wifi: wcn36xx: Add RX frame SNR as a source of system entropy

The signal-to-noise-ratio SNR is returned by the wcn36xx firmware for each
received frame. SNR represents all of the unwanted interference signal
after filtering out the fundamental frequency and harmonics of the
frequency.

Noise can come from various electromagnetic sources, from temperature
affecting the performance hardware components or quantization effects
converting from analog to digital domains.

The SNR value returned by the WiFi firmware then is a good source of
entropy.

Other WiFi drivers offer up the noise component of the FFT as an entropy
source for the random pool e.g.

commit 2aa56cca ("ath9k: Mix the received FFT bins to the random pool")

I attended Jason's talk on sources of randomness at Plumbers and it
occurred to me that SNR is a reasonable candidate to add.

Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220915004117.1562703-2-bryan.odonoghue@linaro.org

drivers/net/wireless/ath/wcn36xx/txrx.c

@@ -16,6 +16,7 @@
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <linux/random.h>
 #include "txrx.h"
 
 static inline int get_rssi0(struct wcn36xx_rx_bd *bd)
@@ -278,6 +279,7 @@ static void wcn36xx_update_survey(struct wcn36xx *wcn, int rssi, int snr,
 	struct ieee80211_supported_band *sband;
 	int idx;
 	int i;
+	u8 snr_sample = snr & 0xff;
 
 	idx = 0;
 	if (band == NL80211_BAND_5GHZ)
@@ -297,6 +299,8 @@ static void wcn36xx_update_survey(struct wcn36xx *wcn, int rssi, int snr,
 	wcn->chan_survey[idx].rssi = rssi;
 	wcn->chan_survey[idx].snr = snr;
 	spin_unlock(&wcn->survey_lock);
+
+	add_device_randomness(&snr_sample, sizeof(snr_sample));
 }
 
 int wcn36xx_rx_skb(struct wcn36xx *wcn, struct sk_buff *skb)