Hi Stanislaw,
On 02/18/2014 04:38 AM, Stanislaw Gruszka wrote:
Hi,
setserial has low_latency option which should minimize receive latency
(scheduler delay). AFAICT it is used if someone talk to external device
via RS-485/RS-232 and need to have quick requests and responses . On
kernel this feature was implemented by direct tty processing from
interrupt context:
void tty_flip_buffer_push(struct tty_port *port)
{
struct tty_bufhead *buf = &port->buf;
buf->tail->commit = buf->tail->used;
if (port->low_latency)
flush_to_ldisc(&buf->work);
else
schedule_work(&buf->work);
}
But after 3.12 tty locking changes, calling flush_to_ldisc() from
interrupt context is a bug (we got scheduling while atomic bug report
here: https://bugzilla.redhat.com/show_bug.cgi?id=1065087 )
I'm not sure how this should be solved. After Peter get rid all of those
race condition in tty layer, we probably don't want go back to use
spin_lock's there. Maybe we can create WQ_HIGHPRI workqueue and schedule
flush_to_ldisc() work there. Or perhaps users that need to low latency,
should switch to thread irq and prioritize serial irq to meat
retirements. Anyway setserial low_latency is now broken and all who use
this feature in the past can not do this any longer on 3.12+ kernels.
Thoughts ?
Can you give me an idea of your device's average and minimum required
latency (please be specific)? Is your target arch x86 [so I can evaluate the
the impact of bus-locked instructions relative to your expected]?
Also, how painful would it be if unsupported termios changes were rejected
if the port was in low_latency mode and/or if low_latency setting was
disallowed because of termios state?
It would be pointless to throttle low_latency, yes?
What would be an acceptable outcome of being unable to accept input?
Corrupted overrun? Dropped i/o? Queued for later? Please explain with
comparison to the outcome of missed minimum latency.
Regards,
Peter Hurley
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