netdev - Re: [PATCH v3 1/7] linux/log2.h: Add roundup/rounddown_pow

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Date:   Wed, 27 Nov 2019 19:24:07 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Robin Murphy <robin.murphy@....com>,
        Leon Romanovsky <leon@...nel.org>
Cc:     andrew.murray@....com, maz@...nel.org,
        linux-kernel@...r.kernel.org,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        james.quinlan@...adcom.com, mbrugger@...e.com,
        f.fainelli@...il.com, phil@...pberrypi.org, wahrenst@....net,
        jeremy.linton@....com, linux-pci@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        devicetree@...r.kernel.org, linux-rockchip@...ts.infradead.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v3 1/7] linux/log2.h: Add roundup/rounddown_pow_two64()
 family of functions

On Wed, 2019-11-27 at 18:06 +0000, Robin Murphy wrote:
> On 26/11/2019 12:51 pm, Leon Romanovsky wrote:
> > On Tue, Nov 26, 2019 at 10:19:39AM +0100, Nicolas Saenz Julienne wrote:
> > > Some users need to make sure their rounding function accepts and returns
> > > 64bit long variables regardless of the architecture. Sadly
> > > roundup/rounddown_pow_two() takes and returns unsigned longs. Create a
> > > new generic 64bit variant of the function and cleanup rougue custom
> > > implementations.
> > 
> > Is it possible to create general roundup/rounddown_pow_two() which will
> > work correctly for any type of variables, instead of creating special
> > variant for every type?
> 
> In fact, that is sort of the case already - roundup_pow_of_two() itself 
> wraps ilog2() such that the constant case *is* type-independent. And 
> since ilog2() handles non-constant values anyway, might it be reasonable 
> to just take the strongly-typed __roundup_pow_of_two() helper out of the 
> loop as below?
> 
> Robin
> 

That looks way better that's for sure. Some questions.

> ----->8-----
> diff --git a/include/linux/log2.h b/include/linux/log2.h
> index 83a4a3ca3e8a..e825f8a6e8b5 100644
> --- a/include/linux/log2.h
> +++ b/include/linux/log2.h
> @@ -172,11 +172,8 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
>    */
>   #define roundup_pow_of_two(n)			\
>   (						\
> -	__builtin_constant_p(n) ? (		\
> -		(n == 1) ? 1 :			\
> -		(1UL << (ilog2((n) - 1) + 1))	\
> -				   ) :		\
> -	__roundup_pow_of_two(n)			\
> +	(__builtin_constant_p(n) && (n == 1)) ?	\
> +	1 : (1UL << (ilog2((n) - 1) + 1))	\

Then here you'd have to use ULL instead of UL, right? I want my 64bit value
everywhere regardless of the CPU arch. The downside is that would affect
performance to some extent (i.e. returning a 64bit value where you used to have
a 32bit one)?

Also, what about callers to this function on platforms with 32bit 'unsigned
longs' that happen to input a 64bit value into this. IIUC we'd have a change of
behaviour.

Regards,
Nicolas


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