote:

> On Mon, Dec 19, 2016 at 8:17 AM, Robert P. J. Day <rpjday at crashcourse.ca> wrote:
>         as a starting point, COMPATIBLE_MACHINE is processed on a
>       per-recipe basis, and if it has no value, then there is no
>       machine restriction being applied to that recipe, correct?
>       that's based on this snippet from base.bbclass:
>
>         need_machine = d.getVar('COMPATIBLE_MACHINE')
>         if need_machine:
>             import re
>             compat_machines = (d.getVar('MACHINEOVERRIDES') or "").split(":")
>             for m in compat_machines:
>                 if re.match(need_machine, m):
>                     break
>             else:
>                 raise bb.parse.SkipPackage("incompatible with machine %s (not in COMPATIBLE_MACHINE)" %
>       d.getVar('MACHINE'))
>
>       so far, so good?
>
>         next, the documentation describes the value of that variable
>       as a regular expression, so the values are processed as RE
>       patterns, but some of the actual uses are confusing. from
>       poky/meta/recipes-kernel/linux:
>
>       $ grep -r COMPATIBLE_MACHINE *
>       linux-dummy.bb:#COMPATIBLE_MACHINE = "your_machine"
>       linux-yocto_4.1.bb:COMPATIBLE_MACHINE = "qemuarm|qemuarm64|qemux86|qemuppc|qemumips|qemumips64|qemux86-64"
>       linux-yocto_4.4.bb:COMPATIBLE_MACHINE = "qemuarm|qemuarm64|qemux86|qemuppc|qemumips|qemumips64|qemux86-64"
>       linux-yocto_4.8.bb:COMPATIBLE_MACHINE = "qemuarm|qemuarm64|qemux86|qemuppc|qemumips|qemumips64|qemux86-64"
>       linux-yocto-dev.bb:COMPATIBLE_MACHINE = "(qemuarm|qemux86|qemuppc|qemumips|qemumips64|qemux86-64)"
>       linux-yocto-rt_4.1.bb:COMPATIBLE_MACHINE = "(qemux86|qemux86-64|qemuarm|qemuppc|qemumips)"
>       linux-yocto-rt_4.4.bb:COMPATIBLE_MACHINE = "(qemux86|qemux86-64|qemuarm|qemuppc|qemumips)"
>       linux-yocto-rt_4.8.bb:COMPATIBLE_MACHINE = "(qemux86|qemux86-64|qemuarm|qemuppc|qemumips)"
>       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$)"
>       $
>
>         first, what is best practice -- to use parentheses or not?
>       i'm assuming it makes no difference, but it does seem
>       inconsistent and could cause some confusion.
>
> They’re not needed in a case like this. 

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

>       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.

>         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.

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================