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Subject: Re: [PATCH] Airprime driver improvements to allow full speed EvDO
	transfers
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Sergei Organov <osv@javad.com>
Cc: Andrew Morton <akpm@osdl.org>, gregkh@suse.de,
       linux-kernel@vger.kernel.org, linux-usb-devel@lists.sourceforge.net
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Date: Mon, 10 Jul 2006 12:10:55 +0100
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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. The tty buffering is
now a proper queuing system.

> 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:

       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);