linux-kernel - uart : lost characters when system is busy

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Date:	Fri, 10 Jun 2011 10:58:17 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<linux-serial@...r.kernel.org>, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: uart : lost characters when system is busy

Hi,

the linux uart layer can loose some characters if the system is busy.

uart_throttle/uart_unthrottle is called from a workqueue.
If the system is busy, and the uart receive lot's of data, we fill the tty
buffer, but the workqueue doesn't run and we never have a chance to call
uart_throttle. So the uart is never slow down.

And because most uart driver call uart_insert_char (that doesn't return if
tty_insert_flip_char manage to push the character), we never detect that there
are some lost characters.

A workaround could be to check the buffer threshold in tty_flip_buffer_push and
call throttle callback if needed.

Matthieu
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