[PATCH 2/4] drm/dp_mst: Only create connector for connected end device

[Lyude Paul]

Sorry about the slow response, this week XDC has been going on and I've been
mostly paying attention to that.

On Tue, 2021-09-14 at 08:46 +0000, Lin, Wayne wrote:
> [Public]
> 
> > -----Original Message-----
> > From: Lyude Paul <lyude at redhat.com>
> > Sent: Thursday, September 2, 2021 6:00 AM
> > To: Lin, Wayne <Wayne.Lin at amd.com>; dri-devel at lists.freedesktop.org
> > Cc: Kazlauskas, Nicholas <Nicholas.Kazlauskas at amd.com>; Wentland, Harry
> > <Harry.Wentland at amd.com>; Zuo, Jerry
> > <Jerry.Zuo at amd.com>; Wu, Hersen <hersenxs.wu at amd.com>; Juston Li
> > <juston.li at intel.com>; Imre Deak <imre.deak at intel.com>;
> > Ville Syrjälä <ville.syrjala at linux.intel.com>; Daniel Vetter
> > <daniel.vetter at ffwll.ch>; Sean Paul <sean at poorly.run>; Maarten Lankhorst
> > <maarten.lankhorst at linux.intel.com>; Maxime Ripard <mripard at kernel.org>;
> > Thomas Zimmermann <tzimmermann at suse.de>;
> > David Airlie <airlied at linux.ie>; Daniel Vetter <daniel at ffwll.ch>; Deucher,
> > Alexander <Alexander.Deucher at amd.com>; Siqueira,
> > Rodrigo <Rodrigo.Siqueira at amd.com>; Pillai, Aurabindo
> > <Aurabindo.Pillai at amd.com>; Bas Nieuwenhuizen
> > <bas at basnieuwenhuizen.nl>; Cornij, Nikola <Nikola.Cornij at amd.com>; Jani
> > Nikula <jani.nikula at intel.com>; Manasi Navare
> > <manasi.d.navare at intel.com>; Ankit Nautiyal <ankit.k.nautiyal at intel.com>;
> > José Roberto de Souza <jose.souza at intel.com>; Sean
> > Paul <seanpaul at chromium.org>; Ben Skeggs <bskeggs at redhat.com>;
> > stable at vger.kernel.org
> > Subject: Re: [PATCH 2/4] drm/dp_mst: Only create connector for connected end
> > device
> > 
> > Actually - did some more thinking, and I think we shouldn't try to make
> > changes like this until we actually know what the problem is
> > here. I could try to figure out what the actual race conditions I was facing
> > before with trying to add/destroy connectors based on PDT,
> > but we still don't even actually have a clear idea of what's broken here.
> > I'd much rather us figure out exactly how this leak is
> > happening before considering making changes like this, because we have no
> > way of knowing if we've properly fixed it or not if we
> > don't know what the problem is in the first place.
> > 
> > I'm still happy to write up the topology debugging stuff I mentioned to you
> > if you think that would help you debug this issue - since
> > that would make it a lot easier for you to track down what references are
> > keeping a connector alive (and additkionally, where those
> > references were taken in code. thanks
> > stack_depot!)
> Hi Lyude,
> Sorry for late response. A bit busy on other stuff recently..
> 
> Really really thankful for all your help : ) I'm also glad to have the
> debugging tool if it won’t bother you too much. But before debugging,

no problem! I will get to it early next week then

> I need to have consensus with you about where do we expect to release resource
> allocated for a stream sink when it's reported as
> disconnected. Previous patch suggests releasing resource when connector is
> destroyed which will happen when topology refcount
> reaches zero (i.e. unplug mstb from topology). But when the case is receiving
> CSN notifying connection change, we don't try to destroy
> connector in this case now. And this is not caused by topology/malloc refcount
> leak since I don't expect neither one of them get
> decrease to zero under this case (topology of mstbs and ports is not changed).
> Hence, my plan was to also try to destroy connector under

Ah - I wonder if this might have been where some of the confusion here came
from. So-both mstbs and ports (assume I'm talking the actual drm_dp_mst_port
and drm_dp_mst_branch structs here) are supposed to have non-zero topology
refcounts as long as there is a valid path between the port or mstb, and our
source. This also means that for ports, the drm_connector associated with
these ports should stay around as long as the port is reachable from the sink
- regardless of whether anything is actually plugged into the port or not.

