Re: [PATCH 4/4] iio: ad7949: fix channels mixups

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 20 Sep 2019 09:45:21 +0200
Andrea Merello <andrea.merello@gmail.com> wrote:

> Il giorno ven 13 set 2019 alle ore 13:30 Couret Charles-Antoine
> <charles-antoine.couret@essensium.com> ha scritto:
> 
> > >> One thing that maybe could be optimized (for the driver), is in `ad7949_spi_read_channel()` to use the current-channel &
> > >> not do a SPI write to change the channel if it doesn't change.
> > >>
> > >> Datasheets (in general) are not always obvious about describing chip behavior for SW (or for writing a driver), but I
> > >> would suspect that if you are reading garbage data, it could be that the channel has changed.
> > >> This is true for some other ADCs.
> > >> And requires testing for this one.  
> > > Yes, it's exactly what I've seen here. If the channel does not change
> > > then the AD is already in acquisition phase on the right channel (I
> > > assume it's OK to keep it in such phase indefinitely), then we can
> > > just trigger a new conversion (CNV low->high, that is a dummy xfer)
> > > and then read the result in following xfer, as the driver already did.
> > >
> > > I craft a V2 that performs the extra (3rd) spi xfer only if the
> > > channel has to change.  
> >
> > This design should be ok. I didn't implement in that way because not
> > enough time to optimize the driver before release (I don't have access
> > to the chip anymore) and for our workflow it was not relevant (we
> > scanned all channels).  
> 
> I was in the process of doing this, but, thinking again about it, I'm
> not completely sure it is really OK:
> Should we guarantee that the value we return as outcome of a IIO read
> request (i.e. we access in_voltage_raw) is sampled _after_ the user
> read request?
> 
> For example, the user might setup some other HW for the measure, or
> somehow wait for the right moment for doing the measure, and then
> perform the read from IIO, thus it's probably not OK if we convert a
> value sampled just before the IIO read request.

Absolutely.  MUX in front of the sensor is a fairly common thing to see.

> 
> If we skip the configuration rewrite when the channel doesn't change -
> as discussed above - then we actually _terminate_ the acquisition when
> the IIO read is triggered, that is we are converting the value sampled
> right before the IIO read.
> 
> If this is OK then I'll go on, otherwise I think that we should always
> do the three cycles (so that triggering IIO read always waits also for
> a new acquisition phase)
An excellent point.  I agree and suspect we may have this wrong in other
sensors.  The question gets more interesting if running in buffered mode
as we have had systems using a trigger generated by an external process.
I suppose in that case we just have to deal with the offset in the fifo
etc in userspace.

Maybe we should think about adding a note to be careful of that.  Not
really sure where we would note it though...

Thanks,

Jonathan