linux-kernel - Re: [PATCH 00/14] drm/tidss: dispc: Convert to FIELD

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Message-ID: <20250731-powerful-termite-of-inspiration-13f36d@houat>
Date: Thu, 31 Jul 2025 15:26:42 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Louis Chauvet <louis.chauvet@...tlin.com>
Cc: Jyri Sarha <jyri.sarha@....fi>, 
	Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/14] drm/tidss: dispc: Convert to FIELD_* API

Hi Louis,

On Thu, Jul 31, 2025 at 03:04:49PM +0200, Louis Chauvet wrote:
> Le 30/07/2025 à 10:57, Maxime Ripard a écrit :
> > Hi,
> > 
> > The tidss driver rolls its own API equivalent to the FIELD_* API already
> > provided the kernel.
> > 
> > Since it's an ad-hoc implementation, it also is less convenient and
> > doesn't provide some useful features like being able to share the field
> > definitions that will come handy in the future.
> > 
> > Thus, this series converts the driver to that API and drops its own
> > version.
>
> I just saw your series after sending mine [2]. I checked, there is only one
> minor conflict that can be easly fixed.
> 
> But when applied on drm-misc/drm-misc-next, your series raises:
> 
> In file included from <command-line>:
> drivers/gpu/drm/tidss/tidss_dispc.c: In function 'FLD_MOD':
> ././include/linux/compiler_types.h:568:45: error: call to
> '__compiletime_assert_589' declared with attribute error: FIELD_PREP: mask
> is not constant
>   568 |         _compiletime_assert(condition, msg, __compiletime_assert_,
> __COUNTER__)
>       |                                             ^
> ././include/linux/compiler_types.h:549:25: note: in definition of macro
> '__compiletime_assert'
>   549 |                         prefix ## suffix();         \
>       |                         ^~~~~~
> ././include/linux/compiler_types.h:568:9: note: in expansion of macro
> '_compiletime_assert'
>   568 |         _compiletime_assert(condition, msg, __compiletime_assert_,
> __COUNTER__)
>       |         ^~~~~~~~~~~~~~~~~~~
> ./include/linux/build_bug.h:39:37: note: in expansion of macro
> 'compiletime_assert'
>    39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
>       |                                     ^~~~~~~~~~~~~~~~~~
> ./include/linux/bitfield.h:65:17: note: in expansion of macro
> 'BUILD_BUG_ON_MSG'
>    65 |                 BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),
> \
>       |                 ^~~~~~~~~~~~~~~~
> ./include/linux/bitfield.h:115:17: note: in expansion of macro
> '__BF_FIELD_CHECK'
>   115 |                 __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ");
> \
>       |                 ^~~~~~~~~~~~~~~~
> drivers/gpu/drm/tidss/tidss_dispc.c:599:33: note: in expansion of macro
> 'FIELD_PREP'
>   599 |         return (orig & ~mask) | FIELD_PREP(mask, val);
>       |                                 ^~~~~~~~~~
> 
> 
> This seems to be a limitation of FIELD_PREP [1].
> I think the only way to avoid this issue is to use macros and not functions.
> 
> [1]:https://elixir.bootlin.com/linux/v6.16/source/include/linux/bitfield.h#L65-L66
> [2]:https://lore.kernel.org/all/20250730-fix-edge-handling-v1-0-1bdfb3fe7922@bootlin.com/

Weird, it compiles without warning for me here. Which compiler do you use?

Thanks!
Maxime

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