Hi Violeta,
On 04/04/2015 09:54 AM, Violeta Menendez Gonzalez wrote:
> Hello everyone,
>
> I'm testing UART with different custom baud rates on a renesas lager board that uses the sh-sci driver.
>
> After discovering that TIOCSSERIAL is deprecated
ioctl(TIOCSSERIAL) is not deprecated but not every driver supports its
(full) functionality. Feel free to add that (missing) functionality to the
sh-sci driver.
> I found the way of doing it with TCSETS2, which seems to work. My problem comes when trying to confirm the speed that I've tried to set, it seems to set any speed I request, when it's obvious that some of those speeds are not even valid. The issue seems to be with set_termios(), the serial driver API does not allow the serial driver to return an error, it must make a best effort. But then when I ask back for the speed again it returns the speed I had requested, instead of the actual one.
Well, there are two issues here:
1) You need to set BOTHER in termios2.c_cflag like this:
termios2.c_cflag &= CBAUD;
termios2.c_cflag |= BOTHER;
That disables nearest baud fitting in the serial core.
2) There's a limit to the baud resolution you can achieve
Using the original 16550 as an example, which has a 1.843Mhz clock,
the following baud rates are achievable:
divisor baud rate
11 10472
12 9599
13 8860
Faster uart clocks achieve smaller intervals but nowhere near +/-13 baud.
You could use the ->base_baud field obtained with ioctl(TIOCGSERIAL) to
determine the actual baud:
div = round_nearest(base_baud / baud);
actual = round_nearest(base_baud / div);
There's some complications with the sh-sci driver because it may be using
oversampling other than 16 (based on port type, which is also in TIOCGSERIAL),
but hopefully you get the general idea.
> So for example, when I run my code trying to set up a speed of 9587bps, I would expect it to actually set 9600, but when I ask for the speed back I still get 9587.
>
> Is this the normal/expected behaviour?
Serial drivers are notoriously bad at updating the termios with what is
actually set, and now the problem has become that userspace now relies
on serial drivers _not_ updating the termios.
For example, no driver I know of masks off CRTSCTS if not available.
Regards,
Peter Hurley
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