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.
-ed
