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

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Date: Mon, 29 Jan 2024 08:49:35 -0600
From: Lucas De Marchi <lucas.demarchi@...el.com>
To: Yury Norov <yury.norov@...il.com>
CC: <intel-gfx@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
	<dri-devel@...ts.freedesktop.org>, Andy Shevchenko
	<andriy.shevchenko@...ux.intel.com>, <intel-xe@...ts.freedesktop.org>
Subject: Re: Re: Re: [PATCH 1/3] bits: introduce fixed-type genmasks

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@...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?

either that or an explicit check as it was suggested. What's your
preference?

Lucas De Marchi

>
>Thanks,
>Yury