drm/i915: Introduce intel_scanlines_to_usecs()

Introduce intel_scanlines_to_usecs() as a counterpart to
intel_usecs_to_scanlines().

We'll have some use for this in DSB code as we want to do
relative scanline waits to evade the delayed vblank, but
unfortunately DSB can't do relative scanline waits (only
absolute). So we'll instead convert the relative scanline
count to usec and do a relative usec wait instead (which the
DSB knows how to do).

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930170415.23841-9-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_crtc.c

@@ -483,6 +483,17 @@ int intel_usecs_to_scanlines(const struct drm_display_mode *adjusted_mode,
 				1000 * adjusted_mode->crtc_htotal);
 }
 
+int intel_scanlines_to_usecs(const struct drm_display_mode *adjusted_mode,
+			     int scanlines)
+{
+	/* paranoia */
+	if (!adjusted_mode->crtc_clock)
+		return 1;
+
+	return DIV_ROUND_UP_ULL(mul_u32_u32(scanlines, adjusted_mode->crtc_htotal * 1000),
+				adjusted_mode->crtc_clock);
+}
+
 /**
  * intel_pipe_update_start() - start update of a set of display registers
  * @state: the atomic state

drivers/gpu/drm/i915/display/intel_crtc.h

@@ -31,6 +31,8 @@ struct intel_display;
 
 int intel_usecs_to_scanlines(const struct drm_display_mode *adjusted_mode,
 			     int usecs);
+int intel_scanlines_to_usecs(const struct drm_display_mode *adjusted_mode,
+			     int scanlines);
 void intel_crtc_arm_vblank_event(struct intel_crtc_state *crtc_state);
 u32 intel_crtc_max_vblank_count(const struct intel_crtc_state *crtc_state);
 int intel_crtc_init(struct drm_i915_private *dev_priv, enum pipe pipe);