Re: [v8] misc: pci_endpoint_test: Fix overflow of bar_size

Arnd Bergmann
2025-01-08T14:10:25Z

On Tue, Jan 7, 2025, at 16:57, Niklas Cassel wrote:
> On Tue, Jan 07, 2025 at 11:44:21PM +0800, Hans Zhang wrote:
>> On 2025/1/7 19:47, Niklas Cassel wrote:
>>
>> Hi Niklas,
>>
>> > The error:
>> > drivers/misc/pci_endpoint_test.c:315: undefined reference to `__udivmoddi4'
>> > sounds like the compiler is using a specialized instruction to do both div
>> > and mod in one. By removing the mod in patch 1/2, I expect that patch 2/2
>> > will no longer get this error.
>>
>> The __udivmoddi4 may be the way div and mod are combined.
>>
>> Delete remain's patch 1/2 according to your suggestion. I compiled it as a
>> KO module for an experiment.
>>
>> There are still __udivdi3 errors, so the do_div API must be used.
>
> Ok. Looking at do_div(), it seems to be the correct API to use
> for this problem. Just change bar_size type to u64 (instead of casting)
> and use do_div() ? That is how it is seems to be used in other drivers.

I think using div_u64_rem() instead of do_div() would make this
more readable as this is always an inline function, so the type can
remain resource_size_t, and the division gets optimized well when
that is a 32-bit type.

Arnd