Re: [PATCH v5 1/2] TTY: Add TTY slave enumeration support

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

On Sat, Jan 26, 2013 at 03:04:33AM +0000, Zheng, Lv wrote:
> Thanks for your suggestions.
> All your concerns are against the needs of creating such slave devices.
> Let me delete all of the others and reply once.
> 
> > > In order to send uevent, a device need to be a class device or a bus
> > > device. This patch introduces a tty_enum bus since the enumerated
> > > slave devices are expected to be physical devices.
> > 
> > Again, tty devices are already class devices, and they send out uevents.
> > You can see this today by watching the uevent stream using a tool like
> > 'udevadmin monitor'.
> 
> Please consider the following real scenario when a ttyS0 driver module gets loaded at kernel runtime:
> 1. the tty controller driver creates ttyS0

Fine.

> 2. user gets uevent notified and doesn't know how to use this tty device

Of course, that's normal.

> 3. a slave ACPI node under the ttyS0 gets enumerated and attributes gets appended

How will that happen?  Who causes that to happen?  When would that
happen?

> 4. user need to do user space effort to regenerate the uevent to get a Bluetooth hci user driver matched

Whatever caused step 3 to happen can send a new uevent saying something
has changed.  That's a one line patch.

> If the step 3 can be put ahead of step 2, there might not be weakness without this enhancement.

Yes.

> Considering a more complex scenario:
> There are two or more target devices under the ttyS0 and can be selected by some external GPIOs.
> Actually, BIOS will do this putting many test devices under the ttyS0.
> The step 3 then can never be put ahead of step 2.
> (It might be implemented using some exist kernel mechanisms like the SCSI contribute container devices...)

No, don't look at SCSI for sysfs ideas or driver core usage, it's a
mess and is wrong in places.  Look at how USB and PCI do it if you are
curious how to implement a bus.

But again, this isn't a bus, this is a TTY class, and again, we already
have one, so use it, don't create another one to stack on top of it,
that's crazy.

greg k-h