Re: [RFC 13/20] drm/i915/dp: Extract drm_dp_downstream_read_info()

[Lyude Paul]

On Wed, 2020-08-19 at 11:15 -0400, Sean Paul wrote:
> On Tue, Aug 11, 2020 at 04:04:50PM -0400, Lyude Paul wrote:
> > We're going to be doing the same probing process in nouveau for
> > determining downstream DP port capabilities, so let's deduplicate the
> > work by moving i915's code for handling this into a shared helper:
> > drm_dp_downstream_read_info().
> > 
> > Note that when we do this, we also do make some functional changes while
> > we're at it:
> > * We always clear the downstream port info before trying to read it,
> >   just to make things easier for the caller
> > * We skip reading downstream port info if the DPCD indicates that we
> >   don't support downstream port info
> > * We only read as many bytes as needed for the reported number of
> >   downstream ports, no sense in reading the whole thing every time
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/drm_dp_helper.c         | 32 +++++++++++++++++++++++++
> >  drivers/gpu/drm/i915/display/intel_dp.c | 14 ++---------
> >  include/drm/drm_dp_helper.h             |  3 +++
> >  3 files changed, 37 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c
> > b/drivers/gpu/drm/drm_dp_helper.c
> > index 4c21cf69dad5a..9703b33599c3b 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -423,6 +423,38 @@ bool drm_dp_send_real_edid_checksum(struct drm_dp_aux
> > *aux,
> >  }
> >  EXPORT_SYMBOL(drm_dp_send_real_edid_checksum);
> >  
> > +/**
> > + * drm_dp_downstream_read_info() - read DPCD downstream port info if
> > available
> > + * @aux: DisplayPort AUX channel
> > + * @dpcd: A cached copy of the port's DPCD
> > + * @downstream_ports: buffer to store the downstream port info in
> > + *
> > + * Returns: 0 if either the downstream port info was read successfully or
> > + * there was no downstream info to read, or a negative error code
> > otherwise.
> > + */
> > +int drm_dp_downstream_read_info(struct drm_dp_aux *aux,
> > +				const u8 dpcd[DP_RECEIVER_CAP_SIZE],
> > +				u8 downstream_ports[DP_MAX_DOWNSTREAM_PORTS])
> > +{
> > +	int ret;
> > +	u8 len;
> > +
> > +	memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
> > +
> > +	/* No downstream info to read */
> > +	if (!drm_dp_is_branch(dpcd) ||
> > +	    dpcd[DP_DPCD_REV] < DP_DPCD_REV_10 ||
> > +	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
> > +		return 0;
> > +
> > +	len = (dpcd[DP_DOWN_STREAM_PORT_COUNT] & DP_PORT_COUNT_MASK) * 4;
> 
> I'm having a hard time rationalizing DP_MAX_DOWNSTREAM_PORTS being 16, but
> only
> having 4 ports worth of data in the DP_DOWNSTREAM_PORT_* registers. Do you
> know
> what's supposed to happen if dpcd[DP_DOWN_STREAM_PORT_COUNT] is > 4?

I thought I had addressed this bit but I guess I missed some parts here.

So; there's actually two different possible lengths for how long each downstream
port's capabilities can be: 1 byte long (if DETAILED_CAP_INFO_AVAILABLE in the
DOWN_STREAM_PORT_PRESENT is 0, e.g. 005h bit 4), and 4 bytes long if that bit is
1. What's unfortunately not as clear, is whether or not 1 byte long cap fields
mean "each port has four bytes, but only one byte is used" or "each port truly
only has one byte". The DP spec says:

   DFPX_CAP
   1 byte/DFP
   X = DFP number. Port_x capability is stored at the DFP number’s address plus
   80h

Which at first seems to imply that each cap is at 80 + X, e.g. only one byte
long. However, the explanation for when DETAILED_CAP_INFO_AVAILABLE == 1 says
almost the same thing:

   DFPX_CAP
   X = DFP number. Port_x capability is stored at the DFP number’s address plus
   80h.

Although right above that unlike the previous section, they mention that DFP0
goes from 80-83, DFP1 84-87, etc...

Not entirely sure what to think here since I don't really have any devices (nor
do I think I've ever seen any) that have more then one DFP. As well, for the
case where we have multiple DFPs (which according to the spec appears to only be
something we need to worry about for SST) they're not really explicit on how to
combine the downstream capabilities from each DFP. My guess is maybe you
determine the max downstream clock and bpp from the lowest clock limits
advertised across each port?
(if you have a DP device with multiple DFPs and can test this, that would rock
:), but I have a feeling you probably don't have one either)
> 
> Sean
> 
> > +	ret = drm_dp_dpcd_read(aux, DP_DOWNSTREAM_PORT_0, downstream_ports,
> > +			       len);
> > +
> > +	return ret == len ? 0 : -EIO;
> > +}
> > +EXPORT_SYMBOL(drm_dp_downstream_read_info);
> > +
> >  /**
> >   * drm_dp_downstream_max_clock() - extract branch device max
> >   *                                 pixel rate for legacy VGA
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c
> > b/drivers/gpu/drm/i915/display/intel_dp.c
> > index 1e29d3a012856..984e49194ca31 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -4685,18 +4685,8 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
> >  			return false;
> >  	}
> >  
> > -	if (!drm_dp_is_branch(intel_dp->dpcd))
> > -		return true; /* native DP sink */
> > -
> > -	if (intel_dp->dpcd[DP_DPCD_REV] == 0x10)
> > -		return true; /* no per-port downstream info */
> > -
> > -	if (drm_dp_dpcd_read(&intel_dp->aux, DP_DOWNSTREAM_PORT_0,
> > -			     intel_dp->downstream_ports,
> > -			     DP_MAX_DOWNSTREAM_PORTS) < 0)
> > -		return false; /* downstream port status fetch failed */
> > -
> > -	return true;
> > +	return drm_dp_downstream_read_info(&intel_dp->aux, intel_dp->dpcd,
> > +					   intel_dp->downstream_ports) == 0;
> >  }
> >  
> >  static bool
> > diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
> > index 5c28199248626..1349f16564ace 100644
> > --- a/include/drm/drm_dp_helper.h
> > +++ b/include/drm/drm_dp_helper.h
> > @@ -1613,6 +1613,9 @@ int drm_dp_dpcd_read_link_status(struct drm_dp_aux
> > *aux,
> >  bool drm_dp_send_real_edid_checksum(struct drm_dp_aux *aux,
> >  				    u8 real_edid_checksum);
> >  
> > +int drm_dp_downstream_read_info(struct drm_dp_aux *aux,
> > +				const u8 dpcd[DP_RECEIVER_CAP_SIZE],
> > +				u8 downstream_ports[DP_MAX_DOWNSTREAM_PORTS]);
> >  int drm_dp_downstream_max_clock(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
> >  				const u8 port_cap[4]);
> >  int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
> > -- 
> > 2.26.2
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
-- 
Sincerely,
      Lyude Paul (she/her)
      Software Engineer at Red Hat