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

From: Andrea Merello <andrea.merello@gmail.com>
To: Couret Charles-Antoine <charles-antoine.couret@essensium.com>
Cc: "Ardelean, Alexandru" <alexandru.Ardelean@analog.com>,
	"Hennerich, Michael" <Michael.Hennerich@analog.com>,
	"lars@metafoo.de" <lars@metafoo.de>,
	"jic23@kernel.org" <jic23@kernel.org>,
	"pmeerw@pmeerw.net" <pmeerw@pmeerw.net>,
	"knaack.h@gmx.de" <knaack.h@gmx.de>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>
Subject: Re: [PATCH 4/4] iio: ad7949: fix channels mixups
Date: Fri, 20 Sep 2019 09:45:21 +0200	[thread overview]
Message-ID: <CAN8YU5ORoM69GDi4VVGf6iWb3A2S1ZjkiLmcV+_hUbG4446yXQ@mail.gmail.com> (raw)
In-Reply-To: <4a25469a-9fe6-a560-b1cb-e9b0af7209e9@essensium.com>

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.

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)