I'm developing software for an embedded device (NXP i.MX 8M mini SoC). It is running Linux (currently 5.15.32 kernel) and I'm providing serial I/O over the SoC's built-in USB interface using a multifunction gadget.
The gadget provides an RNDIS network interface and an ACM serial interface. Both work great, but I found that when my app (which periodically writes data to the ACM device, /dev/ttyGS0) terminates, it sometimes takes 16 seconds to close the ACM device's file descriptor.
I have observed that this delay only happens when there is no application reading bytes from the other end of the ACM interface. If there is anything reading those bytes (even a simple cat /dev/ttyACM0 > /dev/null on the connected host), then the close() call completes immediately.
So I'm pretty sure that the delay is because of the data queued in the USB driver. The close() call (I assume) is waiting for the data to be received by the remote end of the connection, and it times out after 16 seconds.
I have already tried to tcflush(fd, TCIOFLUSH) the file descriptor before the close() operation, but it didn't help.
I also tried setting the bit-rate to B0, which is meant to represent a "hangup" event for a modem, thinking that might purge the data, but it didn't change anything.
Here's the (hopefully relevant) part of my USB gadget setup script:
modproble libcomposite
cd /sys/kernel/config/usb_gadget
mkdir g
cd g
echo 0x1d6b > idVendor # Linux Foundation
echo 0x0104 > idProduct # Multifunction composite gadget
echo 0x0100 > bcdDevice # v1.0.0
echo 0x0200 > bcdUSB # USB 2.0
# Miscellaneous / Wire Adapter Multifunction programming interface
echo 0xEF > bDeviceClass
echo 0x02 > bDeviceSubClass
echo 0x01 > bDeviceProtocol
mkdir -p strings/0x409
echo "my_serial_no" > strings/0x409/serialnumber
echo "my_manufacturer" > strings/0x409/manufacturer
mkdir -p functions/acm.usb0
mkdir -p configs/c.1
echo 250 > configs/c.1/MaxPower
ln -s functions/acm.usb0 configs/c.1
(creation and configuration of RNDIS function omitted)
ln -s configs/c.1 os_desc
Here's a fragment showing how I open and configure the file descriptor (skipping over the error handling checks):
fd = open("/dev/ttyGS0", O_RDWR | O_NONBLOCK);
struct termios settings;
tcgetattr(fd, &settings);
cfsetospeed(&settings, B1000000); // 1Mbps
settings.c_cflag &= ~(CSTOPB | // 1 stop bit
CSIZE | // Clear previous bit-size bits
PARENB); // Don't use parity checking
settings.c_cflag |= (CS8 | // 8 data bits
CLOCAL); // Ignore modem control lines
settings.c_iflag &= ~(IGNBRK | // Don't ignore break
BRKINT | // Don't generate SIGINT on BRK
PARMRK | // Don't mark framing/parity errors
ISTRIP | // Don't strip the 8th bit
INLCR | // Don't translate NL to CR
IGNCR | // Don't ignore CR
ICRNL | // Don't ignore NL
IXON); // Don't use XON/XOFF flow control
settings.c_oflag &= ~OPOST; // Don't use implementation-defined output processing
settings.c_lflag &= ~(ECHO | // Don't echo input characters
ECHONL | // Don't echo NL
ICANON | // Don't use canonical mode
ISIG | // Don't generate signals
IEXTEN); // Don't use implementation-defined input processing
settings.c_cc[VTIME] = 0; // Disable timeout for non canonical read
settings.c_cc[VMIN] = 0; // No minimum chars for non canonical read
tcsetattr(fd, TCSANOW, &settings);
tcflush(fd, TCOFLUSH);
I have tried explicitly enabling and disabling flow control (c_cflag |= CRTSCTS). It changes nothing.
So my questions are:
- Can my problem be solved by changing the ACM device's configuration or the termios configuration?
- If not, is there a way to tell Linux that I don't want to wait when closing the FD?
- If that's not an option, is there a way to tell the ACM driver to flush/discard these buffers so the close operation doesn't have to wait?
- If none of the above is possible, is there a way my device can know when there is an app connected to the other end of the ACM device so I can tell my app to not send data when there is no receiver?
