Re: [PATCH] mtd: maps: fix -Wvoid-pointer-to-enum-cast warning

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 16/08/2023 01:06, Justin Stitt wrote:
> On Tue, Aug 15, 2023 at 2:15 PM Krzysztof Kozlowski
> <krzysztof.kozlowski@linaro.org> wrote:
>>
>> On 15/08/2023 23:11, Justin Stitt wrote:
>>> When building with clang 18 I see the following warning:
>>> |       drivers/mtd/maps/physmap-versatile.c:209:25: warning: cast to smaller
>>> |               integer type 'enum versatile_flashprot' from 'const void *' [-Wvoid-pointer-to-enum-cast]
>>> |         209 |                 versatile_flashprot = (enum versatile_flashprot)devid->data;
>>>
>>> This is due to the fact that `devid->data` is a void* while `enum
>>> versatile_flashprot` has the size of an int. This leads to truncation
>>> and possible data loss.
>>
>> Cast does not solve truncation. This part of commit msg suggests that
>> you actually fix real issue... and that is an issue, because then
>> AUTOSEL will grab it. This is just compiler warning silencing and rather
>> coding standard correctness, no real fix, so please drop the sentence.
> OK, makes sense about this not technically solving an issue and thus
> AUTOSEL may pick it up. Can you elaborate, though, on how the cast
> doesn't solve truncation. 

Because that is no how the C language work?

Casting UINTMAX+1 to unsigned int, does not magically change the
unsigned int into something else...


> Is the initial implementation not a
> pointer-width down to int-width cast?

These are different widths, so cast cannot solve truncation.


> Surely we're losing the top half
> of bits. 

If we are losing top half then how is the truncation and data loss solved?

> I'm still not saying there's data loss, to be clear. Just
> that the compiler is warning because of the truncation.

Sorry, what truncation? The one which will happen always regardless of
the cast and warning?

Best regards,
Krzysztof