linux-kernel - Re: [PATCH v2 1/2] regulator: pcf50633: Don't write to reserved bits of AUTO output voltage select register

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Date:	Mon, 19 Mar 2012 10:07:42 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	axel.lin@...il.com
CC:	linux-kernel@...r.kernel.org, Liam Girdwood <lrg@...com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Subject: Re: [PATCH v2 1/2] regulator: pcf50633: Don't write to reserved bits
 of AUTO output voltage select register

On 03/19/2012 03:53 AM, Axel Lin wrote:
> 2012/3/18 Lars-Peter Clausen <lars@...afoo.de>:
>> On 03/17/2012 01:07 AM, Axel Lin wrote:
>>> The datasheet says 00000000 to 00101110 are reserved, and the min value of the
>>> voltage setting is 1.8 V.
>>> Thus don't write 0 to AUTO output voltage select register (address 1Ah).
>>>
>>> Table 50. AUTOOUT - AUTO output voltage select register (address 1Ah) bit description[1]
>>> Bit Symbol Access Description
>>> 7:0 auto_out R/W VO(prog) = 0.625 + auto_out × 0.025 V
>>> eg. 00000000 to 00101110: reserved
>>> 00101111: 1.8 V (min)
>>> 01010011: 2.7 V
>>> 01101010: 3.275 V
>>> 01101011: 3.300 V
>>> 01101100: 3.325 V
>>> 01111111 : 3.800 V (max)
>>> ..... .....
>>> 11111110 : 3.800 V
>>> 11111111 : 3.800 V
>>>
>>> This patch also fixes a bug in pcf50633_regulator_list_voltage.
>>> It is wrong to do "index += 0x2f" for PCF50633_REGULATOR_AUTO in
>>> pcf50633_regulator_list_voltage. The purpose of adding 0x2f to index is because
>>> current code return 0 in auto_voltage_bits when millivolts < 1800.
>>> For millivolts > 1800, adding 0x2f to index is wrong.
>>>
>>
>> I think you misunderstood what the current code does. The first usable voltage
>> is 1.8V which is equal to a index of of 0x2f. So the driver adds 0x2f to the
>> index so that there is not a headroom of 0x2f unusable voltages. So a selector
>> of 0 translates to 1.8V, a selector of 1 translates to 1.825V and so on.
> I think what I wry to explain is:
> In regulator core _regulator_do_set_voltage function:
> 
>         if (rdev->desc->ops->set_voltage) {
>                 ret = rdev->desc->ops->set_voltage(rdev, min_uV, max_uV,
>                                                    &selector);
> 
>                 if (rdev->desc->ops->list_voltage)
>                         selector = rdev->desc->ops->list_voltage(rdev,
>                                                                  selector);
>                 else
>                         selector = -1;
> 
> The list_voltage call here takes the selector got from set_voltage callback.
> Thus adding 0x2f to the index in pcf50633_regulator_list_voltage looks
> wrong to me.
> 

Ah ok, now I understand what you mean. When the selector parameter to
set_voltage was introduced we already had list_voltage adding the 0x2f, so
from my point of view list_voltage is correct and the bug is in the commit
which introduced the selector parameter to select voltage, that's why I was
confused as to why you think there was a bug in list_voltage. But it all
makes sense now, thanks.

Luckily the bug is not so critical, since selector is not really used in
_regulator_do_set_voltage in this case except for the trace message.

- Lars
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