Re: Accessing specific hardware features of mxs-lradc

[Lars-Peter Clausen <lars@metafoo.de>]

On 07/02/2013 05:21 PM, Lars-Peter Clausen wrote:
> On 07/02/2013 04:55 PM, Hector Palacios wrote:
>> Hello,
>>
>> On 07/02/2013 03:28 PM, Alexandre Belloni wrote:
>>> On 02/07/2013 14:58, Lars-Peter Clausen wrote:
>>>> On 07/02/2013 12:49 PM, Hector Palacios wrote:
>>>>> Greetings,
>>>>>
>>>>> The mxs-lradc has some hardware specific features like the DIVIDE_BY_TWO (an
>>>>> analog divide by two circuit that can be activated per channel).
>>>>> This is not currently implemented in the driver and I was wondering what would
>>>>> be the way to expose and control such setting. A specific sysfs entry? Somehow
>>>>> through the device tree?
>>>>>
>>>>> Is there any example of how to control a hw-specific feature in other similar
>>>>> iio drivers?
>>>>>
>>>>
>>>> This sounds as if the channel scale is programmable. There are quite a few
>>>> drivers for devices which have similar features, just make the scale property
>>>> writable and provide a scale_available.
>>>>
>>>
>>> I was looking at that yesterday as I had to had a scale for the
>>> temperature sensors. You'll have to be careful because the maximum
>>> measured voltage when DIVIDE_BY_TWO is not set is 1.85V and when it is
>>> set, it becomes VDDIO - 50mv.
> 
> I had a quick peak at the datasheet and I think that's a typo. VDDIO is 3.3V.
> This sentence also doesn't make much sense if divide by two would actually
> lower the range. "For input channels (other than BATT, 5V, or VDDIO) with
> signals larger than 1.85 V, the divide-by-two option should be set."

Uhm, ignore this, parse error on my side. I thought the dash was a hyphen not a
minus.

> 
>>>
>>> Also, scaling for quite a lot of channels is fixed. You'll also get
>>> channels with a fixed divider that can take DIVIDE_BY_TWO. That is quite
>>> a mess ;)
>>
>> Thank you, I checked and it's easy to add custom sysfs attributes using
>> IIO_DEVICE_ATTR(). I was thinking whether adding a divide_by_two attribute per
>> channel:
>>
>>     divide_by_two0
>>     divide_by_two1
>>     ...
>>     divide_by_two7
>>
>> or a unique divide_by_two descriptor that admits a mask value for all channels.
>> I think the first approach (one per channel) is cleaner. Opinions?
> 
> Just use the scale attribute for this. That's the standard way to handle this,
> don't invent custom interfaces if there is already a generic one.
> 
> - Lars
>