Re: [PATCH] USB: serial: ftdi_sio: fix extreme low-latency setting

[Johan Hovold <johan@kernel.org>]

On Wed, Jan 25, 2017 at 04:06:47PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Jan 25, 2017 at 03:35:20PM +0100, Johan Hovold wrote:
> > Since commit 557aaa7ffab6 ("ft232: support the ASYNC_LOW_LATENCY
> > flag") the FTDI driver has been using a receive latency-timer value of
> > 1 ms instead of the device default of 16 ms.
> > 
> > The latency timer is used to periodically empty a non-full receive
> > buffer, but a status header is always sent when the timer expires
> > including when the buffer is empty. This means that a two-byte bulk
> > message is received every millisecond also for an otherwise idle port as
> > long as it is open.
> > 
> > Let's restore the pre-2009 behaviour which reduces rate of status
> > messages to 1/16th (e.g. interrupt frequency drops from 1 kHz to 62.5
> > Hz) by not setting ASYNC_LOW_LATENCY by default.
> > 
> > Anyone willing to pay the price for the minimum-latency behaviour should
> > set the flag explicitly instead using the TIOCSSERIAL ioctl or a tool
> > such as setserial (e.g. setserial /dev/ttyUSB0 low_latency).
> > 
> > Note that since commit 0cbd81a9f6ba ("USB: ftdi_sio: remove
> > tty->low_latency") the ASYNC_LOW_LATENCY flag has no other effects but
> > to set a minimal latency timer.
> > 
> > Reported-by: Antoine Aubert <a.aubert@overkiz.com>
> > Fixes: 557aaa7ffab6 ("ft232: support the ASYNC_LOW_LATENCY flag")
> > Cc: stable <stable@vger.kernel.org>	# v2.6.31
> > Signed-off-by: Johan Hovold <johan@kernel.org>
> > ---
> > 
> > Greg,
> > 
> > I've been aware of this overhead for a while, but never realised it was
> > actually a regression introduced in 2009.
> > 
> > Fixing something like this after such a long time obviously means
> > risking a regression for anyone who is now relying on the new default
> > behaviour instead. I still think it's reasonable in this case to restore
> > the earlier behaviour given the penalty everyone else is paying for a
> > minimal-latency behaviour that they likely do not need or want.
> > 
> > Whether this should go to stable is a different question. Perhaps the
> > stable tag is not warranted, and this should just be the default
> > behaviour going forward? What do you think?
> 
> I think the stable tag is warrented here.  Do you want me to take this
> patch now into my usb-linus tree, or will you include it in a pull
> request?

I'll include it in a pull request. I was gonna apply it for -next, but
if you prefer I can send it along with some new device ids I have queued
up for 4.10-rc?

Thanks,
Johan