Re: [PATCH v16 9/9] drm/i915: Compute CMRR and calculate vtotal

[Nathan Chancellor <nathan@kernel.org>]

Hi Mitul,

On Mon, Jun 10, 2024 at 12:52:02PM +0530, Mitul Golani wrote:
...
> diff --git a/drivers/gpu/drm/i915/display/intel_vrr.c b/drivers/gpu/drm/i915/display/intel_vrr.c
> index 4ad99a54aa83..05f67dc9d98d 100644
> --- a/drivers/gpu/drm/i915/display/intel_vrr.c
> +++ b/drivers/gpu/drm/i915/display/intel_vrr.c
> @@ -12,6 +12,9 @@
>  #include "intel_vrr_regs.h"
>  #include "intel_dp.h"
>  
> +#define FIXED_POINT_PRECISION		100
> +#define CMRR_PRECISION_TOLERANCE	10
> +
>  bool intel_vrr_is_capable(struct intel_connector *connector)
>  {
>  	const struct drm_display_info *info = &connector->base.display_info;
> @@ -107,6 +110,52 @@ int intel_vrr_vmax_vblank_start(const struct intel_crtc_state *crtc_state)
>  	return crtc_state->vrr.vmax - intel_vrr_vblank_exit_length(crtc_state);
>  }
>  
> +static bool
> +is_cmrr_frac_required(struct intel_crtc_state *crtc_state)
> +{
> +	int calculated_refresh_k, actual_refresh_k, pixel_clock_per_line;
> +	struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
> +	struct drm_i915_private *i915 = to_i915(crtc_state->uapi.crtc->dev);
> +
> +	if (!HAS_CMRR(i915))
> +		return false;
> +
> +	actual_refresh_k =
> +		drm_mode_vrefresh(adjusted_mode) * FIXED_POINT_PRECISION;
> +	pixel_clock_per_line =
> +		adjusted_mode->crtc_clock * 1000 / adjusted_mode->crtc_htotal;
> +	calculated_refresh_k =
> +		pixel_clock_per_line * FIXED_POINT_PRECISION / adjusted_mode->crtc_vtotal;
> +
> +	if ((actual_refresh_k - calculated_refresh_k) < CMRR_PRECISION_TOLERANCE)
> +		return false;
> +
> +	return true;
> +}
> +
> +static unsigned int
> +cmrr_get_vtotal(struct intel_crtc_state *crtc_state, bool video_mode_required)
> +{
> +	int multiplier_m = 1, multiplier_n = 1, vtotal, desired_refresh_rate;
> +	long long adjusted_pixel_rate;
> +	struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
> +
> +	desired_refresh_rate = drm_mode_vrefresh(adjusted_mode);
> +
> +	if (video_mode_required) {
> +		multiplier_m = 1001;
> +		multiplier_n = 1000;
> +	}
> +
> +	crtc_state->cmrr.cmrr_n =
> +		desired_refresh_rate * adjusted_mode->crtc_htotal * multiplier_n;
> +	vtotal = (adjusted_mode->crtc_clock * 1000 * multiplier_n) / crtc_state->cmrr.cmrr_n;
> +	adjusted_pixel_rate = adjusted_mode->crtc_clock * 1000 * multiplier_m;
> +	crtc_state->cmrr.cmrr_m = do_div(adjusted_pixel_rate, crtc_state->cmrr.cmrr_n);
> +
> +	return vtotal;
> +}

This change is now in -next as commit 1676ecd303ac ("drm/i915: Compute
CMRR and calculate vtotal"), where it breaks the xe build for 32-bit
platforms with:

  $ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- allmodconfig drivers/gpu/drm/xe/i915-display/intel_vrr.o
  In file included from arch/arm/include/asm/div64.h:107,
                   from include/linux/math.h:6,
                   from include/linux/kernel.h:27,
                   from include/linux/cpumask.h:11,
                   from include/linux/smp.h:13,
                   from include/linux/lockdep.h:14,
                   from include/linux/spinlock.h:63,
                   from include/linux/kref.h:16,
                   from include/drm/drm_device.h:5,
                   from include/drm/drm_drv.h:35,
                   from drivers/gpu/drm/xe/compat-i915-headers/i915_drv.h:13,
                   from drivers/gpu/drm/i915/display/intel_vrr.c:7:
  drivers/gpu/drm/i915/display/intel_vrr.c: In function 'cmrr_get_vtotal':
  include/asm-generic/div64.h:222:35: error: comparison of distinct pointer types lacks a cast [-Werror]
    222 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
        |                                   ^~
  drivers/gpu/drm/i915/display/intel_vrr.c:155:35: note: in expansion of macro 'do_div'
    155 |         crtc_state->cmrr.cmrr_m = do_div(adjusted_pixel_rate, crtc_state->cmrr.cmrr_n);
        |                                   ^~~~~~
  cc1: all warnings being treated as errors

Also, is do_div() correct here? It is different from the other div_()
macros in that the "return value" is the remainder, not the result of
the division.

Cheers,
Nathan