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From:   Loic Poulain <loic.poulain@linaro.org>
Date:   Wed, 14 Sep 2022 14:53:35 +0200
Message-ID: <CAMZdPi_jZEHSz9+WaR3N-MANU7YdYK0zqO7VXNwAcES=YkNaxg@mail.gmail.com>
Subject: Re: [PATCH] wcn36xx: Add RX frame SNR as a source of system entropy
To:     "Bryan O'Donoghue" <bryan.odonoghue@linaro.org>
Cc:     kvalo@kernel.org, davem@davemloft.net, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com, wcn36xx@lists.infradead.org,
        linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
        "Jason A . Donenfeld" <Jason@zx2c4.com>
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Hi Bryan,

On Tue, 13 Sept 2022 at 19:42, Bryan O'Donoghue
<bryan.odonoghue@linaro.org> wrote:
>
> 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@zx2c4.com>
> Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.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));

We store the SNR in an integer, but isn't it reported as u8 (or s8) by
the firmware? So maybe we should just inject the LSByte since the upper
ones will always be 0?


>  }
>
>  int wcn36xx_rx_skb(struct wcn36xx *wcn, struct sk_buff *skb)
> --
> 2.37.3
>