Received-SPF: pass (google.com: domain of linux-kernel+bounces-146905-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Date: Tue, 16 Apr 2024 16:44:14 +0300
From: Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>
To: Maxime Ripard <mripard@kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	Jonathan Corbet <corbet@lwn.net>, Sandy Huang <hjc@rock-chips.com>,
	Heiko =?iso-8859-1?Q?St=FCbner?= <heiko@sntech.de>,
	Chen-Yu Tsai <wens@csie.org>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	Samuel Holland <samuel@sholland.org>,
	Hans Verkuil <hverkuil@xs4all.nl>,
	Sebastian Wick <sebastian.wick@redhat.com>,
	dri-devel@lists.freedesktop.org,
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
	linux-rockchip@lists.infradead.org, linux-sunxi@lists.linux.dev,
	Dave Stevenson <dave.stevenson@raspberrypi.com>
Subject: Re: [PATCH v11 09/28] drm/display: hdmi: Add HDMI compute clock
 helper
Message-ID: <Zh6Ars8z1ESz-LQO@intel.com>
References: <20240326-kms-hdmi-connector-state-v11-0-c5680ffcf261@kernel.org>
 <20240326-kms-hdmi-connector-state-v11-9-c5680ffcf261@kernel.org>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20240326-kms-hdmi-connector-state-v11-9-c5680ffcf261@kernel.org>

On Tue, Mar 26, 2024 at 04:40:13PM +0100, 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@raspberrypi.com>
> Signed-off-by: Maxime Ripard <mripard@kernel.org>
> ---
>  drivers/gpu/drm/display/drm_hdmi_helper.c | 70 +++++++++++++++++++++++++++++++
>  include/drm/display/drm_hdmi_helper.h     |  4 ++
>  2 files changed, 74 insertions(+)
> 
> diff --git a/drivers/gpu/drm/display/drm_hdmi_helper.c b/drivers/gpu/drm/display/drm_hdmi_helper.c
> index faf5e9efa7d3..2518dd1a07e7 100644
> --- a/drivers/gpu/drm/display/drm_hdmi_helper.c
> +++ b/drivers/gpu/drm/display/drm_hdmi_helper.c
> @@ -193,5 +193,75 @@ 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.

Everything generally uses kHz. Sticking to common units
would be better.

> + */
> +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;
> +
> +	/*
> +	 * HDMI 2.0 Spec, section 7.1 - YCbCr 4:2:0 Pixel Encoding
> +	 * specifies that YUV420 encoding is only available for those
> +	 * VICs.
> +	 */
> +	if (fmt == HDMI_COLORSPACE_YUV420 &&
> +	    !(vic == 96 || vic == 97 || vic == 101 ||
> +	      vic == 102 || vic == 106 || vic == 107))
> +		return 0;

I believe that is already outdated. I would just rip this out since the 
sink is anyway required to declare for which timings it will support
4:2:0 via the Y420CMDB/VDB data blocks (see
drm_mode_is_420_{only,also}().

> +
> +	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;
> +
> +	clock = clock * bpc;
> +	do_div(clock, 8);

IMO one shouldn't use bare do_div(). There are
more sensible wrappers for it.

In this case I would use DIV_ROUND_CLOSEST_ULL().

Although the 64bit math is not even required if you
just stick to kHz like everyone else.

> +
> +	return clock;
> +}
> +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
> 
> -- 
> 2.44.0

-- 
Ville Syrj?l?
Intel