Re: Linux tty layer hackery: Heads up and RFC

From: Alan Cox
Date: Tue Jul 26 2005 - 05:47:59 EST

On Maw, 2005-07-26 at 10:55 +0100, Mark Underwood wrote:
> What my driver would like to do is to handle its own
> input buffers. It would pass the buffer to the tty
> layer when it is full and the tty layer would pass the

In theory you can do that already, although the locking is a bit screwed
up for it. Actually all the tty locking is broken for rx I believe.
Everyone should be holding the tty read lock when updating flip buffers
but right now we don't

> buffer back once it has drained the data from it.
> The problem is that I don't always receive a block
> worth of characters so I also need to pass the tty
> layer a buffer (which I'm still DMAing into) with a
> count of how many chars there are in the buffer and a
> offset of where to start from.

You can do this now providing you don't do it blindly from IRQ context.

>From a workqueue do

struct tty_ldisc *ld = tty_ldisc_ref(tty);
int space;

if(ld == NULL) /* Bin/defer */
space = ld->receive_room(tty);
if(count > space) count = space;

ld->receive_buf(tty, charbuf, flagbuf, count);

There is a corner case if TTY_DONT_FLIP is set where you should queue
but not all drivers do this and the DONT_FLIP hack 'has to die'

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