Output character lost with 8250 UART driver

Output character lost with 8250 UART driver

[ZhangXiao]

Hi,

When I boot one of my target I find the UART(8250) works improperly.
Sometimes lost output characters. When I dig in the 8250.c I find
something strange in function transmit_chars. 

Below is the very piece of this function:

...
1	count = up->tx_loadsz;
2	do {
3		serial_out(up, UART_TX, xmit->buf[xmit->tail]);
4		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
5		up->port.icount.tx++;
6		if (uart_circ_empty(xmit))
7			break;
8	} while (--count > 0);
...

Based on theses lines, characters should be written to the THR
continuous, no matter if it had already move the character to the TSR. I
think it is not safe in some cases so I add a line

	wait_for_xmitr(up, UART_LSR_THRE);

before line 3. And then the issue in my target was disappeared. This is
a static function that was already exist in the same file. It just check
and wait until the THR was empty.

I noticed that in the same file (8250.c), in console part, all data
written to THR was protected by the "wait_for_xmitr". So for the
function "transmit_chars", does that a bug and should be resolved? Or
there are any reasons that it needn't this protection?

Thanks a lot
Xiao

RE: Output character lost with 8250 UART driver

[Chris Doré]

> -----Original Message-----
> From: ZhangXiao
> Sent: June 25, 2008 4:07 AM
> 
> Based on theses lines, characters should be written to the THR
> continuous, no matter if it had already move the character to the TSR. I
> think it is not safe in some cases so I add a line
> 
> 	wait_for_xmitr(up, UART_LSR_THRE);
> 
> before line 3. And then the issue in my target was disappeared. This is
> a static function that was already exist in the same file. It just check
> and wait until the THR was empty.

Do you have the FIFO's disabled, but transmit_chars() thinks they are
enabled?
Sounds to me like that might be your problem and without the
wait_for_xmitr() call you are overrunning the transmitter.  In other words,
if you have FIFOs disabled, up->tx_loadsz should be 1.

What UART are you using?


..Chris