Re: include/linux/compiler_types.h:597:38: error: call to '__compiletime_assert_437' declared with attribute error: FIELD_PREP: value too large for the field

[David Laight <david.laight.linux@gmail.com>]

On Tue, 30 Dec 2025 03:47:17 +0100
Marek Vasut <marek.vasut@mailbox.org> wrote:

> On 12/5/25 11:16 AM, kernel test robot wrote:
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   2061f18ad76ecaddf8ed17df81b8611ea88dbddd
> > commit: 4f716a1db661cfb31502a0a6d7e62c06daf2e603 drm/rcar-du: dsi: Convert register bitfields to GENMASK() macro
> > date:   5 weeks ago
> > config: arc-randconfig-001-20251205 (https://download.01.org/0day-ci/archive/20251205/202512051834.bESvhDiG-lkp@intel.com/config)
> > compiler: arc-linux-gcc (GCC) 8.5.0
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251205/202512051834.bESvhDiG-lkp@intel.com/reproduce)
> > 
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <lkp@intel.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202512051834.bESvhDiG-lkp@intel.com/
> > 
> > All errors (new ones prefixed by >>):
> > 
> >     In file included from <command-line>:
> >     drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c: In function 'rcar_mipi_dsi_startup.isra.4':  
> >>> include/linux/compiler_types.h:597:38: error: call to '__compiletime_assert_437' declared with attribute error: FIELD_PREP: value too large for the field  
> 
> [...]
> 
> >     rcar_mipi_dsi.c:660:14: note: in expansion of macro 'VCLKSET_DIV_V4H'
> >        vclkset |= VCLKSET_DIV_V4H(__ffs(setup_info.vclk_divider) - 1);
> >                   ^~~~~~~~~~~~~~~  
> I don't think this condition can occur.

The value of vclk_divider makes no difference.
The compiler has generated a code path where a constant is passed to
FIELD_PREP().

At a guess 'arc' doesn't have a definition for __ffs(x) so the compiler
builtin in used (I've forgotten its name), but that probably has looks like:
#define __ffs(x) (__builtin_ffs(x) - 1)
where (effectively):
#define __builtin_ffs(x) (!x ? 0 : 1 + count_trailing_zeros(x))
That ?: condition ends up in the code and gcc moves the FIELD_PREP()
into it (gcc likes optimising arithmetic on constants).
So you end up with FIELD_PREP(mask, -1) - and hence the error.

> 
> In drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c :
> - rcar_mipi_dsi_startup() calls rcar_mipi_dsi_parameters_calc() which is 
> the only site that assigns setup_info.vclk_divider this way:
> 
>   415         switch (dsi->info->model) {
>   416         case RCAR_DSI_V3U:
>   417         default:
>   418                 setup_info->vclk_divider = 1 << ((clk_cfg->vco_cntrl >> 4) & 0x3);
>   419                 break;
>   420
>   421         case RCAR_DSI_V4H:
>   422                 setup_info->vclk_divider = 1 << (((clk_cfg->vco_cntrl >> 3) & 0x7) + 1);
>   423                 break;
>   424         }
> 
> In case of V3U, vclk_divider is 1 << (0..3) .
> In case of V4H, vclk_divider is 1 << (1..8) .
> 
> In drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c :
> - rcar_mipi_dsi_startup() later contains the offending code:
> 
>   653         switch (dsi->info->model) {
>   654         case RCAR_DSI_V3U:
>   655         default:
>   656                 vclkset |= VCLKSET_DIV_V3U(__ffs(setup_info.vclk_divider));
>   657                 break;
>   658
>   659         case RCAR_DSI_V4H:
>   660                 vclkset |= VCLKSET_DIV_V4H(__ffs(setup_info.vclk_divider) - 1);
>   661                 break;
>   662         }
> 
> This does the reverse of the code that assigned the value above, so:
> 
> In case of V3U, field value is (0..3) .
> In case of V4H, field value is (0..7) .
> 
> Finally in drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi_regs.h :
> 
> 256 #define VCLKSET_DIV_V3U_MASK            GENMASK_U32(5, 4)
> 257 #define VCLKSET_DIV_V3U(n) FIELD_PREP(VCLKSET_DIV_V3U_MASK, (n))
> 258 #define VCLKSET_DIV_V4H_MASK            GENMASK_U32(6, 4)
> 259 #define VCLKSET_DIV_V4H(n) FIELD_PREP(VCLKSET_DIV_V4H_MASK, (n))
> 
> The masks fit exactly. For V3U the mask is 2-bit, or V4H the mask is 3-bit .

