2017-08-28 11:49 GMT+03:00 Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>:
> On Mon, Aug 28, 2017 at 11:22:48AM +0300, Matwey V. Kornilov wrote:
>> 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.
>
> But this is USB, you have to wait for a time for stuff like this to get
> out to the device. You just do it once at open() time and you should be
> fine, right?
Hi Greg,
Sorry, I am not sure that I understand you correctly.
Patch c6dce2626606 ("USB: serial: ftdi_sio: fix extreme low-latency
setting") changed default serial info flags.
Before the patch, there was ASYNC_LOW_LATENCY set at open() time by
default, after the patch it is unset at open(), so I have to set it
manually by ioctl() after I opened the device.
Do you say that I don't need 1ms latency? 1ms and 16ms latencies are
considerable different both from USB layer and user-space layer point
of view. 1ms is long enough time interval.
>
> And low-latency with a USB device? You know you have no guarantee at
> all from the hardware that this will happen, you are playing with fire
> here...
>
>> 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.
>
> Why not always just do this at open() time and you should be fine?
>
> thanks,
>
> greg k-h
--
With best regards,
Matwey V. Kornilov
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