Hi all,
I have an issue with the following patch:
c6dce2626606 ("USB: serial: ftdi_sio: fix extreme low-latency setting")
I really need sub 16-ms latency for my peripheral and while I have no
problems with idea of setting ASYNC_LOW_LATENCY I would like point out
that it doesn't work as expected and probably need to be fixed.
My user space code snipped is simple enough
if (ioctl(fd, TIOCGSERIAL, &old_serial) == 0) {
struct serial_struct new_serial = old_serial;
new_serial.flags |= ASYNC_LOW_LATENCY;
if (ioctl(fd, TIOCSSERIAL, &new_serial) < 0) {
return -1;
}
}
My hardware is
0403:6001 Future Technology Devices International, Ltd FT232
USB-Serial (UART) IC
However, currently I see that
1) Application of ASYNC LOW_LATENCY is not taken immediately. That is
what I see from incoming data which arrives with considerable delays
after I TIOCSSERIAL. I suppose that latency timer is applied after the
end of currently running period. I.e I need to wait ~16ms and only
after then 1ms latency started. 16ms is quite considerable time for
nowadays user-space application.
2) There is no generic reliable user-space way to wait until latency
settings are actually applied. I haven't find the way to determine
from user-space that it is ftdi_sio-driven tty-device and I need to
apply msleep-based quirks.
So, my suggestion is that TIOCSSERIAL must block until latency
settings are actually applied which can be checked internally in the
driver.
--
With best regards,
Matwey V. Kornilov
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