Async methods in RawSerial non accessible in UnbufferedSerial

[mcesar-rlacruz]

Description of defect

Some asynchronous methods in the now deprecated class RawSerial are no longer accessible from the new class UnbufferedSerial, as now it inherits privately from SerialBase, whereas previously RawSerial inherited publicly. Such methods are read() (the callback and char matching ones), write() (the callback one) and others.

Target(s) affected by this defect ?

Detected on DISCO_L072CZ_LRWAN1, but apparently universal.

Toolchain(s) (name and version) displaying this defect ?

ARMC6 6.16

What version of Mbed-os are you using (tag or sha) ?

6.16.0-rc1

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed Studio 1.4.4, mbed-cli, Keil uVision 5, apparently universal

How is this defect reproduced ?

UnbufferedSerial gps;
gps.SerialBase::read(...);
g_gps.set_break();

Will report that read() and set_break() are private members of SerialBase.

[0xc0170]

Unbufferedserial provides read/write, exposing some parts of SerialBase but not all. You can describe why it should expose read/write.

What's the use case? Is this about asynch methods read/write from SerialBase?

[mcesar-rlacruz]

Hi,

My particular use case is receiving NMEA messages inside a system with various talkers and, yes, it is about (but not only) the now hidden async read/write methods from SerialBase. RawSerial::read() allowed launching an async RX transfer to be terminated after receiving a fixed amount of data or when some magic char is received, but this last capability is now lost (but the code is there, inside SerialBase!). Maybe I can circumvent that by registering a callback with UnbufferedSerial::sigio() that checks all arriving chars to be equal to the magic one, but this will led to my callback being executed on each char RX, whereas previously all that was managed transparently by RawSerial, that called the callback I supplied and, even more, told me if the amount of data was exhausted, or the magic char was found, or there was a frame error in the port or...

Also, I see no way now to:

• manage break conditions in the serial port. My system has various talkers in the same serial port, sometimes I need to send a break
• abort any ongoing read or write operation in case I'm no longer interested in the remaining messages (in my scenario this happens each time I know, by another means, that there is no GPS fix, I can skip the rest of the sequence —which may be hundreds of chars long— until the next FIX pulse is received
• ascertain the reason a read/write operation failed (or if it failed), previously I had the mechanism of the the SERIALEVENT... passed to the callback registered when calling SerialBase::read/write(), but callbacks registered with sigio do not receive any event information. Again, I have some talkers in the same port, sometimes there are frame errors

These are the reasons I think that SerialBase::read/write (and also xxx_break() and abort_xxx()) should be user accessible.

Finally, I had many problems with this system when migrating from mbed 5 to mbed 6 until I decided to change from private SerialBase to public SerialBase in the declaration of class UnbufferedSerial inside file UnbufferedSerial.h. C'mon, the code is there, let us use it ;-)

Thanks & regards.

EDIT: now in my local mbed 6 I switched back to private SerialBase, but added more using declarations to re-gain visibility for the methods I do need.

[mcesar-rlacruz]

Hi again,

I tried using sigio() to register a callback to be called at each character RX. Maybe it is that I do not understand it, but I cannot get the callback called.

I see no way to solve this communication problem with the new UnbufferedSerial class, a problem that was solved previously with RawSerial.

I have also found a previous issue about this: #13388

And a thread in the forum describing another possible scenario.

Thanks & regards.

[mcesar-rlacruz]

Still looking into this. Table below shows the methods inside UnbufferedSerial and their true meaning:

Method	Does
read	reads 1 char using _base_getc(), blocking
write	writes many chars using _base_putc(), blocking
set_flow_control	calls SerialBase::set_flow_control()
seek	nothing, returns -ESPIPE
close	nothing, returns 0
sync	nothing, returns 0
isatty	nothing, returns true
rewind	nothing, returns -ESPIPE
tell	nothing, returns -ESPIPE
size	nothing, returns -EINVAL
truncate	nothing, returns -EINVAL
set_blocking	nothing, returns 0 or -ENOTTY
is_blocking	nothing, returns true
enable_input	calls SerialBase::enable_input(enabled);
enable_output	calls SerialBase::enable_output(enabled);
poll	calls SerialBase::readable()/writable();
writable	calls SerialBase::readable()
readable	calls SerialBase::writable()
sigio	nothing

So, basically, UnbufferedSerial is an envelope for getc()/putc(), but definitely not a replacement for the deprecated RawSerial.

Knowing this I tried to re-phrase my problem with BufferedSerial. The results are slightly better (at least sigio() works, but it is not even comparable to the old attach() that at least distinguished between RX and TX IRQs), but lacking all of the previous functionality inside RawSerial I described above.

My perception is that now on mbed6, simply, there is no direct way to gain some low level control over serial ports.

Fortunately SerialBase is documented and one can inherit his/her own UART class from it to cope with this. But I wonder if, seeing the current state of affairs, the low level methods on it will remain inside mbed in the future.

[0xc0170]

I have also found a previous issue about this: https://github.com/ARMmbed/mbed-os/issues/13388

Good found. I recall seeing this before but no agreement.

You can use sgio and handle it by yourself (I found few pointers quickly, there should be however the functionality provided somewhere already): https://github.com/ARMmbed/mbed-os/issues/5035

I don't have anything at hand now unfortunately.

[mcesar-rlacruz]

Thanks. Unfortunately the problem described on #5035 was finally solved using a read() method resembling the now hidden read() method inside SerialBase.