Messages in this thread | | | Date | Mon, 29 Jan 2024 07:11:17 -0800 | From | Yury Norov <> | Subject | Re: Re: Re: [PATCH 1/3] bits: introduce fixed-type genmasks |
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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@intel.com> wrote: > > > > > From: Yury Norov <yury.norov@gmail.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? > > 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
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