drm/display: hdmi: Add HDMI compute clock helper

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>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527-kms-hdmi-connector-state-v15-9-c5af16c3aae2@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>

diff --git a/drivers/gpu/drm/display/drm_hdmi_helper.c b/drivers/gpu/drm/display/drm_hdmi_helper.c
index faf5e9efa7d3380b0c76603ab86544a360fe83d5..74dd4d01dd9bb2c9e69ec1c60b0056bd69417e8a 100644 (file)
--- a/drivers/gpu/drm/display/drm_hdmi_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_helper.c
@@ -195,3 +195,64 @@ 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 1.0 Spec, section 6.5 - Pixel Encoding states that
+                * YUV422 sends 24 bits over three channels, with Cb and Cr
+                * components being sent on odd and even pixels, respectively.
+                *
+                * If fewer than 12 bpc are sent, data are left justified.
+                */
+               if (bpc > 12)
+                       return 0;
+
+               /*
+                * HDMI 1.0 Spec, section 6.5 - Pixel Encoding
+                * specifies that YUV422 sends two 12-bits components over
+                * three TMDS channels per pixel clock, which is equivalent to
+                * three 8-bits components over three channels used by RGB as
+                * far as the clock rate goes.
+                */
+               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 76d234826e22b4b34662df11e3b2d94c68e15dcb..57e3b18c15ec79636d89267aba0e88f434c5d4db 100644 (file)
--- a/include/drm/display/drm_hdmi_helper.h
+++ b/include/drm/display/drm_hdmi_helper.h
@@ -24,4 +24,8 @@ drm_hdmi_infoframe_set_hdr_metadata(struct hdmi_drm_infoframe *frame,
 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