linux-kernel - Re: serial flow control appears broken

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Date:	Fri, 27 Jul 2007 18:28:28 -0500
From:	Paul Fulghum <paulkf@...rogate.com>
To:	Lee Howard <faxguy@...ardsilvan.com>
Cc:	Tilman Schmidt <tilman@...p.cc>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Robert Hancock <hancockr@...w.ca>,
	linux-serial@...r.kernel.org, tytso@....edu, rmk@....linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: serial flow control appears broken

On Fri, 2007-07-27 at 13:48 -0700, Lee Howard wrote:
> Here's the output:
> 
>             type: 4
>             line: 1
>             line: 760
>              irq: 3
>            flags: 1358954688
>   xmit_fifo_size: 16
>   custom_divisor: 0
>        baud_base: 115200

OK, the FIFO should be enabled.

What is known:

* The error is a hardware FIFO overrun.
  - observed message is in n_tty due to driver setting TTY_OVERRUN

* The RTS/CTS flow control is not involved
  - this is done only by the ldisc in response to buffer levels
  - you verified crtscts is set
  - you did not observed RTS change when 'overflow error' logged
  - you did observe RTS change when application stopped reading

So this seems to be a latency issue reading the receive
FIFO in the ISR. The current rx FIFO trigger level
should be 8 bytes (UART_FCR_R_TRIG_10) which gives the
ISR 694usec to get the data at 115200bps.

IIRC, in 2.2.X kernels this defaulted to 4 bytes
(TRIG_01) which gave a little more time to service the interrupt.

How does the data rate affect the frequency of the overrun errors?
Does 57600bps make them go away?

--
Paul




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