[PATCH] drm/i915/snps_hdmi_pll: Fix 64-bit divisor truncation by using div64_u64

[Jani Nikula]

On Fri, 13 Jun 2025, Ankit Nautiyal <ankit.k.nautiyal at intel.com> wrote:
> DIV_ROUND_CLOSEST_ULL uses do_div(), which expects a 32-bit divisor.
> When passing a 64-bit constant like CURVE2_MULTIPLIER, the value is
> silently truncated to u32, potentially leading to incorrect results
> on large divisors.
>
> Replace DIV_ROUND_CLOSEST_ULL with div64_u64(), which correctly
> handles full 64-bit division. Since the result is clamped between
> 1 and 127, rounding is unnecessary and truncating division
> is sufficient.

I don't understand how you can make that conclusion. Please explain.

Would it be safer to just use DIV64_U64_ROUND_CLOSEST()?

> Fixes: 5947642004bf ("drm/i915/display: Add support for SNPS PHY HDMI PLL algorithm for DG2")
> Cc: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
> Cc: Suraj Kandpal <suraj.kandpal at intel.com>
> Cc: Jani Nikula <jani.nikula at intel.com>
> Cc: <stable at vger.kernel.org> # v6.15+
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_snps_hdmi_pll.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_snps_hdmi_pll.c b/drivers/gpu/drm/i915/display/intel_snps_hdmi_pll.c
> index 74bb3bedf30f..ac609bdf6653 100644
> --- a/drivers/gpu/drm/i915/display/intel_snps_hdmi_pll.c
> +++ b/drivers/gpu/drm/i915/display/intel_snps_hdmi_pll.c
> @@ -103,8 +103,8 @@ static void get_ana_cp_int_prop(u64 vco_clk,
>  			    DIV_ROUND_DOWN_ULL(curve_1_interpolated, CURVE0_MULTIPLIER)));
>  
>  	ana_cp_int_temp =
> -		DIV_ROUND_CLOSEST_ULL(DIV_ROUND_DOWN_ULL(adjusted_vco_clk1, curve_2_scaled1),
> -				      CURVE2_MULTIPLIER);
> +		div64_u64(DIV_ROUND_DOWN_ULL(adjusted_vco_clk1, curve_2_scaled1),
> +			  CURVE2_MULTIPLIER);
>  
>  	*ana_cp_int = max(1, min(ana_cp_int_temp, 127));

Unrelated to this patch, but this should be:

	*ana_cp_int = clamp(ana_cp_int_temp, 1, 127);

There's a similar issue with ana_cp_prop also in the file.


BR,
Jani.


-- 
Jani Nikula, Intel