netdev - [PATCH] wcn36xx: Add RX frame SNR as a source of system entropy

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Date:   Tue, 13 Sep 2022 18:42:24 +0100
From:   Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To:     loic.poulain@...aro.org, kvalo@...nel.org, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc:     wcn36xx@...ts.infradead.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, bryan.odonoghue@...aro.org,
        "Jason A . Donenfeld" <Jason@...c4.com>
Subject: [PATCH] wcn36xx: Add RX frame SNR as a source of system entropy

The signal-to-noise-ratio of a received frame is a representation of noise
in a given received frame.

RSSI - received signal strength indication can appear pretty static
frame-to-frame but noise will "bounce around" more depending on the EM
environment, temperature or placement of obstacles between the transmitter
and receiver.

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

Commit: 2aa56cca3571 ("ath9k: Mix the received FFT bins to the random pool")

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

Cc: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
---
 drivers/net/wireless/ath/wcn36xx/txrx.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/wireless/ath/wcn36xx/txrx.c b/drivers/net/wireless/ath/wcn36xx/txrx.c
index 8da3955995b6e..f3b77d7ffebe4 100644
--- a/drivers/net/wireless/ath/wcn36xx/txrx.c
+++ b/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)
@@ -297,6 +298,8 @@ 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, sizeof(int));
 }
 
 int wcn36xx_rx_skb(struct wcn36xx *wcn, struct sk_buff *skb)
-- 
2.37.3