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From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: devel@driverdev.osuosl.org, linux-mmc@vger.kernel.org,
	alsa-devel@alsa-project.org,
	Benjamin Fair <benjaminfair@google.com>,
	linux-aspeed@lists.ozlabs.org, Andrew Jeffery <andrew@aj.id.au>,
	Patrick Venture <venture@google.com>,
	openbmc@lists.ozlabs.org, linux-wireless@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org,
	Nancy Yuen <yuenn@google.com>,
	linux-mediatek@lists.infradead.org,
	dri-devel@lists.freedesktop.org, netdev@vger.kernel.org,
	Joe Perches <joe@perches.com>,
	linux-amlogic@lists.infradead.org,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-stm32@st-md-mailman.stormreply.com,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org
Subject: Re: [PATCH 00/12] treewide: Fix GENMASK misuses
Date: Wed, 10 Jul 2019 10:43:37 +0100
Message-ID: <20190710094337.wf2lftxzfjq2etro@shell.armlinux.org.uk> (raw)
In-Reply-To: <5fa1fa6998332642c49e2d5209193ffe2713f333.camel@sipsolutions.net>

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/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up

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Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20190710050444epcas1p250f7aa0f8798a7757df51d66f5970c2a@epcas1p2.samsung.com>
2019-07-10  5:04 ` Joe Perches
2019-07-10  5:04   ` [PATCH 06/12] mmc: meson-mx-sdio: Fix misuse of GENMASK macro Joe Perches
2019-07-10  5:04   ` [PATCH 10/12] phy: amlogic: G12A: " Joe Perches
2019-07-10  9:17   ` [PATCH 00/12] treewide: Fix GENMASK misuses Johannes Berg
2019-07-10  9:43     ` Russell King - ARM Linux admin [this message]
2019-07-10 15:45       ` Joe Perches
2019-07-10 16:01         ` Joe Perches
2019-07-11 21:30   ` David Miller
2019-07-12 12:54   ` Andrzej Hajda

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