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

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Message-ID: <20090112174531.682f49ea@lxorguk.ukuu.org.uk>
Date:	Mon, 12 Jan 2009 17:45:31 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Denis Joseph Barrow <D.Barow@...ion.com>
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: re your change to the low_latency flag in the hso driver in
 2.6.29-rc1

> 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.

The modern buffering and old buffering are rather different so that
shouldn't be a problem.

> 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

In all cases tty_insert_flip_string may return 0 for other reasons (eg
out of memory).

> 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

Will do, but it should have no impact at all on throughput - in fact it
ought to improve it. You can't call tty_flip_buffer_push() from an
IRQ handler with tty->low_latency set anyway...

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