Re: [PATCH] serial: mxs-uart: fix Wvoid-pointer-to-enum-cast warning

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Aug 14, 2023 at 08:58:34AM +0200, Krzysztof Kozlowski wrote:
> On 10/08/2023 17:44, Greg Kroah-Hartman wrote:
> > On Thu, Aug 10, 2023 at 10:50:42AM +0200, Krzysztof Kozlowski wrote:
> >> `devtype` is enum, thus cast of pointer on 64-bit compile test with W=1
> >> causes:
> >>
> >>   mxs-auart.c:1598:15: error: cast to smaller integer type 'enum mxs_auart_type' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]
> >>
> >> Cc: Andi Shyti <andi.shyti@kernel.org>
> >> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> >> ---
> >>  drivers/tty/serial/mxs-auart.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/tty/serial/mxs-auart.c b/drivers/tty/serial/mxs-auart.c
> >> index 8eeecf8ad359..a9b32722b049 100644
> >> --- a/drivers/tty/serial/mxs-auart.c
> >> +++ b/drivers/tty/serial/mxs-auart.c
> >> @@ -1595,7 +1595,7 @@ static int mxs_auart_probe(struct platform_device *pdev)
> >>  		return -EINVAL;
> >>  	}
> >>  
> >> -	s->devtype = (enum mxs_auart_type)of_device_get_match_data(&pdev->dev);
> >> +	s->devtype = (uintptr_t)of_device_get_match_data(&pdev->dev);
> > 
> > This feels like a compiler issue as devtype is a enum mxs_auart_type
> > variable, so the cast shoudl be correct.
> 
> While the cast is obviously safe here, the warning in general is
> reasonable - people were make too many mistakes by assuming pointers are
> integers...

But this isn't a pointer, it's an enumerated type thrown into a void *,
so cast it properly (void * == unsigned long).

> Just for the record (not saying that others doing is proof of correctness):
> 
> https://lore.kernel.org/lkml/20230809-cbl-1903-v1-1-df9d66a3ba3e@google.com/T/
> 
> But maybe Nathan can share his thoughts whether we should just disable
> this warning for kernel?

I thought that Linus had some objection to the compilers doing something
foolish here as well, but I can't find it in my archives at the moment
(am traveling).

> > And if not, unitptr_t isn't a valid kernel type, so that's not a good
> 
> It is in include/linux/types.h, so do you mean that it is not
> recommended for in-kernel usage? I can go with kernel_ulong_t - which is
> a kernel type - if the cast is agreed.

not recommended for in-kernel usage.

thanks,

greg k-h