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
2. setting low latency doesn't affect the latency timer
(FTDI_SIO_SET_LATENCY_TIMER_REQUEST)
The submitted patch makes:
1. The driver start in normal latency mode (previously low latency)
2. Let any user change latency (i.e. no CAP_SYS_ADMIN required)
3. Setting low latency sets the latency timer to 1 ms, normal latency
sets the
latency timer to 16 ms (chip default).
With the patch applied, low latency mode gives a delay of about 3-4 ms
in contrast
to the 16 ms normally encountered.
Best regards
Anders Blomdell
------------------------------------------------------------------------
------
Anders Blomdell
Department of Automatic Control Email:
[EMAIL PROTECTED]
Lund Institute of Technology Phone: +46 46 222 4625
Box 118, S-221 00 Lund, Sweden Fax: +46 46 138118
--- old/ftdi_sio.c 2005-09-08 21:28:50.000000000 +0200
+++ new/ftdi_sio.c 2005-09-19 12:21:20.073885017 +0200
@@ -1184,7 +1184,6 @@ static int set_serial_info(struct usb_se
(new_serial.flags & ASYNC_FLAGS));
priv->custom_divisor = new_serial.custom_divisor;
- port->tty->low_latency = (priv->flags & ASYNC_LOW_LATENCY) ? 1
: 0;
check_and_exit:
if ((old_priv.flags & ASYNC_SPD_MASK) !=
@@ -1206,7 +1205,29 @@ check_and_exit:
(old_priv.custom_divisor != priv->custom_divisor))) {
change_speed(port);
}
+ if (((old_priv.flags & ASYNC_LOW_LATENCY) !=
+ (priv->flags & ASYNC_LOW_LATENCY))) {
+ int latency;
+ char buf[1];
+
+ if (priv->flags & ASYNC_LOW_LATENCY) {
+ port->tty->low_latency = 1;
+ latency = 1;
+ } else {
+ port->tty->low_latency = 0;
+ latency = 16;
+ }
+ if (priv->chip_type == FT232BM || priv->chip_type ==
FT2232C) {
+ dbg("%s: setting latency timer = %i",
__FUNCTION__, latency);
+ usb_control_msg(port->serial->dev,
+
usb_sndctrlpipe(port->serial->dev, 0),
+
FTDI_SIO_SET_LATENCY_TIMER_REQUEST,
+
FTDI_SIO_SET_LATENCY_TIMER_REQUEST_TYPE,
+ latency, priv->interface,
+ buf, 0, WDR_TIMEOUT);
+ }
+ }
return (0);
} /* set_serial_info */
@@ -1373,9 +1394,6 @@ static int ftdi_common_startup (struct u
spin_lock_init(&priv->rx_lock);
init_waitqueue_head(&priv->delta_msr_wait);
- /* This will push the characters through immediately rather
- than queue a task to deliver them */
- priv->flags = ASYNC_LOW_LATENCY;
/* Increase the size of read buffers */
kfree(port->bulk_in_buffer);
@@ -1589,7 +1607,28 @@ static int ftdi_open (struct usb_serial
dbg("%s", __FUNCTION__);
- port->tty->low_latency = (priv->flags & ASYNC_LOW_LATENCY) ? 1
: 0;
+ {
+ int latency;
+ char buf[1];
+
+ if (priv->flags & ASYNC_LOW_LATENCY) {
+ port->tty->low_latency = 1;
+ latency = 1;
+ } else {
+ port->tty->low_latency = 0;
+ latency = 16;
+ }
+ if (priv->chip_type == FT232BM || priv->chip_type ==
FT2232C) {
+ dbg("%s: setting latency timer = %i",
__FUNCTION__, latency);
+
+ usb_control_msg(port->serial->dev,
+
usb_sndctrlpipe(port->serial->dev, 0),
+
FTDI_SIO_SET_LATENCY_TIMER_REQUEST,
+
FTDI_SIO_SET_LATENCY_TIMER_REQUEST_TYPE,
+ latency, priv->interface,
+ buf, 0, WDR_TIMEOUT);
+ }
+ }
/* No error checking for this (will get errors later anyway) */
/* See ftdi_sio.h for description of what is reset */
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