Re: [PATCH 02/13] drm/connector: Add helper to check if a mode requires scrambling

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Maxime Ripard <maxime@cerno.tech>, Emma Anholt <emma@anholt.net>,
	Dom Cobley <dom@raspberrypi.com>,
	Tim Gover <tim.gover@raspberrypi.com>,
	Dave Stevenson <dave.stevenson@raspberrypi.com>,
	David Airlie <airlied@linux.ie>,
	dri-devel@lists.freedesktop.org,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	linux-tegra@vger.kernel.org,
	Daniel Vetter <daniel.vetter@intel.com>,
	Phil Elwell <phil@raspberrypi.com>
Subject: Re: [PATCH 02/13] drm/connector: Add helper to check if a mode requires scrambling
Date: Fri, 5 Nov 2021 20:14:04 +0200	[thread overview]
Message-ID: <YYV0bArjRbvYJ2K2@intel.com> (raw)
In-Reply-To: <YYVxtp9PZiR3ZOjp@phenom.ffwll.local>

On Fri, Nov 05, 2021 at 07:02:30PM +0100, Daniel Vetter wrote:
> On Thu, Nov 04, 2021 at 05:37:21PM +0200, Ville Syrjälä wrote:
> > On Tue, Nov 02, 2021 at 03:59:33PM +0100, Maxime Ripard wrote:
> > > --- a/include/drm/drm_modes.h
> > > +++ b/include/drm/drm_modes.h
> > > @@ -424,6 +424,21 @@ static inline bool drm_mode_is_stereo(const struct drm_display_mode *mode)
> > >  	return mode->flags & DRM_MODE_FLAG_3D_MASK;
> > >  }
> > >  
> > > +/**
> > > + * drm_mode_hdmi_requires_scrambling - Checks if a mode requires HDMI Scrambling
> > > + * @mode: DRM display mode
> > > + *
> > > + * Checks if a given display mode requires the scrambling to be enabled.
> > > + *
> > > + * Returns:
> > > + * A boolean stating whether it's required or not.
> > > + */
> > > +static inline bool
> > > +drm_mode_hdmi_requires_scrambling(const struct drm_display_mode *mode)
> > > +{
> > > +	return mode->clock > DRM_HDMI_14_MAX_TMDS_CLK_KHZ;
> > > +}
> > 
> > That's only correct for 8bpc. The actual limit is on the TMDS clock (or
> > rather TMDS character rate as HDMI 2.0 calls it due to the 1/1 vs. 1/4
> > magic for the actually transmitted TMDS clock).
> 
> Yeah we might need to add the bus format for the cable here too, to make
> this complete.

I don't think we have a usable thing for that on the drm level, so
would need to invent something. Oh, and the mode->clock vs. 
mode->crtc_clock funny business also limits the usability of this
approach. So probably just easiest to pass in the the driver calculated
TMDS clock instead.

-- 
Ville Syrjälä
Intel