[PATCH 1/7] driver: introduce less error-prone dev_get_drvdata alternative

[Ahmad Fatoum]

Hello Sascha,

On 9/30/20 3:13 PM, Ahmad Fatoum wrote:
> Hi,
> 
> On 9/30/20 9:48 AM, Sascha Hauer wrote:
>>> If the match data is a valid pointer:
>>>  -> It doesn't matter, why we have no match data either way.
>>>
>>> If the match data is a casted integer (e.g. enum):
>>>  The driver author should either:
>>>    -> place the default enum value as first one,
>>>       so no match data => default
>>
>> You know that, but "The driver Author" probably doesn't.
> 
> Like with all other functions that return an error code,
> the author should check those errors:
> 
> enum lm75_type type = (enum lm75_type)device_get_match_data(dev);
> if (type == 0)
>    return -ENODEV;
> 
> I'd expect they will see that 0 shouldn't be part of the enumeration.
> 
>>>    -> should add an initial DEVICE_TYPE_UNKNOWN = 0 in the enum
>>>       and handle it appropriately
>>>
>>> I like the function signature like that, I don't really see
>>> a need to adjust it.
>>>
>>>> As you realize in your series some drivers cast the returned value into
>>>> an integer type and use 0 as a valid possibility. These need an extra
>>>> explanation why we can accept that case. I can think of different
>>>> possibilies to get that straight:
>>>>
>>>> - Put a real pointer into matchdata. This is really preferred as it
>>>>   motivates people to put flags into a (struct type) matchdata which
>>>>   doesn't lead to excessive if (type == foo || type == bar || type ==
>>>>   baz) we sometimes see in drivers.
>>>
>>> We have a real pointer there already. The problem is migrating the
>>> existing drivers.
>>
>> Yes, existing drivers would have to be migrated, that is exactly what I
>> am proposing.
>>
>>>
>>>> - Return an ERR_PTR from device_get_match_data(). this is less likely
>>>>   interpreted as a valid int value
>>>
>>> Doesn't cover all cases. Also for the normal use, it means
>>> you need to have to check with IS_ERR_OR_NULL everywhere to
>>> be sure you don't dereference a NULL pointer.
>>
>> Why that? Just don't return NULL when there's no match data, but return
>> -ESOMETHING.
> 
> For the lm75 we would have to do:
> 
> enum lm75_type { TYPE_1, TYPE_2 };
> 
> const void *match = device_get_match_data(dev);
> if (IS_ERR(match))
>    return PTR_ERR(match);
> enum lm75_type type = (enum lm75_type)match;
> 
> The alternative being:
> 
> enum lm75_type { TYPE_UNKNOWN = 0, TYPE_1, TYPE_2 };
> 
> enum lm75_type type = (enum lm75_type)device_get_match_data(dev);
> if (type == TYPE_UNKNOWN)
>    return -ENODEV;
> 
> I prefer the second one very much.

Gentle ping.

> 
>>
>> Sascha
>>
> 

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