linux-kernel - setserial -> serial_core / 8250 interaction

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Date:	Wed, 18 Jul 2007 10:35:08 -0500
From:	"George McCollister" <georgem@...atech-llc.com>
To:	<linux-kernel@...r.kernel.org>
Subject: setserial -> serial_core / 8250 interaction

I've recently found (using 2.6.21.4) that configuring a serial ports
(ST16654) which use the 8250 driver using setserial results in the
UART's FIFOs being disabled (unless you specify autoconfig).

The problem is that setserial doesn't fill out xmit_fifo_size when
calling TIOCSSERIAL to set the port type. There doesn't appear to be any
provision to set the port type and have 8250 use the default fifo_size
value for the selected FIFO from uart_config.

The result is that fifo_size remains 0 and the fifo is disabled. So, I
believe that anyone using setserial to configure a serial port (unless
autoconfig is used) will have very poor serial port performance (I did,
thats how I ran across this).

I haven't been able to think of a cure all solution without introducing
some pretty nasty changes. You could argue that setserial should set
xmit_fifo_size, but it seems unnecessary to require a user level program
to have knowledge of the FIFO sizes for all of the different UARTS. I'm
going to change my copy of 8250.c to always use fifo_size from
uart_config, but some people may not find this acceptable.

Maybe I'm missing something here, but I wanted to bring it up in case
its actually affecting other people.

I'm not subscribed to the list so if you reply please CC to my address.

Regards,
George McCollister

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