Re: [PATCH] USB: serial: use tty_port_register_device_serdev

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, May 02, 2024 at 02:24:41PM +0100, Måns Rullgård wrote:
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
> 
> > On Thu, May 02, 2024 at 11:45:44AM +0100, Måns Rullgård wrote:
> >> Johan Hovold <johan@kernel.org> writes:
> >> 
> >> > On Thu, May 02, 2024 at 11:07:28AM +0100, Mans Rullgard wrote:
> >> >> Use tty_port_register_device_serdev() so that usb-serial devices
> >> >> can be used as serdev controllers.
> >> >
> >> > I'm afraid it's not that easy. The reason serdev is not enabled for
> >> > usb-serial is that there's currently no support for handling hotplug in
> >> > serdev. The device can go away from under you at any time and then you'd
> >> > crash the kernel.
> >> 
> >> Oh, that's unfortunate.  Regular serial ports can go away too, though,
> >> and that seems to be handled fine.  What am I missing?
> >
> > How is it handled?  Normal serial ports can go away but in practice,
> > it's a rare occurance, and usually people use serdev for devices where
> > the ports can not be removed (i.e. internal connections).
> 
> If I unbind a regular serial port from its driver using sysfs, a serdev
> device defined in a device tree gets removed as expected.  Binding the
> serial port makes everything come back again.  I fail to see any problem
> here.  If there is one, you'll have to be less evasive in explaining
> what it is.

Try yanking a usb-serial device out with this patch applied and see what
happens.  I'm pretty sure serdev will not handle that well, just like if
you yank out a pci serial device while it is being used.  Doing
bind/unbind is not a "surprise" removal, but a nice orderly one :)

If this does now work, nice, but I haven't seen the changes to serdev to
make this happen, I wonder what changed...

thanks,

greg k-h