linux-kernel - Re: [PATCH v2 2/3] iio:adc:palmas: add DT support

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Date:	Mon, 5 Oct 2015 16:26:22 +0200
From:	"H. Nikolaus Schaller" <hns@...delico.com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Rob Herring <robh+dt@...nel.org>, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Benoît Cousson <bcousson@...libre.com>,
	Tony Lindgren <tony@...mide.com>,
	Russell King <linux@....linux.org.uk>,
	Marek Belisko <marek@...delico.com>,
	Pradeep Goudagunta <pgoudagunta@...dia.com>,
	Laxman Dewangan <ldewangan@...dia.com>, gg@...mlogic.co.uk,
	jic23@...23.retrosnub.co.uk, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	linux-iio@...r.kernel.org, Grazvydas Ignotas <notasas@...il.com>
Subject: Re: [PATCH v2 2/3] iio:adc:palmas: add DT support


Am 05.10.2015 um 13:17 schrieb Mark Rutland <mark.rutland@....com>:

> On Sun, Oct 04, 2015 at 06:05:59PM +0200, H. Nikolaus Schaller wrote:
>> From: Marek Belisko <marek@...delico.com>
>> 
>> Code was found at:
>> https://android.googlesource.com/kernel/tegra/+/a90856a6626d502d42c6e7abccbdf9d730b36270%5E%21/#F1
>> 
>> Signed-off-by: Laxman Dewangan <ldewangan@...dia.com>
>> [Fixed minor typos + add channels list to documentation]
>> Signed-off-by: Marek Belisko <marek@...delico.com>
>> ---
>> .../devicetree/bindings/iio/adc/palmas-gpadc.txt   | 46 +++++++++++++++++++
>> drivers/iio/adc/palmas_gpadc.c                     | 52 +++++++++++++++++++---
>> 2 files changed, 93 insertions(+), 5 deletions(-)
>> create mode 100644 Documentation/devicetree/bindings/iio/adc/palmas-gpadc.txt
>> 
>> diff --git a/Documentation/devicetree/bindings/iio/adc/palmas-gpadc.txt b/Documentation/devicetree/bindings/iio/adc/palmas-gpadc.txt
>> new file mode 100644
>> index 0000000..2149afe
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/iio/adc/palmas-gpadc.txt
>> @@ -0,0 +1,46 @@
>> +* Palmas general purpose ADC IP block devicetree bindings
>> +
>> +Channels list:
>> +	0 battery type
>> +	1 battery temp NTC (optional current source)
>> +	2 GP
>> +	3 temp (with ext. diode, optional current source)
>> +	4 GP
>> +	5 GP
>> +	6 VBAT_SENSE
>> +	7 VCC_SENSE
>> +	8 Backup Battery voltage
>> +	9 external charger (VCHG)
>> +	10 VBUS
>> +	11 DC-DC current probe (how does this work?)
>> +	12 internal die temp
>> +	13 internal die temp
>> +	14 USB ID pin voltage
>> +	15 test network
>> +
>> +Required properties:
>> +- compatible : Must be "ti,palmas-gpadc".
>> +
>> +Optional sub-nodes:
>> +ti,channel0-current-microamp: Channel 0 current in uA.
>> +	Values are rounded to derive 0uA, 5uA, 15uA, 20uA.
>> +ti,channel3-current-microamp: Channel 3 current in uA.
>> +	Valid are rounded to derive 0uA, 10uA, 400uA, 800uA.
> 
> It's only possible to configure channels 0 and 3 in this manner?

Yes. The other channels have no built-in current source, i.e. these
channels are special.

> 
>> +ti,enable-extended-delay: Enable extended delay.
> 
> What is this? When would I select it? Why does it belong in the DT
> rather than being a runtime option?

The chip allows to extend the time window between channel selection
and sampling by 400µs (according to data sheet). But for all channels
and not each one. As far as I understand, this is - depending on hardware
setup - to get more stable ADC readings.

Most channels have a fixed function (e.g. battery voltage, USB VBUS,
temperature NTC) so it can't be arbitrarily chosen and depends on how
the Palmas is embedded (it is a PMIC with a bank of ADCs and not a
general purpose ADC chip).

So I think it is really a hardware dependent parameter and not something
the user should be able to change.

> 
>> +Example:
>> +
>> +pmic {
>> +	compatible = "ti,twl6035-pmic", "ti,palmas-pmic";
>> +	...
>> +	gpadc {
>> +		compatible = "ti,palmas-gpadc";
>> +		interrupts = <18 0
>> +			      16 0
>> +			      17 0>;
>> +		ti,channel0-current-microamp = <5>;
>> +		ti,channel3-current-microamp = <10>;
>> +		};
>> +	};
>> +	...
>> +};
> 
> I thought you needed #iio-cells for encoding the channel?

Yes, we forgot (we don't have a client for it in our setup yet - so did not test :)

> 
> Mark.

Thanks,
Nikolaus Schaller

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