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

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Message-ID: <20190710094337.wf2lftxzfjq2etro@shell.armlinux.org.uk>
Date:   Wed, 10 Jul 2019 10:43:37 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Joe Perches <joe@...ches.com>,
        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, 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()?

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.

For example:

	GENMASK(6, 2)

would satisify the requirement that a > b, so a BUILD_BUG_ON() will
not trigger, but was the author meaning 0x3c or 0xc0?

Personally, I've decided I am _not_ going to use GENMASK() in my code
because I struggle to get the macro arguments correct - I'm _much_
happier, and it is way more reliable for me to write the mask in hex
notation.

I think this is where use of a ternary operator would come in use.  The
normal way of writing a number of bits tends to be "a:b", so if GENMASK
took something like GENMASK(6:2), then I'd have less issue with it,
because it's argument is then in a familiar notation.

Yes, I'm sure that someone will point out that the GENMASK arguments
are just in the same order, but that doesn't prevent _me_ frequently
getting it wrong - and that's the point.  The macro seems to me to
cause more problems than it solves.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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