RE: [PATCH] [v2] wifi: b43: fix cordic arithmetic

[David Laight <David.Laight@ACULAB.COM>]

From: Dmitry Antipov
> Sent: 27 June 2023 16:14
> 
> In 'lpphy_start_tx_tone()', 'CORDIC_FLOAT((sample.i * max) & 0xFF)'
> is invalid because it is (<32-bit> & 0xff) shifted right by 15 bits
> and so always evaluates to zero. Looking through brcmsmac's
> 'wlc_lcnphy_start_tx_tone()', the result should be masked instead,
> i. e. 'CORDIC_FLOAT(sample[i].max) & 0xFF'.
> 
> Fixes: 6f98e62a9f1b ("b43: update cordic code to match current specs")
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Cc: stable@vger.kernel.org
> Suggested-by: Jonas Gorski <jonas.gorski@gmail.com>
> Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
> ---
> v2: add Cc: stable and Fixes: (Larry Finger)
> ---
>  drivers/net/wireless/broadcom/b43/phy_lp.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/wireless/broadcom/b43/phy_lp.c b/drivers/net/wireless/broadcom/b43/phy_lp.c
> index 0e5c076e7544..e8ef04e509aa 100644
> --- a/drivers/net/wireless/broadcom/b43/phy_lp.c
> +++ b/drivers/net/wireless/broadcom/b43/phy_lp.c
> @@ -1788,8 +1788,8 @@ static void lpphy_start_tx_tone(struct b43_wldev *dev, s32 freq, u16 max)
>  	for (i = 0; i < samples; i++) {
>  		sample = cordic_calc_iq(CORDIC_FIXED(theta));
>  		theta += rotation;
> -		buf[i] = CORDIC_FLOAT((sample.i * max) & 0xFF) << 8;
> -		buf[i] |= CORDIC_FLOAT((sample.q * max) & 0xFF);
> +		buf[i] = (u16)((CORDIC_FLOAT(sample.i * max) & 0xFF) << 8);
> +		buf[i] |= (u16)(CORDIC_FLOAT(sample.q * max) & 0xFF);

What are the (u16) casts for?
This code is actually called exactly once with max == 100.
The .i and .q are the sine and cosine << 16 (signed).
The CORDIC_FLOAT() is basically >> 16 (not 15) so the result should
be between -100 and +100.
The & 0xFF is there to strip the sign.
The sin+cos are then packed into a short[] then unpacked to be
written to the hardware later.

>  	}
> 
>  	b43_lptab_write_bulk(dev, B43_LPTAB16(5, 0), samples, buf);

Don't open the bag of worms that contains the above :-)

	David

> --
> 2.41.0

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