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

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

[Gustavo A. R. Silva]

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@embeddedor.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);
 
 	/* Mask all master interrupt bits */
 	altr_i2c_int_enable(idev, ALTR_I2C_ALL_IRQ, false);
-- 
2.25.0

Re: [PATCH] i2c: altera: Use 64-bit arithmetic instead of 32-bit

[Thor Thayer]

On 2/10/20 1:26 PM, Gustavo A. R. Silva wrote:
> 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@embeddedor.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);
>   
>   	/* Mask all master interrupt bits */
>   	altr_i2c_int_enable(idev, ALTR_I2C_ALL_IRQ, false);
> 
Thank you.

Reviewed-by: Thor Thayer <thor.thayer@linux.intel.com>

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

[David Laight]

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@embeddedor.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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