linux-kernel - Re: [PATCH] Add the ability to layer another driver over the serial driver

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Date:	Mon, 11 Dec 2006 17:15:21 +0000
From:	Alan <alan@...rguk.ukuu.org.uk>
To:	Corey Minyard <cminyard@...sta.com>
Cc:	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Tilman Schmidt <tilman@...p.cc>, linux-serial@...r.kernel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Hansjoerg Lipp <hjlipp@....de>, Russell Doty <rdoty@...hat.com>
Subject: Re: [PATCH] Add the ability to layer another driver over the serial
 driver

> I was actually wrong, flush_to_ldisc does handle reentrancy.
> It can only have one caller to disc->receive_buf() at a time.  So
> long chains of recursion don't seem to be possible, even if called
> from IRQ context.

disc->receive_buf is single threaded but if it then sends characters back
in the same context (eg flow control) you get re-entry in the driver.

> And studying the way ppp does writing, it can bypass the tty_write()
> call and directly call the drivers.  So that bypasses the transmit
> locking problems I saw.

tty_write() is the layer above the ldisc. tty_write() feeds a line
discipline from (usually) user space. Line disciplines write direct to
the tty.

> 
> This is going to require some more thought.  But I believe it can be
> done with adding a poll routine to the tty_operations structure 

What status do you need to poll ?
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