Linus Torvalds wrote:
On Tue, 30 Oct 2007, Arjan van de Ven wrote:
the problem is... you're not supposed to mix both types of accesses.
You have to, anyway. Even now the MMCONFIG stuff uses CONF1 cycles for
startup.
If it does, it's not by necessity. As soon as you read the table
location out of the ACPI tables you can start using it, and that
shouldn't require any config space accesses.
Also, there's reason to believe that mixing things up _has_ to work
anyway, and if the issue is between "works in practice" and "theory says
that you shouldn't mix", I'll take practice every time.
Especially since we *know* that the theory is broken. Right now MMCONFIG
is effectively disabled very aggressively because it's simply unusably
flaky. So the choice is between:
- don't use MMCONFIG at all, because it has so many problems
- use MMCONFIG sparingly enough to hide the problems
Fact is, we don't really know how many of these systems with supposedly
"broken" MMCONFIG were really just suffering from the overlapping
PCI/MMCONFIG address space problem, which is entirely the fault of the
way we do PCI probing. I would bet quite a few of them.
and what "you're supposed to do" is simply trumped by Real Life(tm).
Because Intel screwed up so badly when they designed that piece of shit.
(Where "screwed up badly" is the usual "left it to firmware people" thing,
of course. Dammit, Intel *could* have just made it a real PCI BAR in the
Northbridge, and specified it as such, and we wouldn't have these
problems! But no, it had to be another idiotic "firmware tells where it
is" thing)
This wouldn't have helped anything with the problem in question.
The fact is, CONF1 style accesses are just safer, and *work*.
I would suggest a slight twist then: use CONF1 *until* you're using
something above 256, and then and only then switch to MMCONFIG from
then on for all accesses.
No.
Maybe if you do it per-device, and only *after* probing (ie we have seen
multiple, and successful, accesses), but globally, absolutely not. That
would be useless. The bugs we have had in this area have been exactly the
kinds of things like "we don't know the real size of the MMCONFIG areas"
etc.
I could easily see device driver writers probing to see if something
works, and I absolutely don't think we should just automatically enable
MMCONFIG from then on.
Why per device? It's not like the MSI case where both the platform and
the device are potentially busted. Whether or not MMCONFIG works has
nothing to do with the device, all that matters is whether it works on
the platform. It shouldn't be the driver's responsibility to know this.
But maybe we could have a per-device flag that a driver *can* set. Ie have
the logic be:
- use MMCONFIG if we have to (reg >= 256)
OR
- use MMCONFIG if the driver specifically asked us to
and then drivers that absolutely need it, and know they do, can set that
flag. Preferably after they actually verified that it works.
How will they verify that it works? If it works, then verifying it works
is all well and good. If it doesn't work, trying to verify if it does
could very well blow up the machine.
I've made the point before that if we're going to allow using it at all,
we'd better find out if it works or not early on, not after we've been
running and somebody decides it's a good idea to try using it and
causing a lockup or something.
That way you _can_ get the "this is how you're supposed to do it"
behaviour, but you get it when there is a reasonable chance that it
actually works.
And quite frankly, if you're not supposed to mix these things even across
devices, then I think we are better off just doing what we effectively do
now: mostly ignore the damn thing because it's too broken to use.
Maybe somebody inside Intel could just clarify the documentation, and
change it from "you're not supposed to mix" to "mix all you want".
Intel could say what they want on the subject.. but that doesn't
necessarily reflect what happens with anyone else's chipset implementations.
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