linux-kernel - Re: [PATCH v3 4/5] iio: mxs-lradc: add scale

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Date:	Tue, 23 Jul 2013 00:36:41 +0200
From:	Marek Vasut <marex@...x.de>
To:	Hector Palacios <hector.palacios@...i.com>
Cc:	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org,
	alexandre.belloni@...e-electrons.com, jic23@...nel.org,
	lars@...afoo.de, fabio.estevam@...escale.com
Subject: Re: [PATCH v3 4/5] iio: mxs-lradc: add scale_available file to channels

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@...i.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.



> +			lradc->scale_avail[i][s].nano =
> +					do_div(scale_uv, 100000000) * 10;

Are we not loosing precission here?

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

Best regards,
Marek Vasut
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