What's the rationale behind sending a Xoff character when the port is stopped ?

What's the rationale behind sending a Xoff character when the port is stopped ?

[Karthik Manamcheri]

Hi,

I noticed in drivers/tty/serial/8250.c that when we transmit
characters (by calling transmit_chars), we check for uart_tx_stopped
only after sending the x_char (if any). What's the rationale behind
this ? I would expect the uart not to send ANY characters (including a
Xoff character) if its throttled (or stopped).

-- 
Karthik Manamcheri
Software R&D | Instrument Control Products | National Instruments
http://www.linkedin.com/in/kmanamcheri
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Re: What's the rationale behind sending a Xoff character when the port is stopped ?

[Alan Cox]

On Mon, 7 May 2012 15:00:50 -0500
Karthik Manamcheri <karthik.manamcheri@gmail.com> wrote:

> Hi,
> 
> I noticed in drivers/tty/serial/8250.c that when we transmit
> characters (by calling transmit_chars), we check for uart_tx_stopped
> only after sending the x_char (if any). What's the rationale behind
> this ? I would expect the uart not to send ANY characters (including a
> Xoff character) if its throttled (or stopped).

We implement xon/xoff flow control. Therefore we need to send characters
when throttled.

Think about the case where you are flow controlled for transmit and your
receive buffer fills up...

Alan

Re: What's the rationale behind sending a Xoff character when the port is stopped ?

[Karthik Manamcheri]

In the case when I am flow controlled (meaning I received a Xoff from
the other end) and my receive buffer fills up, sending a Xoff to the
other end might overflow the buffer for the other guy, right ? Is that
Ok ?

On Mon, May 7, 2012 at 3:50 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> On Mon, 7 May 2012 15:00:50 -0500
> Karthik Manamcheri <karthik.manamcheri@gmail.com> wrote:
>
>> Hi,
>>
>> I noticed in drivers/tty/serial/8250.c that when we transmit
>> characters (by calling transmit_chars), we check for uart_tx_stopped
>> only after sending the x_char (if any). What's the rationale behind
>> this ? I would expect the uart not to send ANY characters (including a
>> Xoff character) if its throttled (or stopped).
>
> We implement xon/xoff flow control. Therefore we need to send characters
> when throttled.
>
> Think about the case where you are flow controlled for transmit and your
> receive buffer fills up...
>
> Alan



-- 
Karthik Manamcheri
Software R&D | Instrument Control Products | National Instruments
http://www.karthikmanamcheri.com
http://www.linkedin.com/in/kmanamcheri
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Re: What's the rationale behind sending a Xoff character when the port is stopped ?

[Grant Edwards]

On 2012-05-07, Karthik Manamcheri <karthik.manamcheri@gmail.com> wrote:

> In the case when I am flow controlled (meaning I received a Xoff from
> the other end) and my receive buffer fills up, sending a Xoff to the
> other end might overflow the buffer for the other guy, right?

Not really.  The Xoff doesn't go into the other guy's receive buffer.
It's processed by the flow-control software (or hardware) before it
gets to the receive buffer.

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