Retrieving buffer/packet/payload sizes for USB serial write transfer in userspace Linux C code

Question

Apologies in advance I won't be able to accept an answer here immediately - just thought I'd like to jot this down, while I have the problem...

In brief: I can observe three different buffer sizes, when I initiate a write to an usb-serial port using user-space C code under Linux - and the problem is, I would like to retrieve all these sizes from the user-space C code itself.

---

Let's say, I have an Arduino Duemillanove, with an FTDI FT232 chip - programmed to read incoming bytes from usb/serial connection from PC, and discard them. When I plug in this device in the system (did this on Ubunty 11.04 Natty), I can observe the following via tail -f /var/log/syslog:

Mar 21 08:05:05 mypc kernel: [  679.197982] usbserial: USB Serial Driver core
Mar 21 08:05:05 mypc kernel: [  679.223954] USB Serial support registered for FTDI USB Serial Device
Mar 21 08:05:05 mypc kernel: [  679.227354] ftdi_sio 2-2:1.0: FTDI USB Serial Device converter detected
Mar 21 08:05:05 mypc kernel: [  679.227633] usb 2-2: Detected FT232RL
Mar 21 08:05:05 mypc kernel: [  679.227644] usb 2-2: Number of endpoints 2
Mar 21 08:05:05 mypc kernel: [  679.227652] usb 2-2: Endpoint 1 MaxPacketSize 64
Mar 21 08:05:05 mypc kernel: [  679.227660] usb 2-2: Endpoint 2 MaxPacketSize 64
Mar 21 08:05:05 mypc kernel: [  679.227667] usb 2-2: Setting MaxPacketSize 64
...

This tells me first that the drivers (kernel modules) usbserial and ftdi_sio have been hooked/loaded to handle the device; these create a file (device node) called /dev/ttyUSB0 - essentially a serial port from the OS perspective. It also tells me that there is a MaxPacketSize of 64 bytes attributed to the device's endpoints. I can get the MaxPacketSize also by querying via lsusb:

$ lsusb | grep FT
Bus 002 Device 002: ID 0403:6001 Future Technology Devices International, Ltd FT232 USB-Serial (UART) IC
$ lsusb -t | grep -B1 ft
/:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=uhci_hcd/2p, 12M
    |__ Port 2: Dev 2, If 0, Class=vend., Driver=ftdi_sio, 12M
$ sudo lsusb -v -d 0403:6001 | grep 'bEndpointAddress\|wMaxPacketSize\|idVendor\|idProduct'
  idVendor           0x0403 Future Technology Devices International, Ltd
  idProduct          0x6001 FT232 USB-Serial (UART) IC
        bEndpointAddress     0x81  EP 1 IN
        wMaxPacketSize     0x0040  1x 64 bytes
        bEndpointAddress     0x02  EP 2 OUT
        wMaxPacketSize     0x0040  1x 64 bytes

Now, let's say I want to write to the device node /dev/ttyUSB0 using the following C program, testusw.c:

#include <stdio.h>   /* Standard input/output definitions */
#include <string.h>  /* String function definitions */
#include <unistd.h>  /* UNIX standard function definitions */
#include <fcntl.h>   /* File control definitions */
#include <errno.h>   /* Error number definitions */
#include <termios.h> /* POSIX terminal control definitions */

// testusw.c
// build with: gcc -o testusw -Wall -g testusw.c

int main( int argc, char **argv ) {

  char *serportdevfile;
  int serport_fd;
  char writeData[20000*5]; //100000 bytes data
  unsigned char snippet[] = {0xAA, 0xBB, 0xCC, 0xDD, 0xFE};
  int i;
  int bytesWritten;

  if( argc != 2 ) {
    fprintf(stdout, "Usage:\n");
    fprintf(stdout, "%s port baudrate file/string\n", argv[0]);
    return 1;
  }

  //populate write data
  for (i=0; i<20000; i++) {
    memcpy(&writeData[5*i], &snippet[0], 5);
  }
  // for strlen, fix (after) last byte to 0
  writeData[20000*5] = 0x00;

  // show writeData - truncate to 10 bytes (.10):
  fprintf(stdout, "//%.10s//\n", writeData);

  serportdevfile = argv[1];
  serport_fd = open( serportdevfile, O_RDWR | O_NOCTTY | O_NONBLOCK );
  if ( serport_fd < 0 ) { perror(serportdevfile); return 1; }

  // do a write:
  fprintf(stdout, "Writing %d bytes\n", strlen(writeData));
  bytesWritten = write( serport_fd, writeData, strlen(writeData) );
  fprintf(stdout, " bytes written: %d \n", bytesWritten);

  return 0;
}

This program deliberately writes a big chunk of data in one call. To see what is happening, first let's capture USB URB requests via Linux's usbmon facility - so in one terminal, we run:

$ sudo cat /sys/kernel/debug/usb/usbmon/2u > testusw.2u.mon

... and in another terminal, after compiling and running testusw, we obtain:

