>>>>> "Michael" == Michael Plante <michael.plante@xxxxxxxxx> writes:
Michael> Uwe Bonnes wrote:
>>> Is this your use case? The (microcontroller) device has already sent
>>> some data. Now you do ftdi_read_data(struct ftdi_context *ftdi,
>>> unsigned char *buf, big_size) and expect all data already in the FT
>>> fifo to be returned even when the number of bytes available is
>>> smaller than the requested number.
Michael> I expect what's available to be returned if the FTDI chip
Michael> indicates that it should be. But my read call always blocked
Michael> for a small amount of time, depending on how much data the FTDI
Michael> chip was ready to push out. That was desirable. Blocking for
Michael> the full timeout value is not desirable, and setting it
Michael> extremely small just hogs the CPU.
But again, the behaviour is unexpected compared to all other read() calls I know
of. How would you implement your requirement of reading variable length
response with low latency with a uc connected to a real serial port.
char buf[BIG];
int fd = open ("/dev/tty");
read (fd, buf, BIG);
wouldn't give you the result you expect libftdi() to work.
You would probably read single bytes until you have a sensible
response. Working that way for your use case in libftdi already works, and with
the libftdi internal buffer getting filled with the first sucessfull
libusb_bulk_transfer() wouldn't produce system call overhead. So the same
argument
Michael> Ok, but that's not additional CPU overhead, it's additional code
complexity.
holds also for your use case...
That doesn't mean I understand your requirements, and libftdi should work in a
sensible and _well documented_ way and in consense with as many users as
possible...
...
Michael> The FTDI device has the SI/WU pin. How do you expect that to
Michael> function if you consider the current behavior "broken"?
The SI/WU pin functionality isn't impacted.
What about
- returning when at least some data has been read
- or ftdi->usb_read_timeout has expired?
At present we always return when the latency timer expires.
Bye
--
Uwe Bonnes bon@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
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