linux-kernel - Re: [PATCH v4 1/5] counter: Internalize sysfs interface code

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Date:   Mon, 10 Aug 2020 17:48:07 -0500
From:   David Lechner <david@...hnology.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     jic23@...nel.org, kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com,
        David.Laight@...LAB.COM
Subject: Re: [PATCH v4 1/5] counter: Internalize sysfs interface code


>>>>>     
>>>>>     CPMAC ETHERNET DRIVER
>>>>>     M:	Florian Fainelli <f.fainelli@...il.com>
>>>>> diff --git a/drivers/counter/104-quad-8.c b/drivers/counter/104-quad-8.c
>>>>> index 78766b6ec271..0f20920073d6 100644
>>>>> --- a/drivers/counter/104-quad-8.c
>>>>> +++ b/drivers/counter/104-quad-8.c
>>>>> @@ -621,7 +621,7 @@ static const struct iio_chan_spec quad8_channels[] = {
>>>>>     };
>>>>>     
>>>>>     static int quad8_signal_read(struct counter_device *counter,
>>>>> -	struct counter_signal *signal, enum counter_signal_value *val)
>>>>> +			     struct counter_signal *signal, u8 *val)
>>>>
>>>> I'm not a fan of replacing enum types with u8 everywhere in this patch.
>>>> But if we have to for technical reasons (e.g. causes compiler error if
>>>> we don't) then it would be helpful to add comments giving the enum type
>>>> everywhere like this instance where u8 is actually an enum value.
>>>>
>>>> If we use u32 as the generic type for enums instead of u8, I think the
>>>> compiler will happlily let us use enum type and u32 interchangeably and
>>>> not complain.
>>>
>>> I switched to fixed-width types after the suggestion by David Laight:
>>> https://lkml.org/lkml/2020/5/3/159. I'll CC David Laight just in case he
>>> wants to chime in again.
>>>
>>> Enum types would be nice for making the valid values explicit, but there
>>> is one benefit I have appreciated from the move to fixed-width types:
>>> there has been a significant reduction of duplicate code; before, we had
>>> a different read function for each different enum type, but now we use a
>>> single function to handle them all.
>>
>> Yes, what I was trying to explain is that by using u32 instead of u8, I
>> think we can actually do both.
>>
>> The function pointers in struct counter_device *counter would use u32 as a
>> generic enum value in the declaration, but then the actual implementations
>> could still use the proper enum type.
> 
> Oh, I see what you mean now. So for example:
> 
>      int (*signal_read)(struct counter_device *counter,
>                         struct counter_signal *signal, u8 *val)
> 
> This will become instead:
> 
>      int (*signal_read)(struct counter_device *counter,
>                         struct counter_signal *signal, u32 *val)
> 
> Then in the driver callback implementation we use the enum type we need:
> 
>      enum counter_signal_level signal_level = COUNTER_SIGNAL_HIGH;
>      ...
>      *val = signal_level;
> 
> Is that what you have in mind?
> 

Yes.

Additionally, if we have...


       int (*x_write)(struct counter_device *counter,
                      ..., u32 val)
  
We should be able to define the implementation as:

static int my_driver_x_write(struct counter_device *counter,
                              ..., enum some_type val)
{
	...
}

Not sure if it works if val is a pointer though. Little-
endian systems would probably be fine, but maybe not big-
endian combined with -fshort-enums compiler flag.


       int (*x_read)(struct counter_device *counter,
                     ..., u32 *val)
  

static int my_driver_x_read(struct counter_device *counter,
                             ..., enum some_type *val)
{
	...
}