Hello I am posting this in reference to the thread "usb-serial: tcgetattr, tcsetattr". There, I described a buffering effect that I observed. I investigated this further, and found a dead lock: write_room is called while the write function has not completely finished yet. Consequently write_room indicates that 0 bytes are available, and the tx queue stalls. Setup: ====== Two serial ports, ttyUSB0 and ttyS1 are connected end to end. The USBRSA is device under test and attached to ttyUSB0. The USBRSA is the sending device (echo "xyz" > /dev/ttyUSB0) while ttyS1 receives (cat /dev/ttyS1). Remark: ======= The write function of the USBRSA driver streams the characters to be sent using a preallocated pool of 10 URBs (very much like the whiteheat driver) Observation: =========== If usbrsa_write requires more urbs than available, echo blocks on the command line. I am sending 577 characters (in 10 URBs each containg up to 64 bytes of payload). echo adds "\a\n" which would require urb #11. The tty layer queries the number of available bytes of the send buffer (usbrsa_write_room) BEFORE any of the urbs sent downstream to the USBRSA is returned to the pool. This leads to a deadlock, i.e. echo waits because there is no urb available and I need to terminiate it with CTRL-C. Nothing is dumped on the receiving terminal. Only if I then issue another echo, the string previously sent by is dumped. This is probably related to the "\a\n" that could not directly be sent. Log file: ========= Here is how this looks in the log file: usbrsa_write - port 1; bytes=577 := count=577 : nofTxBytesFree=4096 usbrsa_write - poolsize 10, urb_index 0 usbrsa_write - port 1;URB allocated=0; bytes in urb=64 usbrsa_write - port 1;URB allocated=1; bytes in urb=64 ... usbrsa_write - port 1;URB allocated=9; bytes in urb=1 usbrsa_write() End - sent=577 bytes usbrsa_write_room() port = 1 usbrsa_write_room(): 0 usbrsa_write_callback(): Returned URB 0 usbrsa_write_callback(): Returned URB 1 usbrsa_write_callback(): Returned URB 9 [Here I press control-C. Only then "\a\n" is no sent in URB #0] usbrsa_write - port 1 START usbrsa_write - port 1; bytes=2 := count=2 : nofTxBytesFree=4096 usbrsa_write - poolsize 10, urb_index 0 usbrsa_write - port 1;URB allocated=0;; bytes in urb=2 usbrsa_write() End - sent=2 Questions: ========== Can I trigger another write_room ? Or do I need to wait in usbrsa_write until the first urb of the empty pool is returned to it ? The whiteheat driver does not seem to explicitly handle an empty URB pool either -- how does it avoid this dead lock? Thanks Tilman -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
