Hello all,
I am porting the Davinci linux kernel to another architecture, called Jacinto2.
I've had several problems with the scheduler that I think may be related to
each other.
The first was located in drivers/char/tty_buffer.c. What occurred was that the
kernel would boot as far as the shell, but the keyboard would not take any
input. The serial buffer was indeed receiving characters, but for some reason
the tty_buffer was not passing them to user space.
The temporary workaround I used was located in the function
"tty_flip_buffer_push" in tty_buffer.c.
If the tty had the low latency flag, it would immediately push the characters
to the line descriptor. However, my kernel was taking the other branch, and
passing the flush to the scheduler with "schedule_delayed_work" as seen below:
if (tty->low_latency)
flush_to_ldisc(&tty->buf.work.work);
else
schedule_delayed_work(&tty->buf.work, 1);
When schedule_delayed_work was called, the kernel would never come back to the
process that was scheduled, and so I had no keyboard input. My workaround was
to hard code it to always call flush_to_ldisc. However, I think this is a poor
fix because it may cause starvation of other processes, because the serial
input buffer now operates in real-time and is very low priority compared to
other operations.
The second problem that I had was with the MMC/SD driver I was porting. In the
file /drivers/mmc/core/core.c, during the function mmc_power_up, the mmc_delay
function would be called (located in core.h). Tracing this, mmc_delay called
msleep (in kernel/timer.c) which eventually called schedule_timeout. Once
again, the scheduler would put the MMC setup in the waiting queue for 10mS, and
move onto the next driver to initialize, but never come back to the MMC setup,
so the driver would not function.
My workaround here was to change the original mmc_delay function, shown below,
to always take the first branch and use mdelay, instead of calling msleep.
if (ms < 1000 / HZ) {
cond_resched();
mdelay(ms);
}
else
msleep(ms);
Once again, this seems to be a poor fix, because mdelay is a cycle-wasting
loop, while msleep utilizes the scheduler. Plus, I really would like to have a
working scheduler in my linux kernel. :-p
Has anyone seen any problems with the scheduler like this? Is there a strange
.config option I am missing? I assume this is a new bug on my architecture
which became introduced when I was porting to the Jacinto2 architecture.
Does anyone have any suggestions, insight, or ideas on ways to fix this bug?
Thanks and Regards,
Joe
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