Re: [igt-dev] [PATCH] tests/kms_dp_aux_dev: Handle AUX failures on disconnected MST connectors

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Oct 15, 2020 at 05:29:35PM +0300, Imre Deak wrote:
> On Tue, Oct 13, 2020 at 04:19:06PM +0300, Imre Deak wrote:
> > On Tue, Oct 13, 2020 at 03:56:30PM +0300, Ville Syrjälä wrote:
> > > On Tue, Oct 13, 2020 at 02:36:32PM +0300, Imre Deak wrote:
> > > > On Tue, Oct 13, 2020 at 02:27:12PM +0300, Ville Syrjälä wrote:
> > > > > On Mon, Oct 12, 2020 at 08:56:54PM +0300, Imre Deak wrote:
> > > > > > The DPCD of an MST connector is read out with a REMOTE_DPCD_READ MST
> > > > > > request. If the given connector is disconnected this read will result in
> > > > > > an MST NAK reply and this will be reported as an EIO error to the
> > > > > > initiator of the AUX read.
> > > > > > 
> > > > > > Handle this in the test that attempts to read the DPCD of any exposed
> > > > > > connector, whether they are connected or not.
> > > > > 
> > > > > MST connectors get nuked once disconnected no? So is this just to avoid
> > > > > some race with the connector disappearing during the test?
> > > > 
> > > > During enumeration of the ports of an MSTB (in response to a hotplug
> > > > when at least one port is connected) a connector is added for all
> > > > (output) ports, even if they are not connected. If a port gets unplugged
> > > > the connector is removed yes, but a new connector for this port is added
> > > > back since the parent MSTB is reprobed then.
> > > 
> > > That's quite confusing. Why are we trying to base the lifetime of the
> > > connector on two different things?
> > 
> > That's how MST core currently works: if a branch device is present then
> > all MST output ports on it will be also present along with the
> > corresponding DRM connector for these output ports, whether or not there
> > is anything plugged to these output ports.
> > 
> > > Also I don't remember seeing any extra connectors for disconnected
> > > ports on MST devices. So a bit confused.
> > 
> > But that matches the code and also what I see. After a branch device (a
> > TBT dock in MST mode) is plugged to the host with 1 monitor plugged to
> > it (to port 2) I get:
> > 
> > [ 6451.971549] [drm:drm_dp_send_link_address [drm_kms_helper]] link address reply: 4
> > [ 6451.985722] [drm:drm_dp_send_link_address [drm_kms_helper]] port 0: input 1, pdt: 1, pn: 0, dpcd_rev: 00, mcs: 1, ddps: 1, ldps 0, sdp 0/0
> > [ 6451.985727] [drm:drm_dp_send_link_address [drm_kms_helper]] port 1: input 0, pdt: 0, pn: 1, dpcd_rev: 00, mcs: 0, ddps: 0, ldps 0, sdp 0/0
> > [ 6452.002362] [drm:drm_dp_send_link_address [drm_kms_helper]] port 2: input 0, pdt: 3, pn: 2, dpcd_rev: 11, mcs: 0, ddps: 1, ldps 0, sdp 1/1
> > [ 6452.002366] [drm:drm_dp_send_link_address [drm_kms_helper]] port 3: input 0, pdt: 0, pn: 3, dpcd_rev: 00, mcs: 0, ddps: 0, ldps 0, sdp 0/0
> > 
> > $ ls /sys/class/drm/
> > 
> > card0  card0-DP-1  card0-DP-2  card0-DP-3  card0-DP-4  card0-DP-5  card0-DP-6  card0-DP-7  card0-HDMI-A-1  renderD128  version
> > 
> > card0-DP-[567] being the 3 output ports on the branch device, card0-DP-6
> > in connected and the other 2 in disconnected state.
> 
> I checked now what happens when plugging an SST monitor to port 1 and an
> MST monitor to port 2 on the above dock:
> 
> [ 1456.272370] [drm:drm_dp_send_link_address [drm_kms_helper]] link address reply: 4
> [ 1456.381696] [drm:drm_dp_send_link_address [drm_kms_helper]] port 0: input 1, pdt: 1, pn: 0, dpcd_rev: 00, mcs: 1, ddps: 1, ldps 0, sdp 0/0
> [ 1456.397902] [drm:drm_dp_send_link_address [drm_kms_helper]] port 1: input 0, pdt: 4, pn: 1, dpcd_rev: 00, mcs: 0, ddps: 1, ldps 1, sdp 1/1
> [ 1456.397906] [drm:drm_dp_send_link_address [drm_kms_helper]] port 2: input 0, pdt: 2, pn: 2, dpcd_rev: 12, mcs: 1, ddps: 1, ldps 0, sdp 0/0
> [ 1456.397910] [drm:drm_dp_send_link_address [drm_kms_helper]] port 3: input 0, pdt: 0, pn: 3, dpcd_rev: 00, mcs: 0, ddps: 0, ldps 0, sdp 0/0
> [ 1459.650637] [drm:drm_dp_send_link_address [drm_kms_helper]] link address reply: 3
> [ 1459.660720] [drm:drm_dp_send_link_address [drm_kms_helper]] port 0: input 1, pdt: 2, pn: 0, dpcd_rev: 00, mcs: 1, ddps: 1, ldps 0, sdp 0/0
> [ 1459.687834] [drm:drm_dp_send_link_address [drm_kms_helper]] port 1: input 0, pdt: 3, pn: 8, dpcd_rev: 12, mcs: 0, ddps: 1, ldps 0, sdp 1/1
> [ 1459.717059] [drm:drm_dp_send_link_address [drm_kms_helper]] port 2: input 0, pdt: 0, pn: 1, dpcd_rev: 00, mcs: 0, ddps: 0, ldps 0, sdp 0/0
> 
> The driver exposes a connector for the branch device in the MST monitor
> too, this connector is in disconnected state, but (as expected) we can
> read out the (branch device) DPCD from it.
> 
> The MST monitor has two output ports, on port 1 (pn 8) is its internal
> sink stream on port 2 (pn 1) is a daisy chain DP port. For both of
> these ports the driver exposes a connector, the latter one in
> disconnected state and the remote DPCD read for this will fail with a
> NAK/EIO error (as in the original case).
> 
> So atm the driver exposes a connector for all output ports of branch
> devices and the branch devices themselves have also a connector (both
> for the "root" branch device, card0-DP-3 above, and all downstream
> branch devices). All ports/connectors that have an SST sink plugged into
> them also expose an I2C and AUX device. Ports/connectors that have a
> branch device plugged into them only expose an AUX device.
> 
> I think this is consistent and (if so) it would make sense to document
> this somewhere as well.
> 
> One inconsistency I noticed is connector numbering, it depends on the
> plugging order. That's because all connectors are added first for any
> downstream branch device as they are enumarated on a port, before adding
> connectors on subsequent ports on the parent branch device.

So it uses dfs rather than bfs, if I understood correctly.

> 
> So in the above case where the dock is plugged with everything else
> already plugged to it DP-[569] belongs to the dock while DP-[78] belongs
> to the monitor. This could be made more consistent by adding first all
> the connectors on a branch device before enumerating any other branch
> devices attached to it.

Yeah, bfs approach could avoid a bit of randomness in the numbering
when plugging things in different orders.

-- 
Ville Syrjälä
Intel
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