linux-kernel - Re: [PATCH 4/4] iio: pressure: Add triggered buffer support for BMP280 driver

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Date: Sat, 9 Mar 2024 18:19:42 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Vasileios Amoiridis <vassilisamir@...il.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, lars@...afoo.de,
 ang.iglesiasg@...il.com, mazziesaccount@...il.com, ak@...klinger.de,
 petre.rodan@...dimension.ro, phil@...pberrypi.com, 579lpy@...il.com,
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] iio: pressure: Add triggered buffer support for
 BMP280 driver

On Mon, 4 Mar 2024 21:05:47 +0100
Vasileios Amoiridis <vassilisamir@...il.com> wrote:

> On Mon, Mar 04, 2024 at 09:18:27PM +0200, Andy Shevchenko wrote:
> > On Mon, Mar 04, 2024 at 08:08:38PM +0100, Vasileios Amoiridis wrote:  
> > > On Mon, Mar 04, 2024 at 01:52:05PM +0200, Andy Shevchenko wrote:  
> > > > On Sun, Mar 03, 2024 at 05:53:00PM +0100, Vasileios Amoiridis wrote:  
> > 
> > ...
> >   
> > > > > +	struct {
> > > > > +		s32 temperature;
> > > > > +		u32 pressure;
> > > > > +		u32 humidity;  
> > > >   
> > > > > +		s64 timestamp;  
> > > > 
> > > > Shouldn't this be aligned properly?  
> > > 
> > > I saw that in some drivers it was added and in some it was not. What is the
> > > difference of aligning just the timestamp of the kernel?  
> > 
> > You can count yourself. With provided structure as above there is a high
> > probability of misaligned timeout field. The latter has to be aligned on
> > 8 bytes.
> >   
> 
> I was unaware, but now I am not. Thank you very much for the feedback.

Fun bit of C is that you aren't actually aligning just the timestamp.
A C structure is aligned to the alignment of the maximum element within it.
So by specifying that timestamp is aligned to 8 bytes, you also force the
alignment of the whole structure to 8 bytes.

When you see the outer buffer aligned as well (typically the potentially larger
__aligned (IIO_DMA_MINALIGN)) that's for a different reason.  Used on a trailing
element of a structure via iio_priv() that ensures there is nothing else in the
cacheline (maximum one in the system) on systems where this matters due to non
coherent DMA.  Still need the __aligned(8) on the timestamp though as otherwise
the internal padding may be wrong (like here).

On some architectures small buffers are always bounced - if that were true on
all of them we could get rid of the complexity of IIO_DMA_MINALIGN.

Alignment is so much fun - particularly with x86_32 which does 8 byte values aligned
to 4 bytes. We had a massive set of patches fixing subtle issues around that a
few years ago.

Jonathan

> > > > > +	} iio_buffer;  
> > 
> > -- 
> > With Best Regards,
> > Andy Shevchenko
> > 
> >