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, 13 Sep 2019 13:40:41 +0200	[thread overview]
Message-ID: <CAN8YU5PsnZ-zXOEvK_RgJh0drEXsGSidf_CZpTRU+caE6N6NQg@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:
>
> Hi,
>
> Le 13/09/2019 à 10:30, Andrea Merello a écrit :
> > Il giorno ven 13 set 2019 alle ore 09:19 Ardelean, Alexandru
> > <alexandru.Ardelean@analog.com> ha scritto:
> >
> >> So, at power-up this chip seems to need 2 dummy reads to discard data.
> >> Which seems to happen in ad7949_spi_init()
> >>
> >> 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).
>
>
> About your fix to read / write several times before reading the value
> after channel change seems not relevant. Did you try with the current
> implementation? Because when I developed the driver, I have always got
> the expected value for each channel with this design.

Yes, I did. But I experienced the said problems. I don't have idea
about why it worked for you and it didn't for me..

By scanning the channels in circular fashion, with the current
implementation, I would expect to get values off-by-one (i.e. you read
ch ...,0,1,2,3,4,0,1,2,3,4,... and you get value for ch
...,4,0,1,2,3,4,0,1,2,3,...).

>
> Just to be sure we are not adding useless steps.
>
> >> Added Charles-Antoine, since he wrote the driver.
> >> Shoud have added him on the other patches as well, but I just remembered.
> > I tried on my first answer, but apparently mails to his address bounce
> > back with a failure response..
>
> But now it seems ok. Did you put the right email address?

Sorry, my fault. I've done a mistake in copy-and-paste.

>
> Thank you for the copy.
>
> Regards,
>
> Charles-Antoine Couret
>