Messages in this thread | | | Date | Tue, 23 Jul 2013 09:00:13 +0200 | From | Hector Palacios <> | Subject | Re: [PATCH v3 4/5] iio: mxs-lradc: add scale_available file to channels |
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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
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