Re: [PATCH 1/2] of: add support for reading s32 property value

[Stephen Warren <swarren@wwwdotorg.org>]

On 12/12/2013 06:24 AM, Laxman Dewangan wrote:
> Add of_property_read_s32() to read the signed 32bit number
> from dt property value. This supports to pass the -ve numbers
> from dt. Use 2's complement method for represnting negative number
> and passed as u32 from dts. When reading back the value, again
> converted to 2's complement if msb shows as 1.

> diff --git a/include/linux/of.h b/include/linux/of.h

> +static inline int of_property_read_s32(const struct device_node *np,
> +				       const char *propname,
> +				       s32 *out_value)
> +{
> +	u32 val;
> +	int ret;
> +
> +	ret = of_property_read_u32(np, propname, &val);
> +	if (ret < 0)
> +		return ret;
> +
> +	/* 2's complement if MSB is 1 */
> +	*out_value = (val & 0x80000000U) ? -((val ^ 0xFFFFFFFFU) + 1) : val;

I may not be thinking straight today since I have a cold, but doesn't
patch 2/2 encode negative values as 2's complement, and an s32 variable
in the kernel is also encoded as 2's complement, so all you need here is
a cast:

	*out_value = (s32)val;

... since the cast doesn't change the binary representation?