Re: [lm-sensors] Dealing with optional i2c devices in a devicetree

From: Guenter Roeck <linux@roeck-us.net>
To: Chris Packham
	<Chris.Packham-6g8wRflRTwXFdCa3tKVlE6U/zSkkHjvu@public.gmane.org>,
	"devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Cc: "jdelvare-l3A5Bk7waGM@public.gmane.org"
	<jdelvare-l3A5Bk7waGM@public.gmane.org>,
	"lm-sensors-GZX6beZjE8VD60Wz+7aTrA@public.gmane.org"
	<lm-sensors-GZX6beZjE8VD60Wz+7aTrA@public.gmane.org>,
	"shardy-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org"
	<shardy-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	"guillaume.roguez-4ysUXcep3aM1wj+D4I0NRVaTQe2KTcn/@public.gmane.org"
	<guillaume.roguez-4ysUXcep3aM1wj+D4I0NRVaTQe2KTcn/@public.gmane.org>
Subject: Re: [lm-sensors] Dealing with optional i2c devices in a devicetree
Date: Fri, 05 Jun 2015 07:30:03 +0000	[thread overview]
Message-ID: <55714FFB.3000608@roeck-us.net> (raw)
In-Reply-To: <55713BB1.80004-6g8wRflRTwXFdCa3tKVlE6U/zSkkHjvu@public.gmane.org>

Hi Chris,

On 06/04/2015 11:03 PM, Chris Packham wrote:
> Hi,
>
> I'm working on a new board and one feature it as is a plug-in module
> with an ADS7830 voltage monitor on it. This will be used during
> manufacturing to sanity check that various voltage rails are within
> expected ranges.
>
> I have a dts entry for the device as below (with some omissions for the
> sake of clarity)
>
> 	soc {
> 		internal-regs {
> 			i2c@11000 {
> 				ads7830@48 {
> 					compatible = "ads7830";
> 					reg = <0x48>;
> 				};
> 			};
> 		};
> 	};
>
> The problem is that when the manufacturing card is not installed the
> device still shows up in /sys/class/hwmon/hwmon0/ and
> /sys/bus/i2c/devices/0-0048/ despite it not actually being present. If I
> was using an old style initialization I could use
> i2c_new_probed_device() which I think would stop the drivers probe()
> function from being called
>
> Looking at the ads7828_probe() function it doesn't actually do anything
> with the i2c device before calling hwmon_device_register(). Some hwmon
> drivers like lm73_probe() do attempt to read from the device and bail if
> the read fails. I can probably fix my problem by doing something similar
> in the ads7828_probe(), but there are other drivers that have a similar
> probe function.
>
But that would be artificial in this case, and it could not be used to really
detect the chip but would only prove that there is a chip at the selected i2c
address. This is not really useful.

> Is there a better way of getting the devicetree machinery to avoid the
> call to the driver probe function in the first place?
>

The easiest solution would probably be to drop the entry from the
devicetree file and instantiate the driver manually via sysfs when
needed. Another option would be to load a different devicetree file
if the chip is plugged in.

You might also be able to use a devicetree overlay, but that would
probably add too much complexity if the chip is only used in
manufacturing.

Guenter

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