Re: [RFD] uevent handling for subchannels

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 29 Jun 2020 13:56:31 +0200
Cornelia Huck <cohuck@redhat.com> 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?