linux-kernel - Re: [PATCH 00/12] treewide: Fix GENMASK misuses

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Message-ID: <b9c3b83c9be50286062ae8cefd5d38e2baa0fb22.camel@perches.com>
Date:   Wed, 10 Jul 2019 08:45:53 -0700
From:   Joe Perches <joe@...ches.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Johannes Berg <johannes@...solutions.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Patrick Venture <venture@...gle.com>,
        Nancy Yuen <yuenn@...gle.com>,
        Benjamin Fair <benjaminfair@...gle.com>,
        Andrew Jeffery <andrew@...id.au>, openbmc@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-aspeed@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, netdev@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-wireless@...r.kernel.org, linux-media@...r.kernel.org,
        linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
        alsa-devel@...a-project.org, linux-mmc@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 00/12] treewide: Fix GENMASK misuses

On Wed, 2019-07-10 at 10:43 +0100, Russell King - ARM Linux admin wrote:
> On Wed, Jul 10, 2019 at 11:17:31AM +0200, Johannes Berg wrote:
> > On Tue, 2019-07-09 at 22:04 -0700, Joe Perches wrote:
> > > These GENMASK uses are inverted argument order and the
> > > actual masks produced are incorrect.  Fix them.
> > > 
> > > Add checkpatch tests to help avoid more misuses too.
> > > 
> > > Joe Perches (12):
> > >   checkpatch: Add GENMASK tests
> > 
> > IMHO this doesn't make a lot of sense as a checkpatch test - just throw
> > in a BUILD_BUG_ON()?

I tried that.

It'd can't be done as it's used in declarations
and included in asm files and it uses the UL()
macro.

I also tried just making it do the right thing
whatever the argument order.

Oh well.

> My personal take on this is that GENMASK() is really not useful, it's
> just pure obfuscation and leads to exactly these kinds of mistakes.
> 
> Yes, I fully understand the argument that you can just specify the
> start and end bits, and it _in theory_ makes the code more readable.
> 
> However, the problem is when writing code.  GENMASK(a, b).  Is a the
> starting bit or ending bit?  Is b the number of bits?  It's confusing
> and causes mistakes resulting in incorrect code.  A BUILD_BUG_ON()
> can catch some of the cases, but not all of them.

It's a horrid little macro and I agree with Russell.

I also think if it existed at all it should have been
GENMASK(low, high) not GENMASK(high, low).

I