Re: arm: ERROR: modpost: "__aeabi_uldivmod" [drivers/gpu/drm/sun4i/sun4i-drm-hdmi.ko] undefined!

Arnd Bergmann
2024-03-04T12:35:34Z

On Mon, Mar 4, 2024, at 12:45, Andre Przywara wrote:
> On Mon, 04 Mar 2024 12:26:46 +0100
> "Arnd Bergmann" <arnd@arndb.de> wrote:
>
>> On Mon, Mar 4, 2024, at 12:24, Andre Przywara wrote:
>> > On Mon, 04 Mar 2024 12:11:36 +0100 "Arnd Bergmann" <arnd@arndb.de> wrote:
>> >>
>> >> This used to be a 32-bit division. If the rate is never more than
>> >> 4.2GHz, clock could be turned back into 'unsigned long' to avoid
>> >> the expensive div_u64().
>> >
>> > Wouldn't "div_u64(clock, 200)" solve this problem?
>>
>> Yes, that's why I mentioned it as the worse of the two obvious
>> solutions. ;-)
>
> Argh, should have cleaned my glasses first ;-)
>
> I guess I was put somehow put off by the word "expensive". While it's
> admittedly not trivial, I wonder if we care about the (hidden) complexity
> of that function? I mean it's neither core code nor something called
> frequently?

It's not critical if this is called infrequently, and as Maxime
just replied, the 64-bit division is in fact required here.
Since we are dividing by a constant value (200), there is a good
chance that this will be get turned into fairly efficient
multiply/shift code.

> I don't think we have any clock exceeding 3GHz at the moment, but it
> sounds fishy to rely on that.

Right, it's just important to look at each case individually.
The cost of 64-bit division is crazy if it gets called repeatedly,
which is of course the entire reason we don't provide a
__aeabi_uldivmod() function and require developers to think
before adding div_u64().

Arnd