On Mon, 29 Jun 2020 13:56:31 +0200 Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > 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. As an aside, there's another odd construct: the I/O subchannel driver *always* binds to the subchannel device, even if there is a problem, and schedules an unregistration of the subchannel device on error. This was introduced because events from machine check handling are not processed if there isn't a driver (at least I thought back then that it was a good idea.) I think a more correct way to handle this would be to do the following: * If something doesn't work, clean up and return an error in the probe function. The subchannel device stays around, it's just not bound. * Have the css bus do some basic processing for subchannels not bound to any driver (e.g., check dnv/w). This would also make it possible to unregister dead message subchannels if a machine check is received for them (don't know if that's an actual problem in pratice.) > > 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.) I'm actually not sure if unregistration-by-driver is the right thing for most cases (except for something like disconnected device removal), that should be done by the bus. Maybe something for later (don't fear, I don't plan to work on the common I/O layer again :) > * 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.) Had a quick look, doesn't actually look too bad (most places already check for !cdev.) > > Userspace should be fine with I/O subchannels without ccw device, > that's nothing new. > > Does that sound reasonable?
