[PATCH v6 17/36] drm/connector: hdmi: Add HDMI compute clock helper

[Maxime Ripard <mripard@kernel.org>]

A lot of HDMI drivers have some variation of the formula to calculate
the TMDS character rate from a mode, but few of them actually take all
parameters into account.

Let's create a helper to provide that rate taking all parameters into
account.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
---
 drivers/gpu/drm/drm_connector.c | 71 +++++++++++++++++++++++++++++++++++++++++
 include/drm/drm_connector.h     |  5 +++
 2 files changed, 76 insertions(+)

diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index b895bea667f7..332477c6a987 100644
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -2982,6 +2982,77 @@ void drm_connector_update_privacy_screen(const struct drm_connector_state *conne
 }
 EXPORT_SYMBOL(drm_connector_update_privacy_screen);
 
+/**
+ * drm_connector_hdmi_compute_mode_clock() - Computes the TMDS Character Rate
+ * @mode: Display mode to compute the clock for
+ * @bpc: Bits per character
+ * @fmt: Output Pixel Format used
+ *
+ * Returns the TMDS Character Rate for a given mode, bpc count and output format.
+ *
+ * RETURNS:
+ * The TMDS Character Rate, in Hertz, or 0 on error.
+ */
+unsigned long long
+drm_connector_hdmi_compute_mode_clock(const struct drm_display_mode *mode,
+				      unsigned int bpc,
+				      enum hdmi_colorspace fmt)
+{
+	unsigned long long clock = mode->clock * 1000ULL;
+	unsigned int vic = drm_match_cea_mode(mode);
+
+	/*
+	 * CTA-861-G Spec, section 5.4 - Color Coding and Quantization
+	 * mandates that VIC 1 always uses 8 bpc.
+	 */
+	if (vic == 1 && bpc != 8)
+		return 0;
+
+	/*
+	 * HDMI 2.0 Spec, section 7.1 - YCbCr 4:2:0 Pixel Encoding
+	 * specifies that YUV420 encoding is only available for those
+	 * VICs.
+	 */
+	if (fmt == HDMI_COLORSPACE_YUV420 &&
+	    !(vic == 96 || vic == 97 || vic == 101 ||
+	      vic == 102 || vic == 106 || vic == 107))
+		return 0;
+
+	if (fmt == HDMI_COLORSPACE_YUV422) {
+		/*
+		 * HDMI 1.4b Spec, section 6.2.3 - Pixel Encoding Requirements
+		 * specifies that YUV422 is 36-bit only.
+		 */
+		if (bpc != 12)
+			return 0;
+
+		/*
+		 * HDMI 1.0 Spec, section 6.5 - Pixel Encoding
+		 * specifies that YUV422 requires two 12-bits components per
+		 * pixel clock, which is equivalent in our calculation to three
+		 * 8-bits components
+		 */
+		bpc = 8;
+	}
+
+	/*
+	 * HDMI 2.0 Spec, Section 7.1 - YCbCr 4:2:0 Pixel Encoding
+	 * specifies that YUV420 encoding is carried at a TMDS Character Rate
+	 * equal to half the pixel clock rate.
+	 */
+	if (fmt == HDMI_COLORSPACE_YUV420)
+		clock = clock / 2;
+
+	if (mode->flags & DRM_MODE_FLAG_DBLCLK)
+		clock = clock * 2;
+
+	clock = clock * bpc;
+	do_div(clock, 8);
+
+	return clock;
+}
+EXPORT_SYMBOL(drm_connector_hdmi_compute_mode_clock);
+
 int drm_connector_set_obj_prop(struct drm_mode_object *obj,
 				    struct drm_property *property,
 				    uint64_t value)
diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
index 74db5ce47e01..0cc5a8732664 100644
--- a/include/drm/drm_connector.h
+++ b/include/drm/drm_connector.h
@@ -38,6 +38,7 @@ struct drm_connector_helper_funcs;
 struct drm_modeset_acquire_ctx;
 struct drm_device;
 struct drm_crtc;
+struct drm_display_mode;
 struct drm_encoder;
 struct drm_panel;
 struct drm_property;
@@ -2136,6 +2137,10 @@ void drm_connector_attach_privacy_screen_properties(struct drm_connector *conn);
 void drm_connector_attach_privacy_screen_provider(
 	struct drm_connector *connector, struct drm_privacy_screen *priv);
 void drm_connector_update_privacy_screen(const struct drm_connector_state *connector_state);
+unsigned long long
+drm_connector_hdmi_compute_mode_clock(const struct drm_display_mode *mode,
+				      unsigned int bpc,
+				      enum hdmi_colorspace fmt);
 
 /**
  * struct drm_tile_group - Tile group metadata

-- 
2.43.0