The only other use I can see is:
			fout = div64_u64(fout, setup_info->vclk_divider);

ISTM that there is an unnecessary '1 << n' and __ffs().
Perhaps you should be saving the 'bit number' not the shifted value.
The div64_u64() is then just a >> and fine on 32bit systems.

	David

> 
> ...
> 
> I can however rewrite the code in this way, which should also mitigate 
> this warning. Do you think this makes the code any better ?
> 
> diff --git a/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c 
> b/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c
> index 4ef2e3c129ed7..20e2a4cae5b86 100644
> --- a/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c
> +++ b/drivers/gpu/drm/renesas/rcar-du/rcar_mipi_dsi.c
> @@ -84,7 +84,6 @@ struct dsi_setup_info {
>   	unsigned long fout;
>   	u16 m;
>   	u16 n;
> -	u16 vclk_divider;
>   	const struct dsi_clk_config *clkset;
>   };
> 
> @@ -338,7 +337,8 @@ rcar_mipi_dsi_post_init_phtw_v4h(struct 
> rcar_mipi_dsi *dsi,
>   static void rcar_mipi_dsi_pll_calc(struct rcar_mipi_dsi *dsi,
>   				   unsigned long fin_rate,
>   				   unsigned long fout_target,
> -				   struct dsi_setup_info *setup_info)
> +				   struct dsi_setup_info *setup_info,
> +				   u16 vclk_divider)
>   {
>   	unsigned int best_err = -1;
>   	const struct rcar_mipi_dsi_device_info *info = dsi->info;
> @@ -391,6 +391,7 @@ static void rcar_mipi_dsi_parameters_calc(struct 
> rcar_mipi_dsi *dsi,
>   	unsigned long fin_rate;
>   	unsigned int i;
>   	unsigned int err;
> +	u16 vclk_divider;
> 
>   	/*
>   	 * Calculate Fout = dot clock * ColorDepth / (2 * Lane Count)
> @@ -415,15 +416,15 @@ static void rcar_mipi_dsi_parameters_calc(struct 
> rcar_mipi_dsi *dsi,
>   	switch (dsi->info->model) {
>   	case RCAR_DSI_V3U:
>   	default:
> -		setup_info->vclk_divider = 1 << ((clk_cfg->vco_cntrl >> 4) & 0x3);
> +		vclk_divider = 1 << ((clk_cfg->vco_cntrl >> 4) & 0x3);
>   		break;
> 
>   	case RCAR_DSI_V4H:
> -		setup_info->vclk_divider = 1 << (((clk_cfg->vco_cntrl >> 3) & 0x7) + 1);
> +		vclk_divider = 1 << (((clk_cfg->vco_cntrl >> 3) & 0x7) + 1);
>   		break;
>   	}
> 
> -	rcar_mipi_dsi_pll_calc(dsi, fin_rate, fout_target, setup_info);
> +	rcar_mipi_dsi_pll_calc(dsi, fin_rate, fout_target, setup_info, 
> vclk_divider);
> 
>   	/* Find hsfreqrange */
>   	setup_info->hsfreq = setup_info->fout * 2;
> @@ -439,7 +440,7 @@ static void rcar_mipi_dsi_parameters_calc(struct 
> rcar_mipi_dsi *dsi,
>   	dev_dbg(dsi->dev,
>   		"Fout = %u * %lu / (%u * %u * %u) = %lu (target %lu Hz, error 
> %d.%02u%%)\n",
>   		setup_info->m, fin_rate, dsi->info->n_mul, setup_info->n,
> -		setup_info->vclk_divider, setup_info->fout, fout_target,
> +		vclk_divider, setup_info->fout, fout_target,
>   		err / 100, err % 100);
> 
>   	dev_dbg(dsi->dev,
> @@ -653,11 +654,11 @@ static int rcar_mipi_dsi_startup(struct 
> rcar_mipi_dsi *dsi,
>   	switch (dsi->info->model) {
>   	case RCAR_DSI_V3U:
>   	default:
> -		vclkset |= VCLKSET_DIV_V3U(__ffs(setup_info.vclk_divider));
> +		vclkset |= VCLKSET_DIV_V3U((clk_cfg->vco_cntrl >> 4) & 0x3);
>   		break;
> 
>   	case RCAR_DSI_V4H:
> -		vclkset |= VCLKSET_DIV_V4H(__ffs(setup_info.vclk_divider) - 1);
> +		vclkset |= VCLKSET_DIV_V4H((clk_cfg->vco_cntrl >> 3) & 0x7);
>   		break;
>   	}
>