linux-kernel - Re: [PATCH 2/5] dma-direct: add an explicit dma_direct_get_required

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Date:   Thu, 27 Sep 2018 16:35:21 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/5] dma-direct: add an explicit
 dma_direct_get_required_mask

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 Nitpick rescinded!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.