Re: Regression: ftdi_sio is slow (since Wed Oct 10 15:05:06 2012)

[Johan Hovold]

On Sat, May 04, 2013 at 01:50:42AM +0400, Stas Sergeev wrote:
> 04.05.2013 00:34, Greg KH ÐÐÑÐÑ:
> > On Fri, May 03, 2013 at 10:27:18PM +0400, Stas Sergeev wrote:
> >> 03.05.2013 21:16, Greg KH ÐÐÑÐÑ:

[...]

> >>> There's no guarantee as to how long select or an ioctl will take, and
> >>> now that we have fixed another bug, this device is slower.
> >>>
> >>> If you change hardware types to use a different usb to serial chip, that
> >>> select call might take 4 times as long.  Are we somehow supposed to
> >>> change the kernel to "fix" that?
> >> Previously, the kernel was not calling to a device at all, so
> >> select() was independent of the chip, and it was fast. I was
> >> not aware you changed that willingly.
> > I don't understand, what do you mean by this?  Some drivers just return
> > the value of an internally held number, and don't query the device.
> >
> > The only way the FTDI driver can determine if the hardware buffer on the
> > chip way out on the end of the USB cable is empty or not, is to query
> > it.  So the driver now does so.
> It does so only for one char. And the query takes longer than
> to just xmit that char. So why do you think this even works as
> expected?

The query takes longer than the transmit at decent baudrates (>=38k)
and under the assumption that flow control isn't causing any delays.

But you do have a point, and I have been meaning to look into whether
the added overhead of checking the hardware buffers could be mitigated
by adding wait_until_sent support to usb-serial. This way the we would
only query the hardware buffers on tty_wait_until_sent (e.g. at close)
and select and TIOCMOUTQ would not suffer. This is also the way things
are handled in serial_core.

I'll prepare a series which adds wait_until_sent to usb-serial, but I
doubt it would be stable material (even if it could get into 3.10).

What do you think Greg, is this overhead to chars_in_buffer reason
enough to disable it in the stable trees or should we simply fix it in
3.11 (or 3.10)? (The overhead is about 3-400 us per call when the port
fifo is empty, which makes chars_in_buffer about 100 times slower on my
test system.)

Thanks,
Johan
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