[Bug 2176] New: fishy behavior in termios tx task mode

Tue Apr 15 17:38:51 UTC 2014

https://www.rtems.org/bugzilla/show_bug.cgi?id=2176

             Bug #: 2176
           Summary: fishy behavior in termios tx task mode
    Classification: Unclassified
           Product: RTEMS
           Version: 4.10
          Platform: All
        OS/Version: RTEMS
            Status: NEW
          Severity: normal
          Priority: P3
         Component: cpukit
        AssignedTo: joel.sherrill at oarcorp.com
        ReportedBy: johill at lanl.gov

I have a look around in the drivers in the various BSPs and I notice that none
of the termios drivers appear to transmit characters at task level even if they
are running in termios task mode. Maybe all (most) of them send characters in
the ISR. If there was a large frame of characters to send then this could lock
out task activity for too long.

FWIW, I had a closer look at this today, and maybe something is fishy in the
termios code when the TX part of termios runs in task driven mode. It seems
that in task mode if the UART can accept characters immediately in the write
routine then we wouldn’t need to turn on any interrupts at all. The write
routine would need to somehow tell termios how many characters it sent;
presumably this would occur by calling rtems_termios_dequeue_characters in the
driver's write function. I see in the code that this tries to work,
rtems_termios_dequeue_characters posts the semaphore of termios tx and
increases the characters sent count of termios. However after the write routine
returns it goes badly.

If the transmitter runs in termios TASK mode and the driver's write routine
does not immediately enable an interrupt, then it returns to the code below in
rtems_termios_puts and it sets the transmitter to rob_busy. After that the
termios tx daemon proceeds to step through all of the characters remaining (I
think that I see this in the debugger) and discards them because the
transmitter stays in rob_busy state. 

It's also probably odd that termios calls the write function with interrupts
disabled when it is in task driven mode; we could loop outputting a large frame
of characters in the write routine at task level with interrupts globally
disabled.

        if (tty->rawOutBufState == rob_idle) {
          /* check, whether XOFF has been received */
          if (!(tty->flow_ctrl & FL_ORCVXOF)) {
            (*tty->device.write)(tty->minor,
            (char *)&tty->rawOutBuf.theBuf[tty->rawOutBuf.Tail],1);
          }
          else {
            /* remember that output has been stopped due to flow ctrl*/
            tty->flow_ctrl |= FL_OSTOP;
          }
          tty->rawOutBufState = rob_busy;
        }

[debug]#0  rtems_termios_puts (_buf=0x9ca60, len=1, tty=0x11509c) at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libcsupport/src/termios.c:677
[debug]#1  0x00015c68 in oproc (c=99 'c', tty=0x11509c) at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libcsupport/src/termios.c:748
[debug]#2  0x00015d9c in rtems_termios_write (arg=0x9cad8) at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libcsupport/src/termios.c:770
[debug]#3  0x00002ef0 in console_write (major=0, minor=0, arg=0x9cad8) at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/lib/libbsp/nios2/altera-sys-config/./console/console.c:171
[debug]#4  0x0006c328 in rtems_io_write (major=0, minor=0, argument=0x9cad8) at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/sapi/src/iowrite.c:47
[debug]#5  0x00068044 in device_write (iop=0x114988, buffer=0x3366d4, count=44)
at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libfs/src/imfs/deviceio.c:160
[debug]#6  0x00017ef4 in write (fd=1, buffer=0x3366d4, count=44) at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libcsupport/src/write.c:51
[debug]#7  0x0007f078 in _write_r (ptr=0x9d638, fd=1, buf=0x3366d4, nbytes=44)
at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libcsupport/src/write_r.c:38
[debug]#8  0x0006d488 in _fflush_r (ptr=0x9d638, fp=0x9d98c) at
../../../../../../nios2-rtems/altera/altera11.0/gnu-tools/rtems-patched/gcc-4.1/newlib/libc/stdio/fflush.c:214
[debug]#9  0x000754e8 in __sfvwrite_r (ptr=0x9d638, fp=0x9d98c, uio=0x9cbe0) at
../../../../../../nios2-rtems/altera/altera11.0/gnu-tools/rtems-patched/gcc-4.1/newlib/libc/stdio/fvwrite.c:257
[debug]#10 0x0007830c in __sprint_r (ptr=0x9d638, fp=0x20, uio=0x9cbe0) at
../../../../../../nios2-rtems/altera/altera11.0/gnu-tools/rtems-patched/gcc-4.1/newlib/libc/stdio/vfprintf.c:322
[debug]#11 0x00072fc8 in _vfprintf_r (data=0x9d638, fp=0x9d98c, fmt0=<value
optimized out>, ap=0x0) at
../../../../../../nios2-rtems/altera/altera11.0/gnu-tools/rtems-patched/gcc-4.1/newlib/libc/stdio/vfprintf.c:1501
[debug]#12 0x0006dce8 in printf (fmt=0x1 "") at
../../../../../../nios2-rtems/altera/altera11.0/gnu-tools/rtems-patched/gcc-4.1/newlib/libc/stdio/printf.c:52
[debug]#13 0x00057de0 in bootpc_init (update_files=false, forever=true) at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libnetworking/nfs/bootp_subr.c:997
[debug]#14 0x0002960c in rtems_bsdnet_do_bootp () at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libnetworking/rtems/rtems_bootp.c:23
[debug]#15 0x0002b218 in rtems_bsdnet_initialize_network () at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/libnetworking/rtems/rtems_glue.c:980
[debug]#16 0x00001528 in Init (ignored=565432) at init.c:47
[debug]#17 0x0006ccb0 in _Thread_Handler () at
/home/hill/nios2-rtems/rtems/rtems-git/rtems-4.10/c/src/../../cpukit/score/src/threadhandler.c:145
[debug]#18 0x0006cc28 in _Thread_Is_heir (the_thread=0x6cc28) at
../../cpukit/../../../altera-sys-config/lib/include/rtems/score/thread.inl:82
[debug]Backtrace stopped: frame did not save the PC

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