Re: [PATCH] video: hdmi: request underscan by default

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Daniel Drake <drake@endlessm.com>
Cc: mjg59@srcf.ucam.org, treding@nvidia.com, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH] video: hdmi: request underscan by default
Date: Tue, 25 Feb 2014 12:33:21 +0200	[thread overview]
Message-ID: <20140225103321.GV3852@intel.com> (raw)
In-Reply-To: <1393280404-9066-1-git-send-email-drake@endlessm.com>

On Mon, Feb 24, 2014 at 04:20:04PM -0600, Daniel Drake wrote:
> Working with HDMI TVs is a real pain as they tend to overscan by
> default, meaning that the pixels around the edge of the framebuffer
> are not displayed. This is well explained here:
> http://mjg59.dreamwidth.org/8705.html
> 
> There is a bit in the HDMI info frame that can request that the
> remote display shows the full pixel data ("underscan"). For the
> remote display, the HDMI spec states that this is optional - it
> doesn't have to listen. That means that most TVs will probably ignore
> this.
> 
> But, maybe there are a handful of TVs for which this would help
> the situation. As we live in a digital world, ask the remote
> display not to overscan by default.
> 
> Signed-off-by: Daniel Drake <drake@endlessm.com>

Yeah, underscan would seem the saner default value. CEA-861 says that
the source should always set the S bits appropriately if it knows what
to put there, even if the sink claims that it won't respect those bits.
So we're not violating the spec by doing this.

I'm thinking we should also add a connector property to allow userspace
to choose the scan behaviour. Or perhaps some driver already has such a
property?

I'm not sure we want to put this into hdmi.c though. Maybe better to
avoid such policy in the infoframe code itself, and instead stick this
into drm_hdmi_avi_infoframe_from_display_mode()?

> ---
>  drivers/video/hdmi.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/video/hdmi.c b/drivers/video/hdmi.c
> index 9e758a8..6c2d924 100644
> --- a/drivers/video/hdmi.c
> +++ b/drivers/video/hdmi.c
> @@ -54,6 +54,7 @@ int hdmi_avi_infoframe_init(struct hdmi_avi_infoframe *frame)
>  	frame->type = HDMI_INFOFRAME_TYPE_AVI;
>  	frame->version = 2;
>  	frame->length = HDMI_AVI_INFOFRAME_SIZE;
> +	frame->scan_mode = HDMI_SCAN_MODE_UNDERSCAN;
>  
>  	return 0;
>  }
> -- 
> 1.8.3.2
> 
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-- 
Ville Syrjälä
Intel OTC