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List:       linux-kernel
Subject:    Re: TTY Flip buffers
From:       Linux Lists <lists () cyclades ! com>
Date:       1999-11-30 22:08:43
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On Tue, 30 Nov 1999 tytso@mit.edu wrote:
> 
> How many characters to you read from the Z at a time?  Presumably you
> want to try to read as many characters as possible, in order to optimize
> for processing time, even though this has a negative impact on latency
> --- otherwise, why have a 8k hardware receive buffer in the first place.

Yes, I read as much as I can. The number is defined by:

MIN(char_count, MIN(tty->ldisc.receive_room(tty), buf_size))

, where:

- char_count = number of characters in the on-board (HW) buffer;
- buf_size = size of the intermediary driver buffer;

> If you reading that a large number of characters at a time, and getting
> periodic interrupts, I'd advice that you skip the flip buffers
> altogether and simply call ldisc->receive_buf() directly.  As a bonus,
> this avoids a memory copy as well.  You can use ldisc->receive_room() to
> see how much space is available in the 4k tty buffer.   (This is what
> the Comtrol Rocketport driver does).

The Rocketport driver I have (i.e., the one present in 2.2.13) does _not_
use this function. Probably you have already released a newer driver with
this check. Anyhow, just after I sent the msg to you I figured that out by
looking at n_tty.c.

>    Anyhow, what's this flow control signal the tty driver sends to the serial
>    driver when the buffer starts getting full?? Is this a flag that stays
>    there until the condition change back to normal, or is it a one-time
>    signal??
> 
> The high-level tty code calls driver->throttle() and
> driver->unthrottle() as necessary.  See include/linux/tty_driver.h and
> include/linux/tty_ldisc.h --- the interfaces are fairly well
> documented.  

Yes, I saw that too, just after I sent the msg. Sorry for the unnecessary
questions, and thank you very much for all the detailed answers !!

Regards,
Ivan


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