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

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Message-ID: <20180927152818.GC10566@lst.de>
Date:   Thu, 27 Sep 2018 17:28:18 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, 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 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?