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From: Bjorn Helgaas <helgaas@kernel.org>
To: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Cc: andrew.murray@arm.com, maz@kernel.org,
	linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH v4 7/8] linux/log2.h: Fix 64bit calculations in roundup/down_pow_two()
Date: Thu, 5 Dec 2019 16:30:44 -0600
Message-ID: <20191205223044.GA250573@google.com> (raw)
In-Reply-To: <20191203114743.1294-8-nsaenzjulienne@suse.de>

You got the "n" on "down" in the subject, but still missing "of" ;)

On Tue, Dec 03, 2019 at 12:47:40PM +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. It turns
> out ilog2() already handles 32/64bit calculations properly, and being
> the building block to the round functions we can rework them as a
> wrapper around it.

Missing "of" in the function names here.
s/a wrapper/wrappers/

IIUC the point of this is that roundup_pow_of_two() returned
"unsigned long", which can be either 32 or 64 bits (worth pointing
out, I think), and many callers need something that returns
"unsigned long long" (always 64 bits).

It's a nice simplification to remove the "__" variants.  Just as a
casual reader of this commit message, I'd like to know why we had both
the roundup and the __roundup versions in the first place, and why we
no longer need both.

> -#define roundup_pow_of_two(n)			\
> -(						\
> -	__builtin_constant_p(n) ? (		\
> -		(n == 1) ? 1 :			\
> -		(1UL << (ilog2((n) - 1) + 1))	\
> -				   ) :		\
> -	__roundup_pow_of_two(n)			\
> - )
> +#define roundup_pow_of_two(n)			  \
> +(						  \
> +	(__builtin_constant_p(n) && ((n) == 1)) ? \
> +	1 : (1ULL << (ilog2((n) - 1) + 1))        \
> +)

Should the resulting type of this expression always be a ULL, even
when n==1, i.e., should it be this?

  1ULL : (1ULL << (ilog2((n) - 1) + 1))        \

Or maybe there's no case where that makes a difference?

Bjorn

  parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-03 11:47 [PATCH v4 0/8] Raspberry Pi 4 PCIe support Nicolas Saenz Julienne
2019-12-03 11:47 ` [PATCH v4 7/8] linux/log2.h: Fix 64bit calculations in roundup/down_pow_two() Nicolas Saenz Julienne
2019-12-03 16:39   ` Chuck Lever
2019-12-04 10:56   ` Martin Habets
2019-12-04 14:17   ` Leon Romanovsky
2019-12-05  0:39   ` Lu Baolu
2019-12-05 17:48   ` Robin Murphy
2019-12-12 12:31     ` Nicolas Saenz Julienne
2019-12-05 22:30   ` Bjorn Helgaas [this message]
2019-12-12 13:16     ` Nicolas Saenz Julienne
2019-12-03 11:47 ` [PATCH v4 8/8] linux/log2.h: Use roundup/dow_pow_two() on 64bit calculations Nicolas Saenz Julienne
2019-12-03 15:53   ` Rob Herring
2019-12-03 16:06     ` Nicolas Saenz Julienne
2019-12-05 20:38   ` Bjorn Helgaas
2019-12-12 13:21     ` Nicolas Saenz Julienne

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