Re: [gentoo-dev] Re: Moving CPU flags into USE_EXPAND

[Andrew Savchenko]

On Wed, 21 Jan 2015 01:13:08 +0000 (UTC) Duncan wrote:
> Andrew Savchenko posted on Tue, 20 Jan 2015 23:59:23 +0300 as excerpted:
>
> > On Tue, 20 Jan 2015 12:17:35 -0800 Christopher Head wrote:
> >> On January 20, 2015 12:47:03 AM PST, Alexis Ballier
> >> <aballier@g.o> wrote:
> >> >So, you're telling me that if you have a list of 90 cpu extensions,
> >> >you will from time to time open that list to see if there is a 91st
> >> >one added ? I think most people won't even notice, at best they'll
> >> >look for the changelog.
> >>
> >> No, actually, I’m advocating the exact opposite. I’m saying that, as
> >> long as the list file is kept up to date, then I will look at those 90
> >> flags when I first install and never again. If a 91st flag appears some
> >> day, then as long as the file was maintained as I described in an
> >> earlier message (i.e. flags are added as soon as manufacturers announce
> >> features), I already know I can reliably ignore the new flag. After
> >> all,
> >> if the flag didn’t exist when I installed the system, then my CPU must
> >> necessarily not have that feature—unless CPUs are in the habit of
> >> sprouting new instructions after you buy them!
> >
> > Not exactly. CPUs are not in a habit, but software is. Some brand new
> > instuction set may be supported in (any of) packages with some delay.
> > Thus it is possible that instruction set supported by your CPU will
> > appear in the list of cpu flags after your ininial install.
>
> PMFJI...
>
> chead's idea is (I believe) simply to have the description file updated
> with all current hardware feature flags as soon as they are known (he
> said announced, but sometimes they change between announcement and
> actually appearing in hardware, so "known", as in "known to actually
> appear in hardware", would seem to be better).
>
> That way, no matter what the software supported at the time and what
> flags were thus actually used in packages, when someone first installs
> gentoo on a new machine, or when they first upgrade their CPU to
> something with new features, they can run the script and update their
> use_expand to match their hardware _ONCE_, without worrying about whether
> or when a package with support for it might appear.
>
> If no package with that support ever appears, no harm done, that entry in
> the use_expand is simply never used.
>
> OTOH when some package /does/ get support for new hardware instructions
> and adds the appropriate flag, it'll appear in portage's output, but
> because the use_expand was already set when gentoo was installed or the
> cpu upgraded, the user won't have to worry about needing to look it up
> and decide whether to set it, again, it'll already be done, back when the
> _hardware_ changed, _not_ sometime later, when the _software_ changed to
> support the new hardware.
>
> Of course if the user upgrades hardware after a package supports a
> feature, they'll have to upgrade their use_expand setting appropriately
> or miss support for the new instructions, but that's always the case.
> Just if handled as chead suggests, it'd be the case ONLY when the
> hardware is updated, instead of every time a package upgrades its own
> support.
>
> Correct, chead?  Does that make things clearer, aballier and bircoph?

Yeah, thanks. I misunderstood original intention. It's all clear
now.

Best regards,
Andrew Savchenko