Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753320Ab3GWHAZ (ORCPT ); Tue, 23 Jul 2013 03:00:25 -0400 Received: from mail1.bemta12.messagelabs.com ([216.82.251.17]:45972 "EHLO mail1.bemta12.messagelabs.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751837Ab3GWHAX (ORCPT ); Tue, 23 Jul 2013 03:00:23 -0400 X-Env-Sender: Hector.Palacios@digi.com X-Msg-Ref: server-16.tower-28.messagelabs.com!1374562820!24258914!1 X-Originating-IP: [66.77.174.13] X-StarScan-Received: X-StarScan-Version: 6.9.11; banners=-,-,- X-VirusChecked: Checked Message-ID: <51EE29FD.5070901@digi.com> Date: Tue, 23 Jul 2013 09:00:13 +0200 From: Hector Palacios Organization: Digi International User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130510 Thunderbird/17.0.6 MIME-Version: 1.0 To: Marek Vasut CC: "linux-iio@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "devicetree-discuss@lists.ozlabs.org" , "alexandre.belloni@free-electrons.com" , "jic23@kernel.org" , "lars@metafoo.de" , "fabio.estevam@freescale.com" , Subject: Re: [PATCH v3 4/5] iio: mxs-lradc: add scale_available file to channels References: <1374501843-19651-1-git-send-email-hector.palacios@digi.com> <1374501843-19651-5-git-send-email-hector.palacios@digi.com> <201307230036.42046.marex@denx.de> In-Reply-To: <201307230036.42046.marex@denx.de> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2643 Lines: 86 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 >> --- > > [...] > >> @@ -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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/