[PATCH v8 03/12] drm: Helper to read max clock rate

[Kahola, Mika]

> -----Original Message-----
> From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> Sent: Thursday, September 8, 2016 4:02 PM
> To: Kahola, Mika <mika.kahola at intel.com>
> Cc: intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org;
> jim.bride at linux.intel.com; daniel.vetter at ffwll.ch
> Subject: Re: [PATCH v8 03/12] drm: Helper to read max clock rate
> 
> On Wed, Aug 17, 2016 at 01:49:39PM +0300, Mika Kahola wrote:
> > Helper routine to read out maximum supported pixel rate for
> > DisplayPort legay VGA converter or TMDS clock rate for other digital
> > legacy converters. The helper returns clock rate in kHz.
> >
> > v2: Return early if detailed port cap info is not available.
> >     Replace if-else ladder with switch-case (Ville)
> >
> > Reviewed-by: Jim Bride <jim.bride at linux.intel.com>
> > Signed-off-by: Mika Kahola <mika.kahola at intel.com>
> > ---
> >  drivers/gpu/drm/drm_dp_helper.c | 33
> +++++++++++++++++++++++++++++++++
> >  include/drm/drm_dp_helper.h     |  2 ++
> >  2 files changed, 35 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c
> > b/drivers/gpu/drm/drm_dp_helper.c index 031c4d3..7497490 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -439,6 +439,39 @@ int drm_dp_link_configure(struct drm_dp_aux
> *aux,
> > struct drm_dp_link *link)  }  EXPORT_SYMBOL(drm_dp_link_configure);
> >
> > +/**
> > + * drm_dp_downstream_max_clock() - extract branch device max
> > + *                                 pixel rate for legacy VGA
> > + *                                 converter or max TMDS clock
> > + *                                 rate for others
> > + * @dpcd: DisplayPort configuration data
> > + * @port_cap: port capabilities
> > + *
> > + * Returns max clock in kHz on success or 0 if max clock not defined
> > +*/ int drm_dp_downstream_max_clock(const u8
> > +dpcd[DP_RECEIVER_CAP_SIZE],
> > +				const u8 port_cap[4])
> > +{
> > +	int type = port_cap[0] & DP_DS_PORT_TYPE_MASK;
> > +	bool detailed_cap_info = dpcd[DP_DOWNSTREAMPORT_PRESENT] &
> > +		DP_DETAILED_CAP_INFO_AVAILABLE;
> > +
> > +	if (!detailed_cap_info)
> > +		return 0;
> > +
> > +	switch (type) {
> > +	case DP_DS_PORT_TYPE_VGA:
> > +		return port_cap[1] * 8 * 1000;
> > +	case DP_DS_PORT_TYPE_DVI:
> > +	case DP_DS_PORT_TYPE_HDMI:
> > +	case DP_DS_PORT_TYPE_DP_DUALMODE:
> > +		return port_cap[1] * 2500;
> 
> The spec says that if the detailed_cap_info==0, then DVI/HDMI/DP++ must
> support at least 165 MHz TMDS clock. I was thinking we might want to limit
> things to 165 in that case to guarantee that we advertize only modes
> guaranteed to work.
That is a valid point. What I had in mind was by returning 0 from this function it is indicated that the value is not found. With the remaining patches that call this function the check is in place so we can skip the step if explicit clock rate is not found. 

> 
> > +	default:
> > +		return 0;
> > +	}
> > +}
> > +EXPORT_SYMBOL(drm_dp_downstream_max_clock);
> > +
> >  /*
> >   * I2C-over-AUX implementation
> >   */
> > diff --git a/include/drm/drm_dp_helper.h
> b/include/drm/drm_dp_helper.h
> > index 0d84046..60dd9dc 100644
> > --- a/include/drm/drm_dp_helper.h
> > +++ b/include/drm/drm_dp_helper.h
> > @@ -815,6 +815,8 @@ int drm_dp_link_probe(struct drm_dp_aux *aux,
> > struct drm_dp_link *link);  int drm_dp_link_power_up(struct drm_dp_aux
> > *aux, struct drm_dp_link *link);  int drm_dp_link_power_down(struct
> > drm_dp_aux *aux, struct drm_dp_link *link);  int
> > drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link
> > *link);
> > +int drm_dp_downstream_max_clock(const u8
> dpcd[DP_RECEIVER_CAP_SIZE],
> > +				const u8 port_cap[4]);
> >
> >  void drm_dp_aux_init(struct drm_dp_aux *aux);  int
> > drm_dp_aux_register(struct drm_dp_aux *aux);
> > --
> > 1.9.1
> 
> --
> Ville Syrjälä
> Intel OTC