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

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Date:	Fri, 10 Jun 2011 10:20:26 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Matthieu CASTET <matthieu.castet@...rot.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<linux-serial@...r.kernel.org>
Subject: Re: uart : lost characters when 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.

You should have around 64K of buffering (actually n_tty worst case
should be 63.5Kbyte) that's a lot of slack so what is holding off the
work queue for so long on your problem system ? I think that should be
answered first as it sounds like some other part of your kernel is
misbehaving.

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

tty_flip_buffer_push can be called from an IRQ, the throttle callback
needs to be able to sleep.

What might work if it is needed though is to provide a tty_is_throttled()
method that a driver can use to check the instantaneous throttle state.
Trouble is that will require a lot of care on the drivers part to deal
with asynchronus throttle/unthrottle events while peering at the state in
its IRQ handler as well.

Anyway - question for the case you hit, what tasks or work held off the
serial work queue for 63.5Kbytes of data ?
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