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

[Ahmad Fatoum]

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.

> 
> Sascha
> 

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |