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

Joe Perches
2019-07-10T15:45:53Z

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