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

[Dmitry Baryshkov]

On Tue, May 21, 2024 at 12:13:42PM +0200, Maxime Ripard wrote:
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Maxime Ripard <mripard@xxxxxxxxxx>
> ---
>  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

This is probably 6.2.4, but it doesn't specify that it is 36-bit _only_.

> +		 * specifies that YUV422 is 36-bit only.
> +		 */
> +		if (bpc != 12)
> +			return 0;

6.5.1 allows using less than 12 bits (If fewer than 12 bits are
used...). So I think it would be more correct to allow less than 12 bpc,
but we'd still have to use 8 for the matter of the calculating the
clock.

> +
> +		/*
> +		 * 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
> 

-- 
With best wishes
Dmitry