linux-kernel - Re: [PATCH RFC v2 07/37] drm/connector: hdmi: Add HDMI compute clock helper

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Date:   Wed, 20 Sep 2023 19:15:37 +0200
From:   Jernej Škrabec <jernej.skrabec@...il.com>
To:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, Emma Anholt <emma@...olt.net>,
        Jonathan Corbet <corbet@....net>,
        Sandy Huang <hjc@...k-chips.com>,
        Heiko Stübner <heiko@...ech.de>,
        Chen-Yu Tsai <wens@...e.org>,
        Samuel Holland <samuel@...lland.org>,
        Maxime Ripard <mripard@...nel.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        Maxime Ripard <mripard@...nel.org>
Subject: Re: [PATCH RFC v2 07/37] drm/connector: hdmi: Add HDMI compute clock helper

Hi Maxime!

Dne sreda, 20. september 2023 ob 16:35:22 CEST je Maxime Ripard napisal(a):
> 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.
> 
> Signed-off-by: Maxime Ripard <mripard@...nel.org>
> ---
>  drivers/gpu/drm/drm_connector.c | 31 +++++++++++++++++++++++++++++++
>  include/drm/drm_connector.h     |  5 +++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index b01cb4783ea6..4c1af97971bb 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -2944,6 +2944,37 @@ 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
> + */
> +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;
> +
> +	if (mode->flags & DRM_MODE_FLAG_DBLCLK)
> +		clock = clock * 2;
> +
> +	if (fmt == HDMI_COLORSPACE_YUV422)
> +		bpc = 8;

I think you're missing YUV420 handling, which needs half of clock speed. But
this is a thing of HDMI2.

Best regards,
Jernej

> +
> +	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 a66cb4e35d7b..d74e9c64ee88 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;
> @@ -2115,6 +2116,10 @@ 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
> 
>