On Thu, Sep 27, 2018 at 03:12:25PM +0100, Robin Murphy wrote:

quoted

+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?