linux-kernel - Re: [PATCH v8 5/5] iio: light: Add support for APDS9306 Light Sensor

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Date: Thu, 29 Feb 2024 14:58:52 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Subhajit Ghosh <subhajit.ghosh@...aklogic.com>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Jonathan Cameron <jic23@...nel.org>, Lars-Peter Clausen
 <lars@...afoo.de>, Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Marek Vasut <marex@...x.de>,
 Anshul Dalal <anshulusr@...il.com>,
 Javier Carrasco <javier.carrasco.cruz@...il.com>,
 Matt Ranostay <matt@...ostay.sg>,
 Stefan Windfeldt-Prytz <stefan.windfeldt-prytz@...s.com>,
 linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 5/5] iio: light: Add support for APDS9306 Light Sensor

On 2/29/24 14:34, Subhajit Ghosh wrote:
> On 29/2/24 03:57, Andy Shevchenko wrote:
>> On Wed, Feb 28, 2024 at 03:08:56PM +0200, Matti Vaittinen wrote:
>>> On 2/28/24 14:24, Subhajit Ghosh wrote:
>>
>> ...
>>
>>>> +    ret = iio_gts_find_new_gain_by_old_gain_time(&data->gts, gain_old,
>>>> +                             intg_old, val2, &gain_new);
>>>
>>> You don't use the 'ret' here, so maybe for the clarity, not assign it.
>>> Or, maybe you wan't to try to squeeze out few cycles for succesful 
>>> case and
>>> check the ret for '0' - in which case you should be able to omit the 
>>> check
>>> right below as well as the call to iio_find_closest_gain_low(). OTOH, 
>>> this
>>> is likely not a "hot path" so I don't care too much about the extra 
>>> call if
>>> you think code is clearer this way.
>>>
>>>> +    if (gain_new < 0) {
>>>> +        dev_err_ratelimited(dev, "Unsupported gain with time\n");
>>>> +        return gain_new;
>>>> +    }
>>
>> What is the difference between negative response from the function 
>> itself and
>> similar in gain_new?
>>
> -ve response form the function is an error condition.
> -ve value in gain_new means - no valid gains could be computed.
> In case of error conditions from the function, the gain_new is also set 
> to -1.
> My use case is valid hardware gain so I went for checking only gain_new.
> Matti will be the best person to answer on this.

I now rely on the kerneldoc for the
iio_gts_find_new_gain_by_old_gain_time() as it seems reasonable to me:

* Return: 0 if an exactly matching supported new gain was found. When a
* non-zero value is returned, the @new_gain will be set to a negative or
* positive value. The negative value means that no gain could be computed.
* Positive value will be the "best possible new gain there could be". There
* can be two reasons why finding the "best possible" new gain is not deemed
* successful. 1) This new value cannot be supported by the hardware. 2) 
The new
* gain required to maintain the scale would not be an integer. In this case,
* the "best possible" new gain will be a floored optimal gain, which may or
* may not be supported by the hardware.

Eg, if ret is zero, there is no need to check validity of the gain_new 
but it is guaranteed to be one of the supported gains.

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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