Re: [linux-next:master 13398/13940] drivers/net/ethernet/sfc/ef100_nic.c:610: undefined reference to `__umoddi3'

From: Edward Cree
Date: Tue Aug 11 2020 - 04:55:50 EST

On 10/08/2020 16:51, Guenter Roeck wrote:
> On Thu, Aug 06, 2020 at 07:17:43PM +0100, Edward Cree wrote:
>> Maybe I should add a
>> static inline u32 mod_u64(u64 dividend, u32 divisor)
>> {
>>         return do_div(dividend, divisor);
>> }
> Your proposed function is an exact replicate of do_div()
No, because do_div() is a macro that modifies 'dividend', whereas by
 wrapping it in an inline function mod_u64() implicitly creates a
 local variable.  Thus do_div() cannot be used on a constant, whereas
 mod_u64() can.
> You could try something like
> if (reader->value > EFX_MIN_DMAQ_SIZE || EFX_MIN_DMAQ_SIZE % (u32)reader->value)
I considered that.  It's ugly, so while it will work I think it's
 worthlooking to see if there's a better way.
> If EFX_MIN_DMAQ_SIZE is indeed known to be a power of 2, you could also use
> the knowledge that a 2^n value can only be divided by a smaller 2^n value,
> meaning that reader->value must have exactly one bit set. This would also
> avoid divide-by-0 issues if reader->value can be 0.
> if (reader->value > EFX_MIN_DMAQ_SIZE || hweight64(reader->value) != 1)
This is also ugly and I don't like relying on the power-of-twoness —
 it just feels fragile.

But you're right to point out that there's a div/0 issue, and if I'm
 going to have to check for that, then ugliness is unavoidable.  So
 I think the least painful option available is probably

    if (!reader->value || reader->value > EFX_MIN_DMAQ_SIZE ||
        EFX_MIN_DMAQ_SIZE % (u32)reader->value)

 which only assumes EFX_MIN_DMAQ_SIZE <= U32_MAX, an assumption I'm
 comfortable with baking in.
I'll put together a formal patch with that.

Thanks for the help.