[PATCH v14 09/28] drm/display: hdmi: Add HDMI compute clock helper

[Maxime Ripard]

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.steven...@raspberrypi.com>
Signed-off-by: Maxime Ripard <mrip...@kernel.org>
---
 drivers/gpu/drm/display/drm_hdmi_helper.c | 57 +++++++++++++++++++++++++++++++
 include/drm/display/drm_hdmi_helper.h     |  4 +++
 2 files changed, 61 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_hdmi_helper.c 
b/drivers/gpu/drm/display/drm_hdmi_helper.c
index faf5e9efa7d3..679eb3e81393 100644
--- a/drivers/gpu/drm/display/drm_hdmi_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_helper.c
@@ -193,5 +193,62 @@ void drm_hdmi_avi_infoframe_content_type(struct 
hdmi_avi_infoframe *frame,
        }
 
        frame->itc = conn_state->content_type != DRM_MODE_CONTENT_TYPE_NO_DATA;
 }
 EXPORT_SYMBOL(drm_hdmi_avi_infoframe_content_type);
+
+/**
+ * drm_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_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;
+
+       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;
+
+       return DIV_ROUND_CLOSEST_ULL(clock * bpc, 8);
+}
+EXPORT_SYMBOL(drm_hdmi_compute_mode_clock);
diff --git a/include/drm/display/drm_hdmi_helper.h 
b/include/drm/display/drm_hdmi_helper.h
index 76d234826e22..57e3b18c15ec 100644
--- a/include/drm/display/drm_hdmi_helper.h
+++ b/include/drm/display/drm_hdmi_helper.h
@@ -22,6 +22,10 @@ drm_hdmi_infoframe_set_hdr_metadata(struct 
hdmi_drm_infoframe *frame,
                                    const struct drm_connector_state 
*conn_state);
 
 void drm_hdmi_avi_infoframe_content_type(struct hdmi_avi_infoframe *frame,
                                         const struct drm_connector_state 
*conn_state);
 
+unsigned long long
+drm_hdmi_compute_mode_clock(const struct drm_display_mode *mode,
+                           unsigned int bpc, enum hdmi_colorspace fmt);
+
 #endif

-- 
2.45.0