Re: [PATCH v2 1/8] iio: core: Add opaque_struct_size() helper and use it

[Uwe Kleine-König]

Hello Andy,

On Fri, Jul 21, 2023 at 08:25:12PM +0300, Andy Shevchenko wrote:
> On Fri, Jul 21, 2023 at 08:22:35PM +0300, Andy Shevchenko wrote:
> > On Fri, Jul 21, 2023 at 08:00:15PM +0300, Andy Shevchenko wrote:
> > > Introduce opaque_struct_size() helper, which may be moved
> > > to overflow.h in the future, and use it in the IIO core.
> 
> ...
> 
> > > +#define opaque_struct_size(p, a, s)	({		\
> > > +	size_t _psize = sizeof(*(p));			\
> > > +	size_t _asize = ALIGN(_psize, (a));		\
> > > +	size_t _ssize = s;				\
> > > +	_ssize ? size_add(_asize, _ssize) : _psize;	\
> > > +})
> > > +
> > > +#define opaque_struct_data(p, a)			\
> > > +	((void *)(p) + ALIGN(sizeof(*(p)), (a)))
> > 
> > Hmm... This can potentially evaluate p twice.

I don't think sizeof evaluates its parameter? It should only defer its
type and then calculate the respective size.

> > Perhaps this variant is better:
> > 
> > #define opaque_struct_data(p, a)	ALIGN((p) + 1, (a)))

Still I like this better.

> > Besides, I don't think we should worry about @s evaluation in the main macro
> > which may be simplified to
> > 
> > #define opaque_struct_size(p, a, s)					\
> > 	((s) ? size_add(ALIGN(sizeof(*(p)), (a)), (s)) : sizeof(*(p)))
> > 
> > Thoughts?
> 
> Also we may leave the struct always be aligned which makes the above even
> simpler (but might waste bytes if @s = 0).

Depending on the value of a, the bytes "wasted" here might not be saved
anyhow because the memory allocator will round up the allocation size to
a multiple of some chunk size anyhow.

> #define opaque_struct_size(p, a, s)	size_add(ALIGN(sizeof(*(p)), (a)), (s))
> 
> (with the respective documentation update).

That would be my favourite.

Possible users of this helper include:

	__spi_alloc_controller() in drivers/spi/spi.c
	alloc_netdev_mqs in net/core/dev.c

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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