Re: [PATCH 2/2] Fix console handling during suspend/resume

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> It's not up to the block driver, that's the thing.
> 
> The _user_mode_ requests should just be stopped (you don't need to block 
> the queue - you just stop the processes). Then you wait for the queue to 
> drain. End of story.
> 
> BUT YOU DON*T STOP THE DEVICE QUEUE. Because if you did, that would mean 
> that you couldn't save the state.

My experience has been that relying on userland being stopped -> no more
driver activity isn't going to work. Things like read-ahead or other
niceties (not even talking about filesystems that scrub things in the
background or kernel based web/nfs servers ;)  will defeat that.

> Note the "often". Quite often you can't. Things like cardbus controllers 
> will be set up by the POST to have all the right things, and you literally 
> can't re-generate it, because it depends on the motherboard. That's when 
> you save it.

Yeah, well, video cards enter in that category... I have code that can
bring back some radeon's from D3 cold for powermac provided I saved a
whole bunch of registers beforehand.

> (Or, even more commonly, you save it just because it's easier than 
> regenerating it)
> 
> > Of course they do. Or we have a different notion of what you call
> > "state" here...
> 
> Yes. I think that's the main stumbling block.
> 
> You consider "state" to be everything, whether needed or not. And I don't. 
> I consider "state" to be the things that "resume()" _requires_ to get 
> going again, which is actually a lot lot smaller.
> 
> And exactly because I don't think it means "every bit", _my_ viewpoint 
> actually matches reality. It matches - for example - _exactly_ what we 
> already do wrt X. We (and here, the "we" is obviously mostly the X server) 
> need to save enough state to _recreate_ the state before suspend, but that 
> does not need that we need to save each bit.

I agree that your viewpoint matches reality, it's just that I wouldn't
have called it 'state' :) 

> It's actually also what a lot of drivers already do. Several drivers' 
> suspend routines don't actually need to save anythign at all, they just 
> turn the device off - exactly because they can recreate all the state 
> _without_ saving anything.

Ben.