Re: [PATCH 19/25] iio:adc:ti-ads1015 Fix buffer element alignment

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Tue, May 26, 2020 at 10:11:44AM +0200, Lars-Peter Clausen wrote:
> On 5/25/20 7:52 PM, Andy Shevchenko wrote:
> > On Mon, May 25, 2020 at 06:06:22PM +0100, Jonathan Cameron wrote:
> > > From: Jonathan Cameron <Jonathan.Cameron@huawei.com>

...

> > > -	s16 buf[8]; /* 1x s16 ADC val + 3x s16 padding +  4x s16 timestamp */
> > > +	/* Ensure natural alignment for buffer elements */
> > > +	struct {
> > > +		s16 channel;
> > > +		s64 ts;
> > > +	} scan;
> > Hmm... On x86_32 and x86_64 this will give different padding. Is it okay from
> > iio_push_to_buffers_with_timestamp() point of view?
> 
> No, this is terrible. IIO expects 64 bit words to be 64 bit aligned. Looks
> like we can't rely on implicit padding, but need to always explicitly
> specify it.
> 
> Or maybe we can typedef and IIO timestamp type with an explicit __aligned
> attribute. I wonder if that works... After having a quick look, the kernel
> already defines aligned_u64, so maybe using that is an option.

Another way is simple to provide offset of timestamp member as a parameter.
Though, if it's an ABI, then alas, we need to align it properly.

Also, wouldn't be better to explicitly show the padding?

	struct {
		s16 channel;
		s16 padding[3];
		s64 ts;
	} scan;

(matter of style though, just saying).

-- 
With Best Regards,
Andy Shevchenko