[PATCH] staging: pi433: fix frequency deviation check

[Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>]

rf69 datasheet states that frequency deviation must exceed 600 Hz but
also that frequency deviation + (bitrate / 2) should be less than equal
to 500 kHz to ensure proper modulation.

This patch validates that both conditions are met so RF intersymbol
interference is "avoided" where possible.

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
---
 drivers/staging/pi433/rf69.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/drivers/staging/pi433/rf69.c b/drivers/staging/pi433/rf69.c
index e62e61ef4d27..cc22915fd489 100644
--- a/drivers/staging/pi433/rf69.c
+++ b/drivers/staging/pi433/rf69.c
@@ -255,17 +255,29 @@ int rf69_set_deviation(struct spi_device *spi, u32 deviation)
 	int retval;
 	u64 f_reg;
 	u64 f_step;
+	u32 bit_rate_reg;
+	u32 bit_rate;
 	u8 msb;
 	u8 lsb;
 	u64 factor = 1000000; // to improve precision of calculation
 
-	// TODO: Dependency to bitrate
-	if (deviation < 600 || deviation > 500000) {
-		dev_dbg(&spi->dev, "set_deviation: illegal input param");
+	// calculate bit rate
+	bit_rate_reg = rf69_read_reg(spi, REG_BITRATE_MSB) << 8;
+	bit_rate_reg |= rf69_read_reg(spi, REG_BITRATE_LSB);
+	bit_rate = F_OSC / bit_rate_reg;
+
+	/*
+	 * frequency deviation must exceed 600 Hz and but not exceed
+	 * 500kHz when taking bitrate dependency into consideration
+	 * to ensure proper modulation
+	 */
+	if (deviation < 600 || (deviation + (bit_rate / 2)) > 500000) {
+		dev_dbg(&spi->dev,
+			"set_deviation: illegal input param: %u", deviation);
 		return -EINVAL;
 	}
 
-	// calculat f step
+	// calculate f step
 	f_step = F_OSC * factor;
 	do_div(f_step, 524288); //  524288 = 2^19
 
-- 
2.25.4