linux-kernel - Re: [PATCH v2 2/2] iio: temperature: add driver support for ti tmp117

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Date:   Sat, 3 Apr 2021 17:03:17 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Puranjay Mohan <puranjay12@...il.com>
Cc:     alexandru.ardelean@...log.com, Jonathan Cameron <jic23@...nel.org>,
        devicetree@...r.kernel.org, knaack.h@....de,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] iio: temperature: add driver support for ti tmp117

On 4/3/21 4:58 PM, Puranjay Mohan wrote:
> On Fri, Apr 2, 2021 at 1:43 PM Lars-Peter Clausen <lars@...afoo.de> wrote:
>> On 4/1/21 11:16 AM, Puranjay Mohan wrote:
>>> TMP117 is a Digital temperature sensor with integrated NV memory.
>>>
>>> Add support for tmp117 driver in iio subsystem.
>>> Datasheet:-https://www.ti.com/lit/gpn/tmp117
>>>
>>> Signed-off-by: Puranjay Mohan <puranjay12@...il.com>
>> Nice and clean driver. Just some comments about the CALIBBIAS.
>>
>>> [...]
>>> +#define TMP117_RESOLUTION_10UC               78125
>> Isn't the unit here 100 uC?
> it is 7.8125 milli degrees_C so 78125 x 10^-4 milli degrees_C
> which is 78125 x 10^-4 x 10^3 micro degrees_C
> so it becomes 78125 x 10^-1 micro degrees_C = 78125 10_microdegrees_C.
> Did it in detail so I remember it in the future. I guess you thought
> it as 0.78125 millidegrees_C?
Ah, I get it, it is a tenth micro degree, not tens of micro degrees, 
sorry got confused.
> [...]
>
>> I think that would be quite unexpected behavior. The unit should be the
>> same. Here in the read function you can just return the register value.
> Okay, if you feel that would be right then I will do it.
Yea, I think reading and writing in different units would be a bit 
confusing.
>> Just make sure to properly sign extend like for the RAW property.
>>
>>> +             return IIO_VAL_INT_PLUS_MICRO;
>>> [...]
>>> +}
>>> +
>>> +static int tmp117_write_raw(struct iio_dev *indio_dev,
>>> +             struct iio_chan_spec const *channel, int val,
>>> +             int val2, long mask)
>>> +{
>>> +     struct tmp117_data *data = iio_priv(indio_dev);
>>> +     s16 off;
>>> +
>>> +     switch (mask) {
>>> +     case IIO_CHAN_INFO_CALIBBIAS:
>>> +             off = (s16)val;
>> This should have some input validation to avoid writing bogus values to
>> the register when the value is out of range. You can either reject out
>> of range values or clamp them into the valid range (using the clamp()
>> macro).
> the maximum value which this register takes is 0xffff, so it should
> get clamped automatically when casting to s16?
> I might be wrong here.
Casting will truncate the upper bits. So something like 0x12345 gets 
turned into 0x2345.
>
>>> +             return i2c_smbus_write_word_swapped(data->client,
>>> +                                             TMP117_REG_TEMP_OFFSET, off);
>>> +
>>> +     default:
>>> +             return -EINVAL;
>>> +     }
>>> +}
>>> +
>> [...]
>
>