rf69_set_deviation in rf69.c (pi433 driver)

[Hugo Lefeuvre]

Hi Marcus,

I have been taking a look at the pi433 driver these last days, and started
working on the remaining TODOs. I just stumbled across the following
one (drivers/staging/pi433/rf69.c):

245         // TODO: Dependency to bitrate
246         if (deviation < 600 || deviation > 500000) {
247                 dev_dbg(&spi->dev, "set_deviation: illegal input param");
248                 return -EINVAL;
249         }

According to the datasheet[0], the deviation should always be smaller
than 300kHz, and the following equation should be respected:

  (1) FDA + BRF/2 =< 500 kHz

Why did you choose 500kHz as max for FDA, instead of 300kHz ? It looks like
a bug to me.

Concerning the TODO, I can see two solutions currently:

1. Introduce a new rf69_get_bit_rate function which reads REG_BITRATE_MSB
   and REG_BITRATE_LSB and returns reconstructed BRF.
   We could use this function in rf69_set_deviation to retrieve the BRF.

+ clean
+ intuitive
- heavy / slow

2. Store BRF somewhere in rf69.c, initialize it with the default value
   (4.8 kb/s) and update it when rf69_set_bit_rate is called.

+ easy
- dirty, doesn't fit well with the design of rf69.c (do not store
  anything, simply expose API)

I'd really prefer going for the first one, but I wanted to have your opinion
on this.

Thanks for your work !

Best regards,
 Hugo

[0] http://www.hoperf.com/upload/rf/RFM69CW-V1.1.pdf

[CC-ing Valentin Vidic, he was quite active on the pi433 driver these
last months]

-- 
             Hugo Lefeuvre (hle)    |    www.owl.eu.com
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