Disabling the FIFO on the UART - Bug in 8250 mode?
Hello everybody,
I'm doing some serial-line programming on an embedded device which has a very limited UART, which doesn't feature a FIFO. For my tests I send bytes using "echo" or "cat" over the serial line for the device to receive. Since the processing of the received data can take some time, I've enable hardware flow control (RTS/CTS) which works fine - excepted that the PC naturally empty its 16 bytes FIFO before stopping sending bytes. Which means that when my device sets CTS to low, I'm losing all the bytes that the PC has in the FIFO.
This is a normal behavior and I can't do much about it, excepted disabling the FIFO on the PC. This is what I'd like to do, but unfortunately, as states the Linux Serial Howto:
"As of late 2000 there was no way the Linux user could set these [the FIFO buffers settings] directly (setserial can't do it)."
I've read somewhere else what seems to be a workaround to do it anyway:
(http://groups.google.nl/groups?q=stty+disable+fifo&hl=nl&lr=&ie=UTF-8&selm=Printing-HOWTO.1-24498.841892935%40cc.gatech.edu&rnum=8)
7.2. Older serial printers that drop characters
Jon Luckey points out that some older serial printers with ten-cent serial interfaces and small buffers really mean stop when they say so with flow control. He found that disabling the FIFO in his Linux box's 16550 serial port with setserial corrected the problem of dropped characters (you apparently just specify the uart type as an 8250 to do this).
So I tried doing so (setserial /dev/ttyS0 uart 8250), and that's where the weird things start. My device then only receives one byte on 16 (i.e. the latest to enter the FIFO when the serial interrupt is triggered!). It is clearly stated that a 8250 uart does not have any FIFO, so I'm starting to suspect a bug here. I've tested on two Linux x86 machines, runnning kernels 2.6.3 and 2.6.6, the behavior is the same. I've also tried with UARTs16550 and 16450.
I've also been able to quickly test on a Windows machine. There, I am able to set some advanced settings of the UART (including enabling/disabling the FIFO and tuning its size) and everything works as I'd like.
I really need the ability to disable the FIFO to death. Is there a good reason why it is not possible with Linux? Any workaround to disable it properly? Is the behavior in 8250 mode incorrect/buggy (the quotation seems to ?
Since I'm in bad need for this I'm ready to add support for it myself if necessary.
Any help/pointer would be greatly appreciated.
Thanks,
Alex.
Signature · 3 lines
Alexandre Courbot - PhD student RD2P/LIFL http://www.lifl.fr/~courbot
Quoted 2 lines
So I tried doing so (setserial
/dev/ttyS0uart 8250), and that's where the weird things start. My device then only receives one byte on 16 (i.e. the latest to enter the FIFO when the serial interrupt is triggered!). It is clearly stated that a 8250 uart does not have any FIFO, so I'm starting to suspect a bug here. I've tested on two Linux x86 machines, runnning kernels 2.6.3 and 2.6.6, the behavior is the same. I've also tried with UARTs16550 and 16450.
Sorry for replying to myself - just to point out that with kernel 2.4.25, on the same machines, it behaves properly (i.e. I'm receiving all the bytes). So it looks like there is a bug in the 2.6 series of the serial driver, not disabling the FIFO when setting the 8250 mode.
Would still be nice if it was possible to explicitely disable or tune the 16550A's FIFO with, say, setserial! :)
Alex.
Signature · 3 lines
Alexandre Courbot - PhD student RD2P/LIFL http://www.lifl.fr/~courbot