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@xxxxxxxxxxx>
> Fixes: 557aaa7ffab6 ("ft232: support the ASYNC_LOW_LATENCY flag")
> Cc: stable <stable@xxxxxxxxxxxxxxx> # v2.6.31
> Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
> ---
>
> 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?
thanks,
greg k-h
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