linux-kernel - RE: [PATCH] i2c: altera: Use 64-bit arithmetic instead of 32-bit

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Date:   Tue, 11 Feb 2020 11:44:55 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Gustavo A. R. Silva'" <gustavo@...eddedor.com>,
        Thor Thayer <thor.thayer@...ux.intel.com>
CC:     "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] i2c: altera: Use 64-bit arithmetic instead of 32-bit

From: Gustavo A. R. Silva
> Sent: 10 February 2020 19:27
> 
> Add suffix ULL to constant 300 in order to avoid a potential integer
> overflow and give the compiler complete information about the proper
> arithmetic to use. Notice that this constant is being used in a context
> that expects an expression of type u64, but it's currently evaluated
> using 32-bit arithmetic.
> 
> Addresses-Coverity: 1458369 ("Unintentional integer overflow")
> Fixes: 0560ad576268 ("i2c: altera: Add Altera I2C Controller driver")
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  drivers/i2c/busses/i2c-altera.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/busses/i2c-altera.c b/drivers/i2c/busses/i2c-altera.c
> index 5255d3755411..526f453f0ff7 100644
> --- a/drivers/i2c/busses/i2c-altera.c
> +++ b/drivers/i2c/busses/i2c-altera.c
> @@ -171,7 +171,8 @@ static void altr_i2c_init(struct altr_i2c_dev *idev)
>  	/* SCL Low Time */
>  	writel(t_low, idev->base + ALTR_I2C_SCL_LOW);
>  	/* SDA Hold Time, 300ns */
> -	writel(div_u64(300 * clk_mhz, 1000), idev->base + ALTR_I2C_SDA_HOLD);
> +	writel(div_u64(300ULL * clk_mhz, 1000),
> +	       idev->base + ALTR_I2C_SDA_HOLD);

Why not factor out the 100?
It may then be you can do 32bit arithmetic (3 * clk_mhz / 10).
If clk_mhz is MHz (not mHz) then the sum will never wrap 32 bits.

	David

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