Spliting u16, u32 to u8 array

Spliting u16, u32 to u8 array

[Crt Mori]

Hi,
I am sure at least some of you were faced to split a u16 or u32 value
in u8 array without being affected by host endianness. My search
turned empty for a standardized function for this task, but this seems
quite improbable. My task is to send 16 bit LE data through i2c.

I am looking for something like:
static void u16_to_u8_array(u16 value, u8 *array)
{
        *array = cpu_to_le16(value) >> 8;
        *(++array) = (u8) cpu_to_le16(value);
}

Best regards,
Crt

Re: Spliting u16, u32 to u8 array

[Peter Meerwald]

Hello,

> I am sure at least some of you were faced to split a u16 or u32 value
> in u8 array without being affected by host endianness. My search
> turned empty for a standardized function for this task, but this seems
> quite improbable. My task is to send 16 bit LE data through i2c.
> 
> I am looking for something like:
> static void u16_to_u8_array(u16 value, u8 *array)
> {
>         *array = cpu_to_le16(value) >> 8;
>         *(++array) = (u8) cpu_to_le16(value);
> }

how about

static void u16_to_le16_array(u16 value, __le16 *array)
{
	*array++ = cpu_to_le16(value);
}

and calling it with a cast if need be, such as

char *u8array;
u16_to_le16_array(123, (__le16 *) u8array)

an issue could be alignment of the u8array

I'd try to avoid u8 altogether and work with le16 as a datatype

p.

-- 

Peter Meerwald
+43-664-2444418 (mobile)

Re: Spliting u16, u32 to u8 array

[Lars-Peter Clausen]

On 08/06/2015 12:42 PM, Peter Meerwald wrote:
> Hello,
> 
>> I am sure at least some of you were faced to split a u16 or u32 value
>> in u8 array without being affected by host endianness. My search
>> turned empty for a standardized function for this task, but this seems
>> quite improbable. My task is to send 16 bit LE data through i2c.
>>
>> I am looking for something like:
>> static void u16_to_u8_array(u16 value, u8 *array)
>> {
>>         *array = cpu_to_le16(value) >> 8;
>>         *(++array) = (u8) cpu_to_le16(value);
>> }
> 
> how about
> 
> static void u16_to_le16_array(u16 value, __le16 *array)
> {
> 	*array++ = cpu_to_le16(value);
> }
> 
> and calling it with a cast if need be, such as
> 
> char *u8array;
> u16_to_le16_array(123, (__le16 *) u8array)
> 
> an issue could be alignment of the u8array

There is put_unaligned_le16() and friends for this situation where the
target buffer is just a generic bytestream. This is architecture optimized,
so if the architecture supports unaligned access it will just do a store, if
it doesn't it will split the operation into multiple stores.

> 
> I'd try to avoid u8 altogether and work with le16 as a datatype

That seems to be the best solution in this case.

Re: Spliting u16, u32 to u8 array

[Crt Mori]

On 6 August 2015 at 13:00, Lars-Peter Clausen <lars@metafoo.de> wrote:
> On 08/06/2015 12:42 PM, Peter Meerwald wrote:
>> Hello,
>>
>>> I am sure at least some of you were faced to split a u16 or u32 value
>>> in u8 array without being affected by host endianness. My search
>>> turned empty for a standardized function for this task, but this seems
>>> quite improbable. My task is to send 16 bit LE data through i2c.
>>>
>>> I am looking for something like:
>>> static void u16_to_u8_array(u16 value, u8 *array)
>>> {
>>>         *array = cpu_to_le16(value) >> 8;
>>>         *(++array) = (u8) cpu_to_le16(value);
>>> }
>>
>> how about
>>
>> static void u16_to_le16_array(u16 value, __le16 *array)
>> {
>>       *array++ = cpu_to_le16(value);
>> }
>>
>> and calling it with a cast if need be, such as
>>
>> char *u8array;
>> u16_to_le16_array(123, (__le16 *) u8array)
>>
>> an issue could be alignment of the u8array
>
> There is put_unaligned_le16() and friends for this situation where the
> target buffer is just a generic bytestream. This is architecture optimized,
> so if the architecture supports unaligned access it will just do a store, if
> it doesn't it will split the operation into multiple stores.
>
>>
>> I'd try to avoid u8 altogether and work with le16 as a datatype
>
> That seems to be the best solution in this case.
So i2c_master_send takes a char buf as input, and if I would send a le16
array[2] then the cast would nicely fit values it into that.

Thanks for the put_unaligned_le16() hint - exactly what I wanted, but
will investigate the best solution in this case as it seems to require a lot
less lines.