Re: [PATCH] Airprime driver improvements to allow full speed EvDO transfers

Sergei Organov
2006-07-10T15:54:42Z

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> Ar Llu, 2006-07-10 am 14:36 +0400, ysgrifennodd Sergei Organov:
>> However, the problem is easily seen for USB-to-tty drivers where there
>> are no UARTS anywhere and speeds are rather high so that more than 4096
>> bytes (the line discipline buffer size) could be received before a task
>> has a chance to read from the line discipline buffer, and single flip
>> size is not limited by the hardware.
>
> There are no flip buffers in 2.6.17, they've gone.

Sure they gone, I've used "flip" because of tty_flip_buffer_push() has "flip"
in its name.

> The tty buffering is now a proper queuing system.

Yes, I know it is.

>
>> Moreover, looking into the source code I don't see how tty can take care
>> not to over-stuff the ldisc. ldisc`s receive_buf() routine doesn't tell
>> the caller how many chars it actually consumed and silently throws away
>
> Not in the current kernel tree. The current tree does this:

Which tree? I see rather different code in the 2.6.17.4, see below.

>
> spin_lock_irqsave(&tty->buf.lock, flags);
> head = tty->buf.head;
> if (head != NULL) {
> tty->buf.head = NULL;
> for (;;) {
> int count = head->commit - head->read;
> if (!count) {
> if (head->next == NULL)
> break;
> tbuf = head;
> head = head->next;
> tty_buffer_free(tty, tbuf);
> continue;
> }
> if (!tty->receive_room) {
> schedule_delayed_work(&tty->buf.work, 1);
> break;
> }
> if (count > tty->receive_room)
> count = tty->receive_room;
> char_buf = head->char_buf_ptr + head->read;
> flag_buf = head->flag_buf_ptr + head->read;
> head->read += count;
> spin_unlock_irqrestore(&tty->buf.lock, flags);
> disc->receive_buf(tty, char_buf, flag_buf, count);
> spin_lock_irqsave(&tty->buf.lock, flags);
> }
> tty->buf.head = head;
> }
> spin_unlock_irqrestore(&tty->buf.lock, flags);

Wow! The code you've quoted seems to be correct! But where did you get
it from? The version of flush_to_ldisc() from drivers/char/tty_io.c from
2.17.4 got last Friday from kernel.org does this:

spin_lock_irqsave(&tty->buf.lock, flags);
while((tbuf = tty->buf.head) != NULL) {
while ((count = tbuf->commit - tbuf->read) != 0) {
char_buf = tbuf->char_buf_ptr + tbuf->read;
flag_buf = tbuf->flag_buf_ptr + tbuf->read;
tbuf->read += count;
spin_unlock_irqrestore(&tty->buf.lock, flags);
disc->receive_buf(tty, char_buf, flag_buf, count);
spin_lock_irqsave(&tty->buf.lock, flags);
}
if (tbuf->active)
break;
tty->buf.head = tbuf->next;
if (tty->buf.head == NULL)
tty->buf.tail = NULL;
tty_buffer_free(tty, tbuf);
}
spin_unlock_irqrestore(&tty->buf.lock, flags);

Nowhere tty->receive_room is checked in tty_io.c! Am I missing something
or is this change not yet in the released kernels? If it's not yet
there, where can I get it and when is it expected to reach released
kernels?

--
Sergei.