So - a CSN on it's own shouldn't really get rid of the port it was notifying
us about. But if that CSN results in an MSTB -with- its own ports being
removed, this would mean there would no longer be a valid path between our
source and the ports on said MSTB and as such - the connector for each one of
those ports is removed from the topology. Remember however, when I say
"removed from the topology" what I'm referring to is the fact that the MST
helpers have dropped the main topology reference for a given mstb or port.
Since various MST helpers retrieve temporary topology references to connectors
they work on in order to simplify handling I/O errors, the operations from
those helpers would potentially keep the port or mstb around in the topology
until those helpers have had a chance to abort and drop their refs. And then
once all the topology references are released, a destruction worker gets
scheduled which handles unregistering the drm_connector (not destroying it).
The drm_connector stays around unregistered, up until the point at which all
malloc references to the drm_dp_mst_port have been released.

I think it may also be worth clarifying the lifetime of drm_connector itself
here as well, since that also actually has a refcount. Basically, as long as
userspace has a mode committed which references a drm_connector - that
drm_connector will still exist in memory, and its mode object ID will remain
valid. This means if we were to have a MST topology hooked up with one display
turned on and then suddenly unplugged it, keeping in mind that the port with
said display now becomes inaccessible from the topology, the drm_connector
associated with that display would continue to have a valid mode object ID up
until the point at which userspace has committed a new mode which disables it.
The sysfs paths for the connector however, will disappear immediately once the
connector is unregistered so as to ensure that userspace applications cannot
try to reuse it later or attempt to reprobe it.

Any resource releases beyond this (streams on the driver side, for instance)
are up to the driver, but typically I would expect them to happen in the same
places as they would with an SST connector. Does that answer your question?

