Re: [PATCH v2 2/3] iio: dac: Add Texas Instruments 8/10/12-bit 2/4-channel DAC driver

[Lukas Wunner <lukas@wunner.de>]

On Sat, Oct 21, 2017 at 08:33:08PM +0100, Jonathan Cameron wrote:
> On Tue, 17 Oct 2017 12:42:00 +0200 Lukas Wunner <lukas@wunner.de> wrote:
> > +struct ti_dac_chip {
> > +	struct mutex lock;
> > +	struct regulator *vref;
> > +	struct spi_message mesg;
> > +	struct spi_transfer xfer;
> > +	u8 buf[2] ____cacheline_aligned;
> 
> I missed this the first time.  The whole point of the fun
> of ____cacheline_aligned is to ensure nothing shares a
> cacheline with the buffer used for DMA.  We take care
> when allocating these private structures that this
> will be true, but it relies on nothing that might be
> changed during dma being after the buffer.
> 
> Simple reordering fix so I'll do it whilst applying.

TBH I was a bit confused about how to achieve proper cacheline
alignment.  My limited understanding of the slab allocator is
that it neither guarantees that an allocation begins on a cacheline
nor that no another allocation immediately starts after it.

However taking a closer look at iio_device_alloc() I realize now
that it implicitly takes care of both by inserting the necessary
padding.  Neat.  I was already wondering why the IIO subsystem
takes this somewhat unusual approach to allocate the private struct
behind struct iio_dev, instead of letting the driver embed it
within its private struct and use container_of() to yield the
private struct corresponding to a struct iio_dev, as most other
subsystems do.  But that explains it.

Thanks,

Lukas