Re: [PATCH] net: openvswitch: use do_div() for 64-by-32 divisions:

[Toke HÃiland-JÃrgensen]

Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> writes:

> Hi Eric,
>
> On Fri, Apr 24, 2020 at 5:05 PM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>> On 4/24/20 5:10 AM, Geert Uytterhoeven wrote:
>> > On 32-bit architectures (e.g. m68k):
>> >
>> >     ERROR: modpost: "__udivdi3" [net/openvswitch/openvswitch.ko] undefined!
>> >     ERROR: modpost: "__divdi3" [net/openvswitch/openvswitch.ko] undefined!
>> >
>> > Fixes: e57358873bb5d6ca ("net: openvswitch: use u64 for meter bucket")
>> > Reported-by: noreply@xxxxxxxxxxxxxx
>> > Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
>> > ---
>> >  net/openvswitch/meter.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/net/openvswitch/meter.c b/net/openvswitch/meter.c
>> > index 915f31123f235c03..3498a5ab092ab2b8 100644
>> > --- a/net/openvswitch/meter.c
>> > +++ b/net/openvswitch/meter.c
>> > @@ -393,7 +393,7 @@ static struct dp_meter *dp_meter_create(struct nlattr **a)
>> >                * Start with a full bucket.
>> >                */
>> >               band->bucket = (band->burst_size + band->rate) * 1000ULL;
>> > -             band_max_delta_t = band->bucket / band->rate;
>> > +             band_max_delta_t = do_div(band->bucket, band->rate);
>> >               if (band_max_delta_t > meter->max_delta_t)
>> >                       meter->max_delta_t = band_max_delta_t;
>> >               band++;
>> >
>>
>> This is fascinating... Have you tested this patch ?
>
> Sorry, I should have said this is compile-tested only.
>
>> Please double check what do_div() return value is supposed to be !
>
> I do not have any openvswitch setups, let alone on the poor m68k box.

I think what Eric is referring to is this, from the documentation of
do_div:

 * do_div - returns 2 values: calculate remainder and update new dividend
 * @n: uint64_t dividend (will be updated)
 * @base: uint32_t divisor
 *
 * Summary:
 * ``uint32_t remainder = n % base;``
 * ``n = n / base;``
 *
 * Return: (uint32_t)remainder


Specifically that last part :)

-Toke