lkml.org 
[lkml]   [2013]   [Jul]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v3 4/5] iio: mxs-lradc: add scale_available file to channels
Hello Marek,

On 07/23/2013 12:36 AM, Marek Vasut wrote:
> Dear Hector Palacios,
>
>> Adds in_voltageX_scale_available file for every channel to read
>> the different available scales.
>> There are two scales per channel:
>> [0] = divider_by_two disabled (default)
>> [1] = divider_by_two enabled
>> The scale is a struct made of integer and nano parts to build
>> a long decimal number.
>>
>> Signed-off-by: Hector Palacios <hector.palacios@digi.com>
>> ---
>
> [...]
>
>> @@ -1008,6 +1098,26 @@ static int mxs_lradc_probe(struct platform_device
>> *pdev) if (ret)
>> goto err_trig;
>>
>> + /* 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++) {
>> + /*
>> + * [s=0] = optional divider by two disabled (default)
>> + * [s=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);
>
> Thinking about this, this is basically
>
> vref_mv[CHANNEL] * 100 000 000
> scale_uv = --------------------------------
> 2^(12 - s)
>
> Where s can be either 0 or 1.
>
> Why do you multiply it by 100000000 I don't quite understand, but maybe it's
> fully obvious. It should be documented though.

I copied Michael Hennerich in CC who was the author of this math in
drivers/iio/adc/ad7793.c
drivers/staging/iio/adc/ad7192.c
which I copied. Maybe he can explain. It looks like it is just a question of having a
intermediate large enough number to later do the do_div operation, rather than having
a meaningful figure.

>> + lradc->scale_avail[i][s].nano =
>> + do_div(scale_uv, 100000000) * 10;
>
> Are we not loosing precission here?

Yes, but the do_div is modifying the scale_uv parameter with the integer part so I
guess it might be somewhat justified.
As I said before, I tried to do my own math without success so I just copied what was
in other drivers.

>
>> + lradc->scale_avail[i][s].integer = scale_uv;
>> + }
>> + }
>> +
>> /* Configure the hardware. */
>> mxs_lradc_hw_init(lradc);
>
> Best regards,
> Marek Vasut
>


Best regards,
--
Hector Palacios


\
 
 \ /
  Last update: 2013-07-23 09:41    [W:0.747 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site