Re: comments on irc log

[Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>]

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On Sat, 2005-03-19 at 00:18 +0100, Pavel Machek wrote:
> Hi!
> 
> > > Usually very fast.  Unfortunately for some kinds of USB host controllers 
> > > it can be relatively slow, since quiescing the controller has an 
> > > unavoidable side effect of suspending all devices on the bus.
> > 
> > I wonder if we need to quiesce the controller in fact for FREEZE.
> > Probably not. Just stop all queue processing and refuse URBs. The hcca
> > will still get updated, but who cares ? it will end up beeing saved in
> > an inconsistent state in the suspend image, so what ? On resume, we will
> > have rebooted, we can "clean it up".
> > 
> > This is sort-of breaking the rule of "no DMA", and thus is not suitable
> > for kexec (which is ok, kexec currently uses the separate "shutdown"
> > callback which must switch DMA off), but would fix the problem for
> > suspend to disk...
> 
> What problem? suspend seems to +/- work with suspend-to-disk just
> now. I'd really hate to have to think about "some memory may change
> behind my back" during suspend. I think "no DMA" is a good rule.

Well, it's not that simple. It may work for you and not for others, and
it will definitely introduce complications with the current scheme since
it seems we +/- have to suspend USB busses (and possibly disconnect some
devices) at freeze time...

Note that this will become a non-issue when instead of waking everybody
up, we just wake the devices on the disk path, we can do the real
suspend initially for the others.

For now, it might be interesting to not shut down the OHCI (not sure
about E/UHCI's tho) as just letting it touch the HCCA isn't an issue
(only for the freeze before snapshot tho, not when getting rid of the
loader kernel, but in this case, what we do is more like kexec and we
might prefer using shutdown callbacks to that effect).

Ben.



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