Re: NAK new drivers without proper power management?

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Tilman Schmidt wrote:
If your device requires power management, and you know it requires power
management, why not just implement power management? [...]
Like it or not, power management is far from trivial, and people
writing device drivers have limited resources. [...]
It's not that complex. All we're really talking about is a bit of extra
code to cleanup and configure hardware state; things that the driver
author already knows how to do. S3 might require a bit more
initialisation if firmware needs to be reloaded or more extensive
configuration needs to be done, but if there's firmware to be loaded,
there is a reasonably good probability that we loaded it from Linux to
start with anyway.

You are assuming a perfect world where driver authors have complete
knowledge of their devices. In reality, many drivers (including
those I have the mixed pleasure of maintaining) are based at least
in part on reverse engineering, and managing power states may well
fall into the domain of things not yet sufficiently reverse
engineered.

Also, in your argument you neglected a few cases:
- What if my device does not require power management?
Then you as a generic routine that does nothing but return success
(potentially shared with other drivers that are in the same situation).

But if I just write an empty routine like that I open myself up to
criticism along the lines of "writing dummy routines just in order
to shut up kernel warnings". BTDT.

Well, it couldn't actually be an empty routine (at least not for a PCI device), since the generic PCI suspend/resume handling doesn't get called if you have suspend/resume functions defined, so you'd have to do the pci_save_state, pci_disable_device, pci_set_power_state, etc. in there at least. Using empty functions would prevent those from being called and result in your PCI config space contain crap after resuming, which can break the whole system.

That's a bit messy, actually.. I know we can't just do that stuff unconditionally in the PCI layer since some devices blow up if you disable them, which we normally do, etc. We should just have some generic function you can stick in the .suspend slot that just says "I know how to suspend, but don't need anything more than the generic handling". And if you do need more than that (which is almost always the case), you can just call that from your own function and then add what you need, instead of duplicating the code a million times (sometimes in an incomplete/incorrect fashion) like we do now.

- What if I don't know whether my device requires power management?
The questions are straight forward: Is there hardware state that needs
to be configured if you've just booted the computer and nothing else has
touched it? If so, that needs to be done in a resume method. Do you need
to clean up state prior to doing the things in the resume method, or
otherwise do things to quiesce the driver? If so, they will need to be
done in the suspend method. The result will be roughly similar to what
you do for module load/unload, except maybe less complete in some cases.

I don't doubt your basic assessment. However it doesn't translate that
easily into a real implementation. In my case, I maintain a USB driver,
so I have to deal with USB specifics of suspend/resume which happen not
to be that well documented. My driver provides an isdn4linux device but
isdn4linux knows nothing about suspend/resume so I am on my own on how
to reconcile the two. The device itself, though in turn far from trivial,
is actually the least of my worries.

That is one good excuse for not implementing it - the driver is part of a framework which does not handle suspend/resume itself. (The current FireWire stack is another case like this.) This is the kind of situation where I would say the driver should just define a suspend function that returns -ENOSYS and then the user would know they have to remove that module, etc. before suspending (there is at least some distro script support for doing that automatically based on a config file), since it hasn't a hope of working after resume until the framework is updated to support suspending.

--
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from [email protected]
Home Page: http://www.roberthancock.com/

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