Re: Re: Re: [PATCH 1/3] bits: introduce fixed-type genmasks

[Yury Norov]

On Mon, Jan 29, 2024 at 08:49:35AM -0600, Lucas De Marchi wrote:
> On Wed, Jan 24, 2024 at 07:27:58AM -0800, Yury Norov wrote:
> > On Wed, Jan 24, 2024 at 08:03:53AM -0600, Lucas De Marchi wrote:
> > > On Wed, Jan 24, 2024 at 09:58:26AM +0200, Jani Nikula wrote:
> > > > On Tue, 23 Jan 2024, Lucas De Marchi <lucas.demarchi@xxxxxxxxx> wrote:
> > > > > From: Yury Norov <yury.norov@xxxxxxxxx>
> > > > >
> > > > > Generalize __GENMASK() to support different types, and implement
> > > > > fixed-types versions of GENMASK() based on it. The fixed-type version
> > > > > allows more strict checks to the min/max values accepted, which is
> > > > > useful for defining registers like implemented by i915 and xe drivers
> > > > > with their REG_GENMASK*() macros.
> > > >
> > > > Mmh, the commit message says the fixed-type version allows more strict
> > > > checks, but none are actually added. GENMASK_INPUT_CHECK() remains the
> > > > same.
> > > >
> > > > Compared to the i915 and xe versions, this is more lax now. You could
> > > > specify GENMASK_U32(63,32) without complaints.
> > > 
> > > Doing this on top of the this series:
> > > 
> > > -#define   XELPDP_PORT_M2P_COMMAND_TYPE_MASK            REG_GENMASK(30, 27)
> > > +#define   XELPDP_PORT_M2P_COMMAND_TYPE_MASK            REG_GENMASK(62, 32)
> > > 
> > > and I do get a build failure:
> > > 
> > > ../drivers/gpu/drm/i915/display/intel_cx0_phy.c: In function ‘__intel_cx0_read_once’:
> > > ../include/linux/bits.h:41:31: error: left shift count >= width of type [-Werror=shift-count-overflow]
> > >    41 |          (((t)~0ULL - ((t)(1) << (l)) + 1) & \
> > >       |                               ^~
> > 
> > I would better include this in commit message to avoid people's
> > confusion. If it comes to v2, can you please do it and mention that
> > this trick relies on shift-count-overflow compiler check?
> 
> either that or an explicit check as it was suggested. What's your
> preference?

Let's put a comment in the code. An argument that shift-count-overflow
may be disabled sounds more like a speculation unless we have a solid
example of a build system where the error is disabled for a good sane
reason, but possible GENMASK() overflow is still considered dangerous.

GENMASK() is all about bit shifts, so shift-related error is something
I'd expect when using GENMASK().

Also, the macro is widely used in the kernel:

        yury:linux$ git grep GENMASK | wc -l
        26879

Explicit check would add pressure on the compiler for nothing. 

Thanks,
Yury