Re: [PATCH 2/5] dma-direct: add an explicit dma_direct_get_required_mask

[Robin Murphy]

On 27/09/18 16:28, Christoph Hellwig wrote:
> On Thu, Sep 27, 2018 at 03:12:25PM +0100, Robin Murphy wrote:
> > > +u64 dma_direct_get_required_mask(struct device *dev)
> > > +{
> > > +	u64 max_dma = phys_to_dma_direct(dev, (max_pfn - 1) << PAGE_SHIFT);
> > > +
> > > +	return (1ULL << (fls64(max_dma) - 1)) * 2 - 1;
> >
> > I think that may as well just use __fls64() - it seems reasonable to assume
> > max_dma > 0. Otherwise,
>
> Is there any good reason to micro-optimize given that this isn't
> a fast path?

Not at all, I wasn't even thinking in terms of optimisation other than 
in terms of number of source characters and levels of parentheses.

But more importantly I was also being a big idiot because no matter how 
much I have the fls()/__fls() thing in mind, __fls64() doesn't actually 
exist. Nitpick rescinded!

Robin.