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semi-blocking serial read
- To: ecos-discuss at sources dot redhat dot com
- Subject: [ECOS] semi-blocking serial read
- From: Grant Edwards <grante at visi dot com>
- Date: Wed, 15 Aug 2001 17:10:01 -0500
My serial.c driver code has diverged somewhat from the standard
serial.c. Most of the things I've done have either evolved in
parallel in the official one (like a non-blocking mode) or are
things that other people don't want.
One unique thing that mine does have that is both general and
(IMO) pretty useful is semi-blocking reads.
A semi-blocking read will block until _some_ amount of data is
available and return after transferring whatever is available.
This is primarily useful for UARTs with FIFOs such that receive
data comes in "chunks". A semi-blocking read waits for the
next "chunk". This allows you to process incoming data with
the lowest possible latency without doing a non-blocking read
in a busy-wait loop.
The change is pretty trivial. In serial_read() you do
something like this:
1 while (size < *len) {
2 if (cbuf->nb > 0) {
3 #ifdef CYGPKG_IO_SERIAL_FLOW_CONTROL
4 if ( (cbuf->nb <= cbuf->low_water) &&
5 (chan->flow_desc.flags & CYG_SERIAL_FLOW_IN_THROTTLED) )
6 restart_rx( chan, false );
7 #endif
8 *buf++ = cbuf->data[cbuf->get];
9 if (++cbuf->get == cbuf->len) cbuf->get = 0;
10 cbuf->nb--;
11 size++;
12 } else {
13 #ifdef CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING
--- 14 if (!cbuf->blocking) {
+++ 14 if (cbuf->nonblocking || (cbuf->semiblocking && size))) {
15 *len = size; // characters actually read
16 res = -EAGAIN;
17 break;
18 }
19 #endif // CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING
The cbuf struct needs to change a little to accomodate the two
flags "nonblocking" and "semiblocking", but I think you see
what I mean.
I don't use serial.c and probably won't get around to
submitting a patch for some time, but I thought I'd toss the
idea out in case anybody else could use something like it.
--
Grant Edwards
grante@visi.com