Messages in this thread | | | Date | Tue, 18 Feb 2014 17:12:13 -0500 | From | Peter Hurley <> | Subject | Re: locking changes in tty broke low latency feature |
| |
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
| |