Linux UART does not time out while receiving (Raspberry PI)

[StWandl]

Hello everybody,

I successfully built and used the library on my Raspberry PI.
But one thing I noticed is that read() in hal_uart_receive() (hal_linux_uart_userspace.c) did not time out when nothing is received. So it just blocked my application.

In hal_uart_open_file() the read timeout is set to 500ms.
// hal_linux_uart_userspace.c line 146
tty.c_cc[VTIME] = 5;

But read() still does not time out, so I did some digging and found a solution by setting VMIN in the termios struct to zero.
tty.c_cc[VMIN] = 0;

Some infos for VTIME and VMIN in the termios struct:
http://unixwiz.net/techtips/termios-vmin-vtime.html

VMIN represents the minimum number of received bytes.
So if VMIN > 0 --> read() never times out because nothing is received.

With VMIN set to 0 the read() call timed out after 500ms.

So I think setting VMIN to 0 should be added to hal_uart_open_file().
Or did I miss something that would be negatively affected by this change?

[bryan-hunt]

It would have to be tested - there are a couple of usecases for the uart usage. Kit protocol (which is basic ascii frames) and SWI which is more complicated (oversampling). In the later case the rx & tx lines are tied together.

Were you using kit protocol to communicate with a development board (i.e. with the kit_host from the app folder on the other side) or were you trying to work with a SWI device?

[StWandl]

Sorry, I forgot to mention. I was communicating with a SWI device (ATSHA204A) and had RX & TX tied together.
The communication worked fine, but one time I forgot to power the device and I noticed that the program got stuck due to the blocking read() call.

[bryan-hunt]

Thanks for the additional details. This does make sense to modify given the SWI usecase generally revolves around removable components.