Message-ID: <597b83ace909f97bfefbe15ffbb0370c2101ff0f.camel@redhat.com>
Date:   Fri, 21 Aug 2020 13:43:39 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     imre.deak@...el.com
Cc:     Sean Paul <sean@...rly.run>,
        Ville Syrjala <ville.syrjala@...ux.intel.com>,
        nouveau@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>,
        Manasi Navare <manasi.d.navare@...el.com>,
        Uma Shankar <uma.shankar@...el.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        José Roberto de Souza 
        <jose.souza@...el.com>, Wambui Karuga <wambui.karugax@...il.com>
Subject: Re: [RFC 13/20] drm/i915/dp: Extract drm_dp_downstream_read_info()

On Fri, 2020-08-21 at 01:37 +0300, Imre Deak wrote:
> On Wed, Aug 19, 2020 at 05:34:15PM -0400, Lyude Paul wrote:
> > (adding Ville and Imre to the cc here, they might be interested to know
> > about
> > this, comments down below)
> > 
> > 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@...hat.com>
> > > > ---
> > > >  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?
> > > 
> > ok!! Taking a lesson from our available_pbn/full_pbn confusion in the past,
> > I
> > squinted very hard at the specification and eventually found something that
> > I
> > think clears this up. Surprise - we definitely had this implemented
> > incorrectly
> > in i915
> 
> To me it looks correct, only DFP0's cap info is used, by also handling
> the DP_DETAILED_CAP_INFO_AVAILABLE=0/1 cases.
Ended up realizing this right after I sent this version of the RFC - yeah, it
definitely shouldn't be causing any real problems as of now

> 
> The wording is a bit unclear, but as I understand the Standard only
> calls for the above:
> 
> """
> A DP upstream device shall read the capability from DPCD Addresses 00080h
> through 00083h. A DP Branch device with multiple DFPs shall report the
> detailed
> capability information of the lowest DFP number to which a downstream device
> is connected, consistent with the DisplayID or legacy EDID access routing
> policy
> of an SST-only DP Branch device as described in Section 2.1.4.1.
> """

So-I saw this too, but notice the use of the language "A /DP Branch/ device with
multiple DFPs shall report the detailed…". This makes me think it's implying
that this is a requirement for MSTBs and not SST sinks, just a guess.
> 
> > From section 5.3.3.1:
> > 
> >    Either one or four bytes are used, per DFP type indication. Therefore, up
> > to
> >    16 (with 1-byte descriptor) or four (with 4-byte descriptor) DFP
> > capabilities
> >    can be stored.
> > 
> > So, a couple takeaways from this:
> > 
> >  * A DisplayPort connector can have *multiple* different downstream port
> > types,
> >    which I think actually makes sense as I've seen an adapter like this
> > before.
> >  * We actually added the ability to determine the downstream port type for
> > DP
> >    connectors using the subconnector prop, but it seems like if we want to
> > aim
> >    for completeness we're going to need to come up with a new prop that can
> >    report multiple downstream port types :\.
> 
> This makes sense to me.
> 
> >  * It's not explicitly mentioned, but I'm assuming the correct way of
> > handling
> >    multiple downstream BPC/pixel clock capabilities is to assume the max
> >    BPC/pixel clock is derived from the lowest max BPC/pixel clock we find on
> >    *connected* downstream ports (anything else wouldn't really make sense,
> > imho)
> 
> This would limit the case where the user only cares about the output
> with a higher BW requirement on a DFP even if another DFP with a lower
> BW cap is also connected. Not sure if it's a real world use-case though.

hm, true.
> 
> > So I'm going to rewrite this so we handle this properly in
> > drm_dp_downstream_read_info() and related helpers. I don't currently have
> > the
> > time to do this, but if there's interest upstream in properly reporting the
> > downstream port types of DP ports in userspace someone might want to
> > consider
> > coming up with another prop that accounts for multiple different downstream
> > port
> > types.
> > 
> > > 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@...ts.freedesktop.org
> > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > -- 
> > Sincerely,
> >       Lyude Paul (she/her)
> >       Software Engineer at Red Hat
> > 
-- 
Sincerely,
      Lyude Paul (she/her)
      Software Engineer at Red Hat

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