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

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 29 Feb 2024 17:35:01 +0200
Matti Vaittinen <mazziesaccount@gmail.com> wrote:

> On 2/29/24 15:42, Andy Shevchenko wrote:
> > On Thu, Feb 29, 2024 at 02:58:52PM +0200, Matti Vaittinen wrote:  
> >> 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:  
> > 
> > ...
> >   
> >>>>>> +    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.  
> > 
> > Right, but this kernel doc despite being so verbose does not fully answer my
> > question. What is the semantic of that "negative value"?   
> 
> Current approach is to always investigate the function return value as 
> error if the 'new_gain' is negative. Or, caller specific error if 
> new_gain is unsuitable in some other way. When this is done, the 
> absolute value of the negative 'new_gain' does not matter.
> 
> > I would expect to have
> > the error code there (maybe different to what the function returns), but at
> > least be able to return it to the upper layers if needed.  
> 
> I am not sure I see the benefit of returning the new_gain over returning 
> the error returned by the function. Well, maybe the benefit to be able 
> to not evaluate the value returned by the 
> iio_gts_find_new_gain_by_old_gain_time() - although I'm not sure I love it.
> 
> > Hence 2 ARs I see:
> > 1) clarify the kernel documentation there;
> > 2) update the semantic of the gain_new to simplify caller's code.  
> 
> Yes, I agree. Patches welcome :) By the very least the kerneldoc can be 
> improved. I'm undecided on benefits of having the error code in 'new_gain'.

It's definitely a weird bit of API and would benefit from a rethink.
> 
> The GTS API fixes shouldn't be required in the context of this driver 
> series though.
Agreed.
> 
> Yours,
> 	--Matti
>