Messages in this thread | | | Date | Fri, 13 Dec 2013 12:02:43 +0530 | From | Laxman Dewangan <> | Subject | Re: [PATCH 1/2] of: add support for reading s32 property value |
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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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