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

[Laxman Dewangan]

On Thursday 12 December 2013 11:31 PM, Stephen Warren wrote:
> 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?

This is correct if all architecture follows the 2's complement for -Ve 
number representation.
If this is true then the patch will become more easy.
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