03.05.2013 20:52, Greg KH пишет:
On Fri, May 03, 2013 at 09:38:50PM +0400, Stas Sergeev wrote:
03.05.2013 20:30, Greg KH пишет:
We need some way to check the chars in the buffer, is the device you are
using just very slow to respond to this request? How slow? Do you have
a test case that we can see how it is affected?
Greg, unfortunately, I do have nothing.
The customer is in CC list, so maybe he will
provide the test-case, but I doubt.
Please, what are your concerns here?
The patch in question does this:
---
+ ret = usb_control_msg(port->serial->dev,
+ usb_rcvctrlpipe(port->serial->dev, 0),
+ FTDI_SIO_GET_MODEM_STATUS_REQUEST,
+ FTDI_SIO_GET_MODEM_STATUS_REQUEST_TYPE,
+ 0, priv->interface,
+ buf, 2, WDR_TIMEOUT);
---
Obviously, this is too expensive to call too frequently,
or am I missing something?
Why do you think that is too expensive to call? Does it somehow stop
the data being sent to the device through the serial endpoints? Is
userspace calling this function too much slowing something else down?
No, it doesn't slow down the data transfer.
But it makes a select() call to take much longer to complete,
and the same goes to TIOCOUTQ ioctl. Yes, the app calls select()
quite too much, and it is single-threaded, too. :)
I asked the customer to comment out
tty_chars_in_buffer(tty) < WAKEUP_CHARS
line in n_tty.c, and he said that cured his problems,
so I think my guess was right.
What exactly is the "problem" being seen?
No idea.
Well, I can make a test-case that does 1000000 select() calls
in a loop and time it. This is probably the best I can do.
The patch claims it only affects tcdrain() and close().
Its trivial to see it also affects poll(), select() and TIOCOUTQ
ioctl, so even from that it is already broken.
Why do you need a test-case for this?
Because I don't know what the problem really is :)
Slow select() call (and other calls).
Can we just use usb_serial_generic_chars_in_buffer()
in ftdi_sio? What was the reason behind the patch at all,
why it is so importand to query TEMT? I can write some
test-case, but it would be better if I at least understand
why the patch was needed at all. I don't understand why
quering TEMT is that important.
I can code up the patch that will just stop quering TEMT,
test it with the customer and send it to you (basically, a revert
of the patch in question).
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