linux-kernel - Re: [PATCH] bq2415x_charger: fix max battery regulation voltage

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Date:	Tue, 15 Oct 2013 16:54:14 +0200
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Pali Rohár <pali.rohar@...il.com>
CC:	Anton Vorontsov <anton@...msg.org>, linux-kernel@...r.kernel.org,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] bq2415x_charger: fix max battery regulation voltage


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On 15/10/2013 15:56, Pali Rohár wrote:
> On Wednesday 02 October 2013 12:24:22 Alexandre Belloni wrote:
>> As per the datasheets, maximum battery regulation voltage is
>> 4440mV.
>>
>> The formula is (voltage - offset) / step, so the maximum value
>> is: (4440 - 3500) / 20 = 47
>>
>> Signed-off-by: Alexandre Belloni
>> <alexandre.belloni@...e-electrons.com> ---
>>  drivers/power/bq2415x_charger.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/power/bq2415x_charger.c
>> b/drivers/power/bq2415x_charger.c index 0727f92..0aa643e
>> 100644
>> --- a/drivers/power/bq2415x_charger.c
>> +++ b/drivers/power/bq2415x_charger.c
>> @@ -607,7 +607,7 @@ static int
>> bq2415x_set_battery_regulation_voltage(struct bq2415x_device
>> *bq,
>>
>>      if (val < 0)
>>          val = 0;
>> -    else if (val > 94) /* FIXME: Max is 94 or 122 ? Set max
>> value ? */ +    else if (val > 47)
>>          return -EINVAL;
>>
>>      return bq2415x_i2c_write_mask(bq, BQ2415X_REG_VOLTAGE, val,
>
> Right, max is 47 (101111) and not 94 (1011110). But if you set
> all six registers to 1 then you get 63 (111111) and not 122.
>
> I do not know from where I got above bad values, but you should
> add comment to code that according to datasheet max value is
> 4440mV (101111) and not 4760mV (111111).
>
You probably missed the division by 2. BTW, I much prefer int val = (mV
- 3500) / 20;
which is clearer: offset is 3500mV and step is 20mV. I find your
calculation quite weird ;)

I'll send V2 with a comment then. Thank you for your review.

- -- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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