Re: [PATCH 3/4] iio: mxs-lradc: add scale_available file to channels

[Hector Palacios <hector.palacios@digi.com>]

Dear Marek,

On 07/05/2013 01:46 PM, Marek Vasut wrote:
[...]
>> +	/* Populate available ADC input ranges */
>> +	for (i = 0; i < LRADC_MAX_TOTAL_CHANS; i++) {
>> +		for (s = 0; s < ARRAY_SIZE(lradc->scale_avail[i]); s++) {
>> +			/*
>> +			 * [0] = optional divider by two disabled (default)
>> +			 * [1] = optional divider by two enabled
>> +			 *
>> +			 * The scale is calculated by doing:
>> +			 *   Vref >> (realbits - s)
>> +			 * which multiplies by two on the second component
>> +			 * of the array.
>> +			 */
>> +			scale_uv = ((u64)lradc->vref_mv[i] * 100000000) >>
>> +				   (iio->channels[i].scan_type.realbits - s);
>
> Given that you do have a table of values already, can this table not be computed
> at compile-time as well?

Yes and no. On one hand, it would be a little redundant and ugly to have two tables 
(one computed out of the other). Considering there are two CPUs, it would force you to 
maintain four tables.
On the other hand the formula uses the 'realbits' (yeah, well it won't change, but still).
I copied the code from ad7192.c. The operation is only done during probe() so I don't 
think the time penalty is worth the effort of having another table.

>> +			lradc->scale_avail[i][s][1] = do_div(scale_uv,
>> +							     100000000) * 10;
>> +			lradc->scale_avail[i][s][0] = scale_uv;
>
> Is this correct? Why is one set to "scale_uv" and the other set to scale_uv /
> 100000000 ? Maybe I just don't understand what each of the fields in the array
> stand for.

The [0] component is the integer (volts) part of the scale. The [1] component is the 
nanoV part.
To be honest, after some unsuccessful tries of doing my own math here, I just copied 
the one from ad7192.c. It is a bit unintelligible because it is doing 64bit math 
operations, but it works. Remember do_div() modifies the first parameter (scale_uv) 
during the operation, apart from returning a value. That may have fooled you.

Best regards,
--
Hector Palacios