On Thu, 21 Dec 2023 15:44:22 -0800 Mina Almasry wrote:
> The warning is like so:
>
> ./include/net/page_pool/helpers.h: In function ‘page_pool_alloc’:
> ./include/linux/stddef.h:8:14: warning: returning ‘void *’ from a
> function with return type ‘netmem_ref’ {aka ‘long unsigned int’} makes
> integer from pointer without a cast [-Wint-conversion]
> 8 | #define NULL ((void *)0)
> | ^
> ./include/net/page_pool/helpers.h:132:24: note: in expansion of macro
> ‘NULL’
> 132 | return NULL;
> | ^~~~
>
> And happens in all the code where:
>
> netmem_ref func()
> {
> return NULL;
> }
>
> It's fixable by changing the return to `return (netmem_ref NULL);` or
> `return 0;`, but I feel like netmem_ref should be some type which
> allows a cast from NULL implicitly.
Why do you think we should be able to cast NULL implicitly?
netmem_ref is a handle, it could possibly be some form of
an ID in the future, rather than a pointer. Or have more low
bits stolen for specific use cases.
unsigned long, and returning 0 as "no handle" makes perfect sense to me.
Note that 0 is a special case, bitwise types are allowed to convert
to 0/bool and 0 is implicitly allowed to become a bitwise type.
This will pass without a warning:
typedef unsigned long __bitwise netmem_ref;
netmem_ref some_code(netmem_ref ref)
{
// direct test is fine
if (!ref)
// 0 "upgrades" without casts
return 0;
// 1 does not, we need __force
return (__force netmem_ref)1 | ref;
}
The __bitwise annotation will make catching people trying
to cast to struct page * trivial.
You seem to be trying hard to make struct netmem a thing.
Perhaps you have a reason I'm not getting?
