linux-kernel - Re: [PATCH v2 1/3] iio: magnetometer: Add support for MEMSIC MMC35240 sensor

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Message-ID: <CAEnQRZDJDfdABW3Y=fUZ8pVmJoy11-FUBtmBFgfvtH_kJMCzPg@mail.gmail.com>
Date:	Tue, 30 Jun 2015 12:35:08 +0300
From:	Daniel Baluta <daniel.baluta@...el.com>
To:	Hartmut Knaack <knaack.h@....de>
Cc:	Daniel Baluta <daniel.baluta@...el.com>,
	Jonathan Cameron <jic23@...nel.org>,
	Peter Meerwald <pmeerw@...erw.net>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] iio: magnetometer: Add support for MEMSIC MMC35240 sensor

On Mon, Jun 29, 2015 at 10:17 PM, Hartmut Knaack <knaack.h@....de> wrote:
> Daniel Baluta schrieb am 29.06.2015 um 09:52:
>> On Mon, Jun 29, 2015 at 2:07 AM, Hartmut Knaack <knaack.h@....de> wrote:
>>> Daniel Baluta schrieb am 24.04.2015 um 17:58:
>>>> Minimal implementation for MMC35240 3-axis magnetometer
>>>> sensor. It provides processed readings and possiblity to change
>>>> the sampling frequency.
>>>>
>>>
>>> Hi Daniel,
>>> please find a few issues inline, which you could address in a next
>>> rework patch set. I would have sent a patch for the critical stuff,
>>> but was obviously too stupid to find a data sheet :-(
>>
>> Well, there is no public datasheet. We are discussing with the vendor
>> to make it public.
>>
> <...>
>>>> +static int mmc35240_hw_set(struct mmc35240_data *data, bool set)
>>>> +{
>>>> +     int ret;
>>>> +     u8 coil_bit;
>>>> +
>>>> +     /*
>>>> +      * Recharge the capacitor at VCAP pin, requested to be issued
>>>> +      * before a SET/RESET command.
>>>> +      */
>>>> +     ret = regmap_update_bits(data->regmap, MMC35240_REG_CTRL0,
>>>> +                              MMC35240_CTRL0_REFILL_BIT,
>>>> +                              MMC35240_CTRL0_REFILL_BIT);
>>>> +     if (ret < 0)
>>>> +             return ret;
>>>> +     usleep_range(MMC35240_WAIT_CHARGE_PUMP, MMC35240_WAIT_CHARGE_PUMP + 1);
>>>> +
>>>> +     if (set)
>>>> +             coil_bit = MMC35240_CTRL0_SET_BIT;
>>>> +     else
>>>> +             coil_bit = MMC35240_CTRL0_RESET_BIT;
>>>> +
>>>> +     return regmap_update_bits(data->regmap, MMC35240_REG_CTRL0,
>>>> +                               MMC35240_CTRL0_REFILL_BIT,
>>>> +                               coil_bit);
>>>
>>> coil_bit is in any case outside the mask of MMC35240_CTRL0_REFILL_BIT.
>>> Not sure about the whole intention, meaning in which state
>>> MMC35240_CTRL0_REFILL_BIT is supposed to be (set) when either
>>> MMC35240_CTRL0_SET_BIT or MMC35240_CTRL0_RESET_BIT is written.
>>
>> Yes, this is a bug. We have a patch prepared for it. Will send once Jonathan is
>> ready to accept bugfixes. Together with this:
>>
>> http://marc.info/?l=linux-iio&m=143489464403101&w=2
>>
>
> Sending it out earlier gives people more time to review (or may allow more people
> to review).
>
>>
>>>
>>>> +}
>
> <...>
>>>> +
>>>> +static int mmc35240_take_measurement(struct mmc35240_data *data)
>>>> +{
>>>> +     int ret, tries = 100;
>>>> +     unsigned int reg_status;
>>>> +
>>>> +     ret = regmap_write(data->regmap, MMC35240_REG_CTRL0,
>>>> +                        MMC35240_CTRL0_TM_BIT);
>>>> +     if (ret < 0)
>>>> +             return ret;
>>>> +
>>>> +     while (tries-- > 0) {
>>>> +             ret = regmap_read(data->regmap, MMC35240_REG_STATUS,
>>>> +                               &reg_status);
>>>> +             if (ret < 0)
>>>> +                     return ret;
>>>> +             if (reg_status & MMC35240_STATUS_MEAS_DONE_BIT)
>>>> +                     break;
>>>
>>> I would actually return 0 here, as measurement was successful. That
>>> would mean that getting outside the loop is the error case and would
>>> make the check obsolete.
>>
>> You are right, this could also work. Anyhow, I think code is easier to
>> understand as it is. The check for (tries < 0) makes it very clear, that the
>> data was not ready.
>>
>> Getting outside the loop in the error case is harder to understand at a first
>> glance.
>>
>
> I can not really agree. The mission is accomplished at the break, so better
> take the shortest way out (return 0 usually reflects that). Still going through
> a check that won't trigger in this case just adds bloat without any benefit.
> It's not a bug, so I don't feel too strong to fix it myself (still too much
> reviews to be done). Sorry for annoying with such issues, spending my childhood
> with slow and low memory 8 bit machines just left a mark ;-)
>
>>>
>>>> +             msleep(20);
>>>> +     }
>>>> +
>>>> +     if (tries < 0) {
>>>> +             dev_err(&data->client->dev, "data not ready\n");
>>>> +             return -EIO;
>>>
>>> Doesn't this qualify more for a timeout error, rather than I/O?
>>
>> Looking at the IIO drivers, most of them return EIO in such case.
>> (grep for EIO in drivers/iio/light for example)
>>
>
> I don't feel too strong about this. I just regard I/O errors as indication
> that communication with the device went wrong. But when getting here, it
> always told to be busy.

Ok, I will you have a good point for both issues. Will try to address them
in a few days.

thanks,
Daniel.
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