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

[Yury Norov]

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?
 
Thanks,
Yury