Re: [PATCH v4 4/4] iio: health: Add driver for the TI AFE4403 heart monitor

[Andrew F. Davis]

On 01/30/2016 09:10 AM, Jonathan Cameron wrote:
> On 25/01/16 17:29, Andrew F. Davis wrote:
> > +
> > +static int afe4403_read(struct afe4403_data *afe, unsigned int reg, u32 *val)
> > +{
> > +	u8 tx[4] ____cacheline_aligned = {AFE440X_CONTROL0, 0x0, 0x0,
> > +					  AFE440X_CONTROL0_READ};
> hmm. Can you do this on the stack?  Don't think so but maybe I'm wrong..
> The cachline aligned trick relies on the start of the allocation on the
> heap being aligned and then pads to ensure that the element so tagged
> is also aligned appropriately.

I am not sure ether, I think I borrowed this from some example that did
it like this, but I can't find it now. So, I'll change this to be safe.

> > +	u8 rx[3];
> > +	int ret;
> Even if this were possible, ret is in the same cacheline as tx and rx so
> chaos may well occur.
>
> If you really want to avoid having allocations elsewhere, just use
> spi_write_then_read(afe->spi, tx, 4, NULL, 0) and you should be fine
> as spi_write_then_read uses safe bounce buffers.

spi_write_then_read performs a memcpy to the rx buffer, even though
the length is 0 I believe it is still technically undefined behavior,
but it doesn't seem to cause any issues with the current implementation
so I'll do this.

Thanks,
Andrew