[riot-devel] Driver design rules in RIOT
Juan Ignacio Carrano
j.carrano at fu-berlin.de
Wed Sep 26 12:16:08 CEST 2018
I'm not very experienced on the driver development side, but enough as a
user to see some issues.
On 9/26/18 9:27 AM, Gunar Schorcht wrote:
> - Should a driver be as complete as possible, which of cource produces
> more code, or should it be kept simple to produce small code? One option
> would be to use the pseudomodule approach to enable additional features.
Part of keeping it small is omitting conversion code (see answer below).
How often does it happen that one runs out of flash space? I'm asking
because I honestly don't know. I do know that it's probably easier for
the user to remove stuff if he runs out of flash than to read the device
manual and add the missing functions if the driver is incomplete.
> On some platforms unused code is not linked into the binary.
Unused functions, where the linker can determine the function is not
used. If you have a big function for configuring device modes, but you
never call it with certain parameters and a bit chunk goes unused, it
may not be optimized away (I'm not sure if LTO changes this).
> - Should a driver support at least data-ready interrupts (if possible at
> all) to realize event-driven data retrieval?
Yes. Totally yes. Polling is dumb:
* Goes against low power goals.
* The data is not polled with a clock that is synchronized with the
sensor clock (if the sensor has an ADC), meaning unpredictable jitter.
> - Should a driver always return normalized/converted data, or rather
> return the raw data and the application needs to convert them? The
> conversion is sometimes quite complex. I saw both approaches of them for
> similar sensors.
* Conversion usually results in loss of precision, especially if one
limits the word length to something like 16 bits (see answer below).
* Doing conversion "right" (in an unbiased way) is non trivial. You
cannot just go around truncating digits.
* Is is beyond the scope of the driver, which should handle device
* If the converted value is not needed, the conversion cannot be undone.
* In SAUL, conversion to and from the base-10 floating point format used
is really painful.
I think the measurement should be raw, and there should be a way to
query the conversion constant. This way the user can choose, and there
are not unnecessary computations done.
In control applications, for example, the conversion is totally not
necessary, as the conversion constants can be folded into the control
> The design rules that are clear to me are:
> - Drivers have to provide an interface for polling with init and read
> that is compatible with SAUL.
Yes. It makes all interfaces consistent. That being said, it is sad that
there is no unified way for configuring and for interrupt driven
> - Output are always 16 bit integers.
I think it is a bad idea to limit output to 16 bits. ADCs meant for
scales, for example, usually have 24 bit . Other applications also
demand higher that 16 bits. Keep in mind that 16 bits is equivalent to
4,8 decimal digits, take 1 bit for the sign and you are left with 4,5.
> What else?
Maybe off topic, but I think we need a IO layer (think SAUL, but more
complete) so that the user does not have to directly interact with
drivers. I would answer many of your questions, as in that case there
would be a well defined interface that device drivers would have to
expose. It is an OS, after all.
Juan I Carrano.
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