linux-kernel - Re: [PATCH] clk: sunxi: clean up rate counting

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Message-ID: <2385242.n0HT0TaD9V@jernej-laptop>
Date: Thu, 20 Feb 2025 19:12:43 +0100
From: Jernej Škrabec <jernej.skrabec@...il.com>
To: Emilio López <emilio@...pez.com.ar>,
 David Laight <david.laight.linux@...il.com>,
 Anastasia Belova <abelova@...ralinux.ru>
Cc: Anastasia Belova <abelova@...ralinux.ru>,
 Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>,
 Chen-Yu Tsai <wens@...e.org>, Samuel Holland <samuel@...lland.org>,
 Maxime Ripard <mripard@...nel.org>, Hans de Goede <hdegoede@...hat.com>,
 linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org,
 lvc-project@...uxtesting.org
Subject: Re: [PATCH] clk: sunxi: clean up rate counting

Dne ponedeljek, 3. februar 2025 ob 12:29:28 Srednjeevropski standardni čas je Anastasia Belova napisal(a):
> If n = 255, the result of multiplication of n and 24000000
> may not fit int type. Swap division and shift with multiplication.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: 6424e0aeebc4 ("clk: sunxi: rewrite sun9i_a80_get_pll4_factors()")
> Signed-off-by: Anastasia Belova <abelova@...ralinux.ru>

While this should work, should we try to remove (old) sunxi clock drivers
instead? Drivers we converted 8 years ago, except A20 GMAC and A80 PRCM clocks.
Even if we convert them now, we need some transition time for them.

Best regards,
Jernej

> ---
>  drivers/clk/sunxi/clk-sun9i-core.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/clk/sunxi/clk-sun9i-core.c b/drivers/clk/sunxi/clk-sun9i-core.c
> index d93c7a53c6c0..639c83ed63b8 100644
> --- a/drivers/clk/sunxi/clk-sun9i-core.c
> +++ b/drivers/clk/sunxi/clk-sun9i-core.c
> @@ -25,12 +25,12 @@
>  
>  static void sun9i_a80_get_pll4_factors(struct factors_request *req)
>  {
> -	int n;
> -	int m = 1;
> -	int p = 1;
> +	unsigned int n;
> +	unsigned int m = 1;
> +	unsigned int p = 1;
>  
>  	/* Normalize value to a 6 MHz multiple (24 MHz / 4) */
> -	n = DIV_ROUND_UP(req->rate, 6000000);
> +	n = DIV_ROUND_UP(req->rate, 6000000ul);
>  
>  	/* If n is too large switch to steps of 12 MHz */
>  	if (n > 255) {
> @@ -50,7 +50,11 @@ static void sun9i_a80_get_pll4_factors(struct factors_request *req)
>  	else if (n < 12)
>  		n = 12;
>  
> -	req->rate = ((24000000 * n) >> p) / (m + 1);
> +	/* Division and shift should be done before multiplication to
> +	 * avoid overflow. The result will be correct because '>> p' and
> +	 * '/ (m + 1)' are both just conditional 'divide by 2'
> +	 */
> +	req->rate = ((24000000ul >> p) / (m + 1)) * n;
>  	req->n = n;
>  	req->m = m;
>  	req->p = p;
>