kms howto - is there one?

Felix Miata mrmazda at earthlink.net
Fri Mar 19 21:21:12 UTC 2010


On 2010/03/19 12:00 (GMT-0700) Adam Williamson composed:

> On Fri, 2010-03-19 at 13:32 -0400, Felix Miata wrote:

>> > And as I said, that behaviour is equally 'wrong'. The other distros
>> > you've tried probably aren't using kernel modesetting, hence the
>> > differing behaviour.

>> I'm having a problem understanding how something that has long been
>> functioning acceptably can be considered wrong. 

> It allows you to set modes that the monitor isn't actually capable of.

Who's to decide what a display is capable of? It's fine for a fully automatic
system to assume some super low safety threshhold. It's quite another to
disregard configuration data provided to enable that which cannot be done
automatically.

>> I posit that KMS
>> configurations simply aren't yet mature enough to deal with absent DDC/EDID.

> You posit incorrectly. :) The case of absent monitor information is
> explicitly handled by using a generic monitor configuration which is
> restricted to a maximum 1024x768 resolution, in order to avoid the
> possibility of hardware damage.

Basically the safety threshold has been reduced as a consequence of switching
to KMS. There already was a more sophisticated safety threshold employed via
HorizSync and VertRefresh ranges. Before, the computer was actually allowed
to compute something based on supplied data. Now that data gets ignored in
the absence of considerable additional data acquired from the same computer,
but from a separate utility whose output must be manually copied into a
config file instead of generated automatically and internally as before.

> It occurs to me that drakx11 in Mandriva probably still allows manual
> monitor configuration; do you have to do this on Mandriva to make it
> work?

Sure! I provide HorizSync & VertRefresh ranges, either manually (my normal
routine), or via drakx11, which does little more than provide a brand label,
and HorizSync & VertRefresh ranges, by using the brand and model I provide it
to look them up in some database. Then, X/video driver replicates all by
itself from those two data ranges whatever modelines it requires, without
manual intervention, which in F13 requires several runs of gtf and/or cvt and
a bunch of copy & paste into a config file (aka regression).

> If so, that would explain why it 'works' in Mandriva, selecting a
> monitor (or high-resolution-capable 'generic' monitor definition) in
> Mandriva is equivalent to manually sticking a monitor definition in your
> xorg.conf in Fedora.

But the monitor definition in Mandriva is sufficient by providing HorizSync
and VertRefresh ranges exclusively (same as in F13 with mga). The only
modelines I've ever seen drakx11 insert are the same two old 768x576 at 79 &
768x576 at 100 it did back when the whole distro fit on one CD.

> I don't know anything about how other distributions
> handle it.

Basically the same way Fedora did before KMS, an approximate equivalent to
system-config-display before it was stripped down to output only a very
minimalist xorg.conf.
-- 
"The wise are known for their understanding, and pleasant
words are persuasive." Proverbs 16:21 (New Living Translation)

 Team OS/2 ** Reg. Linux User #211409

Felix Miata  ***  http://fm.no-ip.com/


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