netdev - re your change to the low_latency flag in the hso driver in 2.6.29-rc1

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Date:	Mon, 12 Jan 2009 18:23:53 +0100
From:	Denis Joseph Barrow <D.Barow@...ion.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Linux USB kernel mailing list <linux-usb@...r.kernel.org>,
	Linux netdev Mailing list <netdev@...r.kernel.org>,
	Paul Hardwick <P.Hardwick@...ion.com>
Subject: re your change to the low_latency flag in the hso driver in 2.6.29-rc1

Hi Alan,
I remember removing the low_latency flag to 0 once & if I recall correctly performance 
was severely impacted & there may have been other problems which I don't recall.
even worse I believe the code can now hang the kernel, the fix is simple, in
put_rxbuf_data I'm dependent on the TTY_THROTTLED bit to exit the loop.
Now tty_insert_flip_string can return 0 bytes leading to an infinite loop if tty buffers are full,
as write_length_remaining never goes to zero as I am dependent on the line
discipline being called from tty_flip_buffer_push to set TTY_THROTTLED by calling flush_to_ldisc
checking if curr_write_len=0 from tty_insert_flip_string might fix the problem
Please look at put_rxbuf_data carefully, there might be other gremlins. The serial port
is supposed to be high performance this fix will impact this

-- 
best regards,
D.J. Barrow

Linux Kernel Developer
Option NV, Gaston Geenslaan 14, 3001 Leuven, Belgium

T: +32 16 311 621
F: +32 16 207 164
d.barow@...ion.com
www.option.com

Disclaimer:
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