[PATCH v2] 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 less likely to happen due to misconfiguration of the uC

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
---
v2: remove typo correction from this patch - Req Greg k-h, Fabio A.
v1: https://lore.kernel.org/lkml/20220103081135.GA11642@mail.google.com/
---
 drivers/staging/pi433/rf69.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/pi433/rf69.c b/drivers/staging/pi433/rf69.c
index e62e61ef4d27..d64df072d8e8 100644
--- a/drivers/staging/pi433/rf69.c
+++ b/drivers/staging/pi433/rf69.c
@@ -255,13 +255,25 @@ 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 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;
 	}
 
-- 
2.25.4