[PATCH v13 09/28] drm/display: hdmi: Add HDMI compute clock helper

Maxime Ripard
2024-05-07T14:31:12Z

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/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