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

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Message-ID: <ZbEsfl0tGLY+xJl0@yury-ThinkPad>
Date: Wed, 24 Jan 2024 07:27:58 -0800
From: Yury Norov <yury.norov@...il.com>
To: Lucas De Marchi <lucas.demarchi@...el.com>
Cc: Jani Nikula <jani.nikula@...ux.intel.com>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	intel-xe@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org
Subject: Re: Re: [PATCH 1/3] bits: introduce fixed-type genmasks

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@...el.com> wrote:
> > > From: Yury Norov <yury.norov@...il.com>
> > > 
> > > 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