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
4608bytes are written - effectively acting as a first buffer size usbmonthen reports this chunk is sequenced into 18 URB requests of256bytes each- finally, MaxPacketSize tells me that each URB request is (probably) seqenced into (four) packets of
64bytes 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;
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-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!