netdev - Re: [PATCH v2 1/6] linux/log2.h: Add roundup/rounddown_pow

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Message-ID: <20191119165502.GB4991@ziepe.ca>
Date:   Tue, 19 Nov 2019 12:55:02 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Andrew Murray <andrew.murray@....com>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Heiko Stuebner <heiko@...ech.de>, linux-pci@...r.kernel.org,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        linux-rdma@...r.kernel.org, maz@...nel.org, phil@...pberrypi.org,
        iommu@...ts.linux-foundation.org,
        linux-rockchip@...ts.infradead.org, f.fainelli@...il.com,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, mbrugger@...e.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        jeremy.linton@....com, Tom Joseph <tjoseph@...ence.com>,
        wahrenst@....net, james.quinlan@...adcom.com,
        Robin Murphy <robin.murphy@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [PATCH v2 1/6] linux/log2.h: Add roundup/rounddown_pow_two64()
 family of functions

On Tue, Nov 19, 2019 at 04:28:50PM +0000, Andrew Murray wrote:
> On Tue, Nov 19, 2019 at 01:43:39PM +0100, Nicolas Saenz Julienne wrote:
> > On Tue, 2019-11-19 at 12:30 +0100, Nicolas Saenz Julienne wrote:
> > > Hi Andrew, thanks for the review.
> > > > > +/**
> > > > > + * __roundup_pow_of_two64() - round 64bit value up to nearest power of
> > > > > two
> > > > > + * @n: value to round up
> > > > > + */
> > > > > +static inline __attribute__((const)) __u64 __roundup_pow_of_two64(__u64
> > > > > n)
> > > > 
> > > > To be consistent with other functions in the same file (__ilog_u64) you may
> > > > want to rename this to __roundup_pow_of_two_u64.
> > > 
> > > Sounds good to me.
> > > 
> > > > Also do you know why u64 is used in some places and __u64 in others?
> > > 
> > > That's unwarranted, it should be __u64 everywhere.
> > 
> > Sorry, now that I look deeper into it, it should be u64.
> 
> Do you know the reason why? I'd be interested to know.

__u64 must be used in header files that are under uapi - ie it is the
name of the symbol in userspace, and u64 does not exist.

u64 should be used in all code that is only inside the kernel, ie .c
files, internal headers, etc

I routinely discourage use of __uXX in kernel native code.

Jason