On Thu, 30 Apr 2020 12:43:16 +0200 Cornelia Huck <cohuck@xxxxxxxxxx> wrote: <It's been some time, but this topic has recently popped up again.> > On Mon, 27 Apr 2020 12:10:17 +0200 > Peter Oberparleiter <oberpar@xxxxxxxxxxxxx> wrote: > > > On 23.04.2020 18:20, Cornelia Huck wrote: > > > On Thu, 23 Apr 2020 16:52:24 +0200 > > > Vineeth Vijayan <vneethv@xxxxxxxxxxxxxxxxxx> wrote: > > >> Then we could also change the way ccw_device_call_sch_unregister() > > >> works, where > > >> the subchannel-unregister is happening from an upper layer. > > > > > > Hm, what's the problem here? This seems to be mostly a case of "we did > > > I/O to the device and it appeared not operational; so we go ahead and > > > unregister the subchannel"? Childless I/O subchannels are a bit useless. > > > > Hey Conny, > > > > sparked by your proposal, Vineeth and myself looked at the corresponding > > CIO code and wondered if things couldn't be done in a generally > > better/cleaner way. So here we'd like to get your opinion. > > > > In particular, as it is currently, a child-driver (IO subchannel driver, > > vfio-ccw, etc.) unregisters a device owned by a parent-device-driver > > (CSS), which feels from a high-level-view like a layering violation: > > only the parent driver should register and unregister the parent device. > > Also in case no subchannel driver is available (e.g. due to > > driver_override=none), there would be no subchannel ADD event at all. > > Doesn't the base css code generate the uevent in that case? Just checked again, the code in css_register_subchannel() should indeed take care of the !driver case. But still, even better if we can get rid of it :) > > > > > So, tapping into you historical expertise about CIO, is there any reason > > for doing it this way beyond being nice to userspace tooling that > > subchannels with non-working CCW devices are automatically hidden by > > unregistering them? > > We always had ccw devices behind I/O subchannels, but that has not been > the case since we introduced vfio-ccw, so hopefully everybody can deal > with that. The rationale behind this was that device-less I/O > subchannels were deemed to be useless; I currently can't remember > another reason. > > What about EADM, btw? CHSC does not have a device, and message does not > have a driver. Just checked EADM; it does not have a child device. > > > > > Removing the child-unregisters-parent logic this would also enable > > manual rebind of subchannels for which only a different driver than the > > default one can successfully talk to the child device, though I'm > > unaware of any current application for that. > > Yes. > > Let me think about that some more (no clear head currently, sorry.) Ok, so I've resumed the thinking process, and I think getting rid of the "no I/O subchannel without functional device" approach is a good idea, and allows us to make handling driver matches more similar to everyone else. What changes would be needed? * The whole logic to suppress uevents for subchannels and generate one later will go. (Touches the various subchannel driver, including vfio-ccw.) * ccw_device_todo() can just unregister the ccw device, and there's no longer a need for ccw_device_call_sch_unregister(). (IIUC, this also covers setting disconnected devices offline.) * As the I/O subchannel driver now needs to deal with cases where no ccw device is available, the code for that needs to be checked. (That's probably the most time-consuming task.) Userspace should be fine with I/O subchannels without ccw device, that's nothing new. Does that sound reasonable?
