svn commit: r294362 - in head/sys: dev/uart kern sys

[Ian Lepore]

On Tue, 2016-01-19 at 23:34 +0000, Marius Strobl wrote:
> Author: marius
> Date: Tue Jan 19 23:34:27 2016
> New Revision: 294362
> URL: https://svnweb.freebsd.org/changeset/base/294362
> 
> Log:
>   Fix tty_drain() and, thus, TIOCDRAIN of the current tty(4) incarnation
>   to actually wait until the TX FIFOs of UARTs have be drained before
>   returning. This is done by bringing the equivalent of the TS_BUSY flag
>   found in the previous implementation back in an ABI-preserving way.
>   Reported and tested by: Patrick Powell
>   
>   Most likely, drivers for USB-serial-adapters likewise incorporating
>   TX FIFOs as well as other terminal devices that buffer output in some
>   form should also provide implementations of tsw_busy.
>   
>   MFC after:> 	> 3 days
> 
> Modified:
>   head/sys/dev/uart/uart_tty.c
>   head/sys/kern/tty.c
>   head/sys/sys/ttydevsw.h

This rocks.

It would rock even more if uart's sc_txbusy actually meant what its
name implies, but for some hardware what it really indicates is "the
hardware can/cannot accept more data" which is not the same as "all
data has been transmitted."  Even for hardware that doesn't signal
txidle until the fifo is completely empty (not just at a lowater mark),
there may still be a last byte on the way out from a tx holding
register.

-- Ian