Re: [dpdk-dev] [PATCH 1/2] eal: add static endianness conversion macros

[Thomas Monjalon <thomas@monjalon.net>]

08/06/2017 11:14, Adrien Mazarguil:
> On Wed, Jun 07, 2017 at 04:16:58PM +0200, Thomas Monjalon wrote:
> > Hi, some comments below:
> > 
> > 18/05/2017 12:14, Adrien Mazarguil:
> > > +#define RTE_STATIC_BSWAP64(v) \
> > > +	((((uint64_t)(v) & UINT64_C(0x00000000000000ff)) << 56) | \
> > > +	 (((uint64_t)(v) & UINT64_C(0x000000000000ff00)) << 40) | \
> > > +	 (((uint64_t)(v) & UINT64_C(0x0000000000ff0000)) << 24) | \
> > > +	 (((uint64_t)(v) & UINT64_C(0x00000000ff000000)) << 8) | \
> > > +	 (((uint64_t)(v) & UINT64_C(0x000000ff00000000)) >> 8) | \
> > > +	 (((uint64_t)(v) & UINT64_C(0x0000ff0000000000)) >> 24) | \
> > > +	 (((uint64_t)(v) & UINT64_C(0x00ff000000000000)) >> 40) | \
> > > +	 (((uint64_t)(v) & UINT64_C(0xff00000000000000)) >> 56))
> > 
> > Minor nit: you could align lines by inserting a space before 8.
> 
> I think alignment attempts past the mandatory line indentation often end up
> in a failure (e.g. when grouping macros by name, one of them inevitably
> happens to be longer than initially envisioned, same for structure fields
> and trailing comment blocks, etc.) Since I'm not convinced it improves
> readability, I tend to avoid them altogether for consistency.

I agree
Here it is just adding a space in front of the single digit to make
bits numbers aligned on 2 digits :)

> It's a matter of style but I can change that if you prefer.
> 
> > > +#if RTE_BYTE_ORDER == RTE_BIG_ENDIAN
> > > +#define RTE_BE16(v) (uint16_t)(v)
> > > +#define RTE_BE32(v) (uint32_t)(v)
> > > +#define RTE_BE64(v) (uint64_t)(v)
> > > +#define RTE_LE16(v) RTE_STATIC_BSWAP16(v)
> > > +#define RTE_LE32(v) RTE_STATIC_BSWAP32(v)
> > > +#define RTE_LE64(v) RTE_STATIC_BSWAP64(v)
> > > +#elif RTE_BYTE_ORDER == RTE_LITTLE_ENDIAN
> > > +#define RTE_BE16(v) RTE_STATIC_BSWAP16(v)
> > > +#define RTE_BE32(v) RTE_STATIC_BSWAP32(v)
> > > +#define RTE_BE64(v) RTE_STATIC_BSWAP64(v)
> > > +#define RTE_LE16(v) (uint16_t)(v)
> > > +#define RTE_LE32(v) (uint32_t)(v)
> > > +#define RTE_LE64(v) (uint64_t)(v)
> > 
> > This naming is confusing.
> > Let's take RTE_BE16() as example, it does not say wether the input value
> > is big endian or the output value will be big endian.
> > I think we should mimic the wording of run-time conversions:
> > 	RTE_BE_TO_CPU_16()
> > 
> > Any other ideas?
> 
> First I'd like to keep those macro names as short as possible, ideally not
> much larger than simply casting the provided value to the target type for
> usability and readability purposes. Think about files full of static
> initializers, while there are not many examples right now, the definition of
> static rte_flow rules and capability trees will need to use these macros
> extensively.
> 
> The fact you suggested RTE_BE_TO_CPU_16() instead of RTE_CPU_TO_BE_16() as a
> replacement for RTE_BE16() highlights the misunderstanding. However I find
> "CPU_TO" overly verbose, particularly since the reverse macros won't exist,
> remember these are made for static conversions of integer constants resolved
> at compilation time, not variables. Users may additionally confuse
> RTE_CPU_TO_BE_16() with its similarly-named inline function counterpart.

You're right.
RTE_BE_TO_CPU_16 does not make sense.
I think you could add a comment like that:
RTE_XE_NN is equivalent to rte_cpu_to_Xe_NN run-time conversion

> Functions and macros are typically named after their output, not their
> input. In that sense and without further precision, RTE_BE16() is fine in my
> opinion.

Good point.

> Remember this [1]? I think we could make everything clearer by perhaps
> applying it and casting the results of these macros to the proper type,
> e.g.:
> 
>  #define RTE_BE16(v) (rte_be16_t)(v)
> 
> I can probably modify this series to introduce the new types first, use them
> in the conversion macro and then later clarify existing structure
> fields. How about this?

Yes good idea.