On 19/09/2005 11:52, Anders Blomdell wrote:
Hi,
I have an application the relies on setting the serial port into low
latency mode, but this doesn't work well with ftdi_sio.c, because:
1. setting low latency (priv->flags & ASYNC_LOW_LATENCY) requires
CAP_SYS_ADMIN
That has always been the case with Linux serial drivers.
2. setting low latency doesn't affect the latency timer
(FTDI_SIO_SET_LATENCY_TIMER_REQUEST)
(Note for regulars: this is a hardware timer that tells the FTDI chip
how often to report received data.)
I guess it could be argued that there should be some link between the
two. Then again, it could also be argued that there should be some link
with c_cc[VTIME] in the termios structure in non-canonical mode (though
that has too large a range and too low a resolution to be of much use here).
The submitted patch makes:
1. The driver start in normal latency mode (previously low latency)
I'd rather it not do that. Low latency mode has some advantages that
would be lost by default with this patch.
2. Let any user change latency (i.e. no CAP_SYS_ADMIN required)
That goes against the grain of existing Linux serial drivers.
3. Setting low latency sets the latency timer to 1 ms, normal latency
sets the
latency timer to 16 ms (chip default).
There is some merit in that, but keeping the port initialixed to low
latency mode as I recommend would result in an increased overhead as
data may be received in shorter chunks. This would reduce the maximum
throughput. So it boils down to whether the port should be initialized
for lowest latency or maximum throughput by default.
Note that there is an "out of band" solution to tweaking the FTDI chip's
latency timer via a sysfs parameter.
With the patch applied, low latency mode gives a delay of about 3-4 ms
in contrast
to the 16 ms normally encountered.
Just wondering: as you still get a latency of 3-4 ms, have you tried
setting the FTDI chip's latency timer to a similar number to see how
much difference it makes?
Regards,
Ian Abbott.
--
-=( Ian Abbott @ MEV Ltd. E-mail: <[EMAIL PROTECTED]> )=-
-=( Tel: +44 (0)161 477 1898 FAX: +44 (0)161 718 3587 )=-
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