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@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?
> >
> 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.
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.
"""
>
> 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.
> 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@xxxxxxxxxxxxxxxxxxxxx
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> --
> Sincerely,
> Lyude Paul (she/her)
> Software Engineer at Red Hat
>
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