[1.] Calling tcsetattr prevents any thread suspended in ioctl TIOCMIWAIT from ever resuming. [2.] If a thread is suspended inside a call to ioctl TIOCMIWAIT, waiting for a modem status change, the 8250 driver enables modem status interrupts. The modem status interrupt service routine resumes the suspended thread on the next modem status irq. If in the mean time, another thread calls tcsetattr then the 8250 driver disables modem status interrupts (unless CTS/RTS handshaking is enabled). This prevents the suspended thread from ever being resumed. [3.] Keywords serial, 8250 [4.] Kernel 2.6.24-21 [4.1.] cat /proc/version: Linux version 2.6.24-21-lvr (root@Collins) (gcc version 4.2.3 (Ubuntu 4.2.3-2ubuntu7)) #2 Wed Oct 22 19:42:04 CEST 2008 [7.] Example C program to demonstrate: /* gcc -o test test.c -l pthread */ #include <stdio.h> #include <errno.h> #include <unistd.h> #include <fcntl.h> #include <pthread.h> #include <termios.h> #include <sys/ioctl.h> #include <linux/serial.h> static void* monitor( void* pv); static int s_fd; int main( void) { const char kszDev[] = "/dev/ttyS0"; pthread_t t; struct termios tio; s_fd = open( kszDev, O_RDWR | O_NONBLOCK); if ( s_fd < 0) return fprintf( stderr, "Error(%d) opening %s: %s\n", errno, kszDev, strerror( errno)), 1; pthread_create( &t, NULL, &monitor, NULL); /* Modem status changes seen here */ sleep( 5); tcgetattr( s_fd, &tio); tio.c_cflag ^= CSTOPB; /* But not after here */ puts( "Main: tcsetattr"); tcsetattr( s_fd, TCSANOW, &tio); for (;;) sleep( 1); } static void* monitor( void* pv) { (void)pv; for(;;) { unsigned uModem; struct serial_icounter_struct cnt; if ( ioctl( s_fd, TIOCMGET, &uModem) < 0) fprintf( stderr, "Error(%d) in TIOCMGET: %s\n", errno, strerror( errno)); printf( "Modem status:%s%s%s%s%s%s\n", (uModem & TIOCM_RTS) ? " RTS" : "", (uModem & TIOCM_DTR) ? " DTR" : "", (uModem & TIOCM_CTS) ? " CTS" : "", (uModem & TIOCM_DSR) ? " DSR" : "", (uModem & TIOCM_CD) ? " CD" : "", (uModem & TIOCM_RI) ? " RI" : "" ); if ( ioctl( s_fd, TIOCGICOUNT, &cnt) < 0) fprintf( stderr, "Error(%d) in TIOCGICOUNT: %s\n", errno, strerror( errno)); printf( "Irqs: CTS:%d DSR:%d RNG:%d DCD:%d Rx:%d Tx:%d Frame:%d Orun:%d Par:%d Brk:%d Oflow:%d\n", cnt.cts, cnt.dsr, cnt.rng, cnt.dcd, cnt.rx, cnt.tx, cnt.frame, cnt.overrun, cnt.parity, cnt.brk, cnt.buf_overrun ); fputs( "Waiting...", stdout), fflush( stdout); if ( 0 > ioctl( s_fd, TIOCMIWAIT, (unsigned long)(TIOCM_CAR | TIOCM_RNG | TIOCM_DSR | TIOCM_CTS))) fprintf( stderr, "\nError(%d) in TIOCMIWAIT: %s\n", errno, strerror( errno)); fputs( "\n", stdout); } return NULL; } [8.] 8250.c line 2112: /* * CTS flow control flag and modem status interrupts */ up->ier &= ~UART_IER_MSI; if (!(up->bugs & UART_BUG_NOMSR) && UART_ENABLE_MS(&up->port, termios->c_cflag)) up->ier |= UART_IER_MSI; This code disables the MS irq. Should take into account if up->port.info->delta_msr_wait is in use. -- Lawrence Rust -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html