Update the CDC-ACM gadget code to support the peripheral-to-host
notifications when the tty is opened or closed, or issues a BREAK.
The serial framework code calls new generic hooks; right now only
CDC-ACM uses those hooks.  This resolves several REVISIT comments
in the code.  (Based on a patch from Felipe Balbi.)

Note that this doesn't expose USB_CDC_CAP_BRK to the host, since
this code still rejects USB_CDC_REQ_SEND_BREAK control requests
for host-to-peripheral BREAK signaling (received via /dev/ttyGS*).
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Cc: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Update RX path handling in new serial gadget code to cope better with
RX blockage:  queue every RX packet until its contents can safely be
passed up to the ldisc.  Most of the RX path work is now done in the
RX tasklet, instead of just the final "push to ldisc" step.  This
addresses some cases of data loss:

  - A longstanding serial gadget bug: when tty_insert_flip_string()
    didn't copy the entire buffer, the rest of the characters were
    dropped!  Now that packet stays queued until the rest of its data
    is pushed to the ldisc.

  - Another longstanding issue:  in the unlikely case that an RX
    transfer returns data and also reports a fault, that data is
    no longer discarded.

  - In the recently added RX throttling logic:  it needs to stop
    pushing data into the TTY layer, instead of just not submitting
    new USB read requests.  When the TTY is throttled long enough,
    backpressure will eventually make the OUT endpoint NAK.

Also: an #ifdef is removed (no longer necessary); and start switching
to a better convention for debug messages (prefix them with tty name).
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

These two fixes ensure the new "CDC Composite Device" gadget
fails cleanly when it's loaded on hardware that can't support
this particular gadget driver.
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

This abstracts the "gadget serial" driver TTY glue into a separate
component, cleaning it up and disentangling it from connection state.

It also changed some behaviors for the better:

  - Stops using "experimental" major #127, and switches over to
    having the TTY layer allocate the dev_t numbers.
    
  - Provides /sys/class/tty/ttyGS* nodes, thus mdev/udev support.
    (Note "mdev" hotplug bug in Busybox v1.7.2: /dev/ttyGS0 will
    be a *block* device without CONFIG_SYSFS_DEPRECATED_V2.)

  - The tty nodes no longer reject opens when there's no host.
    Now they can support normal getty configs in /etc/inttab...

  - Now implements RX throttling.  When the line discipline says
    it doesn't want any more data, only packets in flight will be
    delivered (currently, max 1K/8K at full/high speeds) until it
    unthrottles the data.

  - Supports low_latency.  This is a good policy for all USB serial
    adapters, since it eliminates scheduler overhead on RX paths.

This also includes much cleanup including better comments, fixing
memory leaks and other bugs (including some locking fixes), messaging
cleanup, and an interface audit and tightening.  This added up to a
significant object code shrinkage, on the order of 20% (!) depending
on CPU and compiler.

A separate patch actually kicks in this new code, using the functions
declared in this new header, and removes the previous glue.
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>