Changeset b3fd1641 in rtems for c/src/exec/libcsupport/src/termios.c

Fix from Eric Norum <eric@…>:

"Thomas Doerfler" <td@…> wrote:

While implementing/testing the console/termios support for
PPC403 in RTEMS-4.0.0-beta3, I am stuck at a certain location in
termios.c:

During "rtems_termios_initialize", the main control data structure
"*tty" is allocated using malloc(). (Note, that malloc does not
clear the allocated memory and my BSP does not clear memory during
startup). Furtheron, a lot of fields of that structure are
initialized, but the field "rawOutBufState" is not, and therefore
keeps an arbitrary contents.

When "osend()" is called the first time(with the serial device
driver working in interrupt mode), termios gets stuck and will not
call the device drivers output function.

My questions now are:

• anybody already experienced this bug?
• is it a bug at all or did I do anything fundamentally wrong?
• is there already a common bugfix for that?

I don't like poking around in other people code, as long as I am
not absolutely sure, what I do...

Yes, there's a bug there.
I thought that Joel had patched this already, but here's a patch to
fix this. This patch also addresses a concern that many others have
raised regarding enabling and disabling of transmitter interrupts.

First, here's the example I've been using of a simple UART-style
interrupt-driven driver:
===============================================================
void
device_write_routine (int minor, char *buf, int count)
{

UART->control_register &= ~UART_TRANSMITTER_READY;
UART->output_register = *buf;
UART->control_register |= UART_TRANSMIT_INTERRUPT_ENABLE;

}

void
device_transmit_interrupt_routine (int vector)
{

UART->control_register &= ~UART_TRANSMIT_INTERRUPT_ENABLE;
rtems_termios_dequeue_characters (device_ttyp, 1);

}
==============================================================

Several people have expressed their concern about the disable/enable
of transmitter interrupts for every character. On some machines
this disable/enable is an expensive operation. With the attached
patch applied you can write the two routines as:
==============================================================
void
device_write_routine (int minor, char *buf, int count)
{

code_to_clear_transmitter_ready_status ();
if (device_ttyp->rawOutBufState == rob_idle)

code_to_enable_transmitter_interrupts ();

code_to_send_one_character_to_transmitter (*buf);

}

void
device_transmit_interrupt_routine (int vector)
{

rtems_termios_dequeue_characters (device_ttyp, 1);
if (device_ttyp->rawOutBufState == rob_idle)

code_to_disable_transmitter_interrupts ();

}
===============================================================