RE: [PATCH v2 7/8] iio: core: simplify alloc alignment code

["Sa, Nuno" <Nuno.Sa@analog.com>]

> From: Ardelean, Alexandru <alexandru.Ardelean@analog.com>
> Sent: Freitag, 15. Mai 2020 13:48
> To: linux-arm-kernel@lists.infradead.org; linux-stm32@st-md-
> mailman.stormreply.com; Sa, Nuno <Nuno.Sa@analog.com>; linux-
> kernel@vger.kernel.org; linux-iio@vger.kernel.org
> Cc: ludovic.desroches@microchip.com; nicolas.ferre@microchip.com;
> alexandre.torgue@st.com; ak@it-klinger.de; jic23@kernel.org;
> eugen.hristev@microchip.com; mcoquelin.stm32@gmail.com;
> alexandre.belloni@bootlin.com
> Subject: Re: [PATCH v2 7/8] iio: core: simplify alloc alignment code
> 
> On Fri, 2020-05-15 at 07:12 +0000, Sa, Nuno wrote:
> > Hey Alex,
> >
> > Just a small question...
> >
> > > From: linux-iio-owner@vger.kernel.org <linux-iio-
> owner@vger.kernel.org>
> > > On Behalf Of Alexandru Ardelean
> > > Sent: Donnerstag, 14. Mai 2020 15:17
> > > To: linux-iio@vger.kernel.org; linux-arm-kernel@lists.infradead.org;
> linux-
> > > stm32@st-md-mailman.stormreply.com; linux-kernel@vger.kernel.org
> > > Cc: ludovic.desroches@microchip.com; eugen.hristev@microchip.com;
> > > jic23@kernel.org; nicolas.ferre@microchip.com;
> > > alexandre.belloni@bootlin.com; alexandre.torgue@st.com;
> > > mcoquelin.stm32@gmail.com; ak@it-klinger.de; Ardelean, Alexandru
> > > <alexandru.Ardelean@analog.com>
> > > Subject: [PATCH v2 7/8] iio: core: simplify alloc alignment code
> > >
> > > There was a recent discussion about this code:
> > >   https://urldefense.com/v3/__https://lore.kernel.org/linux-
> > >
> iio/20200322165317.0b1f0674@archlinux/__;!!A3Ni8CS0y2Y!pgdUSayJCfxMiE
> > > w8Fpv0LkEZurCSkX0sEcLnXeDSCLmhpu1xont6-vBQj3ZbCw$
> > >
> > > This looks like a good time to rework this, since any issues about it
> > > should pop-up under testing, because the iio_dev is having a bit of an
> > > overhaul and stuff being moved to iio_dev_priv.
> > >
> > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> > > ---
> > >  drivers/iio/industrialio-core.c | 10 +++-------
> > >  1 file changed, 3 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-
> > > core.c
> > > index a1b29e0f8fd6..7671d36efae7 100644
> > > --- a/drivers/iio/industrialio-core.c
> > > +++ b/drivers/iio/industrialio-core.c
> > > @@ -1514,13 +1514,9 @@ struct iio_dev *iio_device_alloc(int
> sizeof_priv)
> > >  	struct iio_dev *dev;
> > >  	size_t alloc_size;
> > >
> > > -	alloc_size = sizeof(struct iio_dev_opaque);
> > > -	if (sizeof_priv) {
> > > -		alloc_size = ALIGN(alloc_size, IIO_ALIGN);
> > > -		alloc_size += sizeof_priv;
> > > -	}
> > > -	/* ensure 32-byte alignment of whole construct ? */
> > > -	alloc_size += IIO_ALIGN - 1;
> > > +	alloc_size = ALIGN(sizeof(struct iio_dev_opaque), IIO_ALIGN);
> > > +	if (sizeof_priv)
> > > +		alloc_size += ALIGN(sizeof_priv, IIO_ALIGN);
> >
> > Do we actually need to do the `ALIGN` again? It seems to me that
> `alloc_size
> > += sizeof_priv`
> > would be enough or am I missing something obvious?
> 
> Well, it's not always clear what value 'sizeof_priv' has, and whether it is
> provided already aligned.
> The requirement is usually that this data be cacheline aligned.
> 
> So, sizeof(struct iio_dev_opaque) is aligned already a few lines above, but
> the
> private information should also be aligned [given that it's an unknown value
> provided by the driver].
> I think this is mostly important, if we need to do DMA access to buffers
> allocated on the driver's state-struct, which is allocated here, and which is
> usually provided as sizeof_priv.

Yes, AFAIU this is to guarantee that the priv struct will start at an address that is 
DMA safe (cacheline-aligned). Hence, if there is any data in 'priv' that needs to be DMA
safe, we are fine...

Well, I was also misreading the code. Still, I think it should look something like:

````
alloc_size = sizeof(struct iio_dev_opaque)
if (sizeof_priv)
	alloc_size += ALIGN(alloc_size, IIO_ALIGN);
````

If there is no priv, I think we don't need the padding bytes...

- Nuno Sá