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

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Message-ID: <20130503165231.GA1966@kroah.com>
Date:	Fri, 3 May 2013 09:52:31 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Jarkko Huijts <jarkko.huijts@...il.com>,
	Alan Cox <alan@...ux.intel.com>, linux-usb@...r.kernel.org,
	linux-serial@...r.kernel.org,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Caylan Van Larson <i@...lan.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: Regression: ftdi_sio is slow (since Wed Oct 10 15:05:06 2012)

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?

> 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?

> 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 :)

thanks,

greg k-h
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