$ gcc -o testusw -Wall -g testusw.c
$ ./testusw /dev/ttyUSB0
//ª»ÌÝþª»ÌÝþ//
Writing 100000 bytes
 bytes written: 4608
$

(Note that the testusw call above will likely reset the Arduino). After testusw, we can go back to the first terminal, and interrupt the cat process with CTRL+C; we are left with a logfile, testusw.2u.mon. We can open this logfile with Virtual USB Analyzer:

$ ./vusb-analyzer testusw.2u.mon

... and obtain the following visualization:

Note that there are 2*9 = 18 URB requests shown for "EP2 OUT" that perform the writing, carrying 0x0100 = 256 bytes each; so in total, 18*256 = 4608 bytes were written - as reported by "bytes written" by testusw above. Also, ignore the data on EP1 IN (that is some junk my Arduino code is sending - which ends with a "Status: -2" error).

---

Thus, I can observe the following:

• From the C program, I initiate a write of 100000 bytes
• As a result, only 4608 bytes are written - effectively acting as a first buffer size
• usbmon then reports this chunk is sequenced into 18 URB requests of 256 bytes each
• finally, MaxPacketSize tells me that each URB request is (probably) seqenced into (four) packets of 64 bytes on USB wire

In effect, I have three buffer sizes: 4608, 256 and 64 bytes; similar to what is mentioned in Serial HOWTO: Serial Port Basics: 4.7 Data Flow Path; Buffers:

application     8k-byte         16-byte        1k-byte        tele-
BROWSER ------- MEMORY -------- FIFO --------- MODEM -------- phone
program         buffer          buffer         buffer         line

So, my question is: how can these buffer sizes be retrieved from the userspace C code itself - however, only from the device node path /dev/ttyUSB0 as the only input parameter?

I would be OK with running external programs via a system popen command, and parsing the output. For instance, I could obtain MaxPacketSize via lsusb -v -d 0403:6001 | grep MaxPacketSize - but that requires vendor/product ID, and I don't know how to obtain that, if only piece of information is the device node path /dev/ttyUSB0.

Given that /dev/ttyUSB0 is essentially treated as a serial port, I thought querying via stty would provide something - however, I cannot see anything related to buffer sizes there:

$ stty -a -F /dev/ttyUSB0
speed 115200 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^A; eol = <undef>;
eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread -clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff
-iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl -onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt
-echoctl -echoke

I also know I can use udevadm to query for data related to the device node path /dev/ttyUSB0:

$ udevadm info --query=all --name=/dev/ttyUSB0
P: /devices/pci0000:00/0000:00:1d.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0
N: ttyUSB0
S: serial/by-path/pci-0000:00:1d.0-usb-0:2:1.0-port0
S: serial/by-id/usb-FTDI_FT232R_USB_UART_A9007OH3-if00-port0
E: UDEV_LOG=3
E: DEVPATH=/devices/pci0000:00/0000:00:1d.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0
E: MAJOR=188
E: MINOR=0
E: DEVNAME=/dev/ttyUSB0
E: SUBSYSTEM=tty
E: ID_PORT=0
E: ID_PATH=pci-0000:00:1d.0-usb-0:2:1.0
E: ID_VENDOR=FTDI
E: ID_VENDOR_ENC=FTDI
E: ID_VENDOR_ID=0403
E: ID_MODEL=FT232R_USB_UART
E: ID_MODEL_ENC=FT232R\x20USB\x20UART
E: ID_MODEL_ID=6001
E: ID_REVISION=0600
E: ID_SERIAL=FTDI_FT232R_USB_UART_A9007OH3
E: ID_SERIAL_SHORT=A9007OH3
E: ID_TYPE=generic
E: ID_BUS=usb
E: ID_USB_INTERFACES=:ffffff:
E: ID_USB_INTERFACE_NUM=00
E: ID_USB_DRIVER=ftdi_sio
E: ID_IFACE=00
E: ID_VENDOR_FROM_DATABASE=Future Technology Devices International, Ltd
E: ID_MODEL_FROM_DATABASE=FT232 USB-Serial (UART) IC
E: ID_MM_CANDIDATE=1
E: DEVLINKS=/dev/serial/by-path/pci-0000:00:1d.0-usb-0:2:1.0-port0 /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_A9007OH3-if00-port0

# the below has huge output, so pipe it to `less`
$ udevadm info --attribute-walk --name=/dev/ttyUSB0 | less

... but again, I cannot see much related to the encountered buffer sizes.

To wrap this up, the question again: can I retrieve the encountered buffer sizes related to the usb-serial write transfer from a user-space C application; and if so - how?

Many thanks in advance for any answers,
Cheers!

Answer 1

Don't see why you'd want to know this. Linux allows you to use the TIOCGSERIAL ioctl to retrieve a struct serial_struct which has a xmit_fifo_size field. Though I'd be surprised if many USB serial driver bother to write something meaningful there.