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

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Date:   Wed, 27 Nov 2019 18:06:18 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Leon Romanovsky <leon@...nel.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
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 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

----->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))	\
   )

  /**