> this case and the reason is reasonable to me as described in previous mail.
> With this patch set, I can see connectors eventually get
> successfully destroyed after userspace committing set_crtc() to free
> connectors (although also need a fix on the connector refcount
> grabbed by drm_client_modeset_probe() under specific scenario).
> 
> I think the main problem I encountered here is that I couldn't find a place
> that notify us to release resource allocated for a disconnected
> stream sink when receive CSN. If we decide not to destroy connector under this
> case, then I probably need some guidance about where
> to do the release work.
> 
> Thanks again Lyude!
> > 
> > On Tue, 2021-08-31 at 18:47 -0400, Lyude Paul wrote:
> > > (I am going to try responding to this tomorrow btw. I haven't been
> > > super busy this week, but this has been a surprisingly difficult email
> > > to respond to because I need to actually need to do a deep dive some
> > > of the MST helpers tomorrow to figure out more of the specifics on why
> > > I realized we couldn't just hot add/remove port->connector here).
> > > 
> > > On Wed, 2021-08-25 at 03:35 +0000, Lin, Wayne wrote:
> > > > [Public]
> > > > 
> > > > > -----Original Message-----
> > > > > From: Lyude Paul <lyude at redhat.com>
> > > > > Sent: Tuesday, August 24, 2021 5:18 AM
> > > > > To: Lin, Wayne <Wayne.Lin at amd.com>;
> > > > > dri-devel at lists.freedesktop.org
> > > > > Cc: Kazlauskas, Nicholas <Nicholas.Kazlauskas at amd.com>; Wentland,
> > > > > Harry <Harry.Wentland at amd.com>; Zuo, Jerry <Jerry.Zuo at amd.com>;
> > > > > Wu, Hersen <hersenxs.wu at amd.com>; Juston Li <juston.li at intel.com>;
> > > > > Imre Deak <imre.deak at intel.com>; Ville Syrjälä
> > > > > <ville.syrjala at linux.intel.com>; Daniel Vetter
> > > > > <daniel.vetter at ffwll.ch>; Sean Paul <sean at poorly.run>; Maarten
> > > > > Lankhorst <maarten.lankhorst at linux.intel.com>; Maxime Ripard
> > > > > <mripard at kernel.org>; Thomas Zimmermann <tzimmermann at suse.de>;
> > > > > David Airlie <airlied at linux.ie>; Daniel Vetter <daniel at ffwll.ch>;
> > > > > Deucher, Alexander <Alexander.Deucher at amd.com>; Siqueira, Rodrigo
> > > > > <Rodrigo.Siqueira at amd.com>; Pillai, Aurabindo
> > > > > <Aurabindo.Pillai at amd.com>; Bas Nieuwenhuizen
> > > > > <bas at basnieuwenhuizen.nl>; Cornij, Nikola <Nikola.Cornij at amd.com>;
> > > > > Jani Nikula <jani.nikula at intel.com>; Manasi Navare
> > > > > <manasi.d.navare at intel.com>; Ankit Nautiyal
> > > > > <ankit.k.nautiyal at intel.com>; José Roberto de Souza
> > > > > <jose.souza at intel.com>; Sean Paul <seanpaul at chromium.org>; Ben
> > > > > Skeggs <bskeggs at redhat.com>; stable at vger.kernel.org
> > > > > Subject: Re: [PATCH 2/4] drm/dp_mst: Only create connector for
> > > > > connected end device
> > > > > 
> > > > > [snip]
> > > > > 
> > > > > I think I might still be misunderstanding something, some comments
> > > > > below
> > > > > 
> > > > > On Mon, 2021-08-23 at 06:33 +0000, Lin, Wayne wrote:
> > > > > > > > Hi Lyude,
> > > > > > > > 
> > > > > > > > Really thankful for willing to explain in such details.
> > > > > > > > Really appreciate.
> > > > > > > > 
> > > > > > > > I'm trying to fix some problems that observed after these 2
> > > > > > > > patches
> > > > > > > > * 09b974e8983 drm/amd/amdgpu_dm/mst: Remove
> > > > > > > > ->destroy_connector() callback
> > > > > > > > * 72dc0f51591 drm/dp_mst: Remove
> > > > > > > > drm_dp_mst_topology_cbs.destroy_connector
> > > > > > > > 
> > > > > > > > With above patches, we now change to remove dc_sink when
> > > > > > > > connector is about to be destroyed. However, we found out
> > > > > > > > that connectors won't get destroyed after hotplugs. Thus,
> > > > > > > > after few times hotplugs, we won't create any new dc_sink
> > > > > > > > since number of sink is exceeding our limitation. As the
> > > > > > > > result of that, I'm trying to figure out why the refcount of
> > > > > > > > connectors won't get zero.
> > > > > > > > 
> > > > > > > > Based on my analysis, I found out that if we connect a sst
> > > > > > > > monitor to a mst hub then connect the hub to the system, and
> > > > > > > > then unplug the sst monitor from the hub. E.g.
> > > > > > > > src - mst hub - sst monitor => src - mst hub  (unplug) sst
> > > > > > > > monitor
> > > > > > > > 
> > > > > > > > Within this case, we won't try to put refcount of the sst
> > > > > > > > monitor.
> > > > > > > > Which is what I tried to resolve by [PATCH 3/4].
> > > > > > > > But here comes a problem which is confusing me that if I can
> > > > > > > > destroy connector in this case. By comparing to another
> > > > > > > > case, if now mst hub is connected with a mst monitor like this:
> > > > > > > > src - mst hub - mst monitor => src - mst hub  (unplug) mst
> > > > > > > > monitor
> > > > > > > > 
> > > > > > > > We will put the topology refcount of mst monitor's branching
> > > > > > > > unit in and
> > > > > > > > drm_dp_port_set_pdt() and eventually call
> > > > > > > > drm_dp_delayed_destroy_port() to unregister the connector of
> > > > > > > > the logical port. So following the same rule, I think to
> > > > > > > > dynamically unregister a mst connector is what we want and
> > > > > > > > should be reasonable to also destroy sst connectors in my
> > > > > > > > case. But this conflicts the idea what we have here. We want
> > > > > > > > to create connectors for all output ports.
> > > > > > > > So if dynamically creating/destroying connectors is what we
> > > > > > > > want, when is the appropriate time for us to create one is
> > > > > > > > what I'm considering.
> > > > > > > > 
> > > > > > > > Take the StartTech hub DP 1to4 DP output ports for instance.
> > > > > > > > This hub, internally, is constructed by  3 1-to-2 mst branch
> > > > > > > > chips. 2 output ports of 1st chip are hardwired to another 2
> > > > > > > > chips. It's how it makes it to support 1-to-4 mst branching.
> > > > > > > > So within this case, the internal
> > > > > > > > 2 output ports of 1st chip is not connecting to a stream
> > > > > > > > sink and will never get connected to one.  Thus, I'm
> > > > > > > > thinking maybe the best timing to attach a connector to a
> > > > > > > > port is when the port is connected, and the connected PDT is
> > > > > > > > determined as a stream sink.
> > > > > > > > 
> > > > > > > > Sorry if I misunderstand anything here and really thanks for
> > > > > > > > your time to shed light on this : ) Thanks Lyude.
> > > > > > > 
> > > > > > > It's no problem, it is my job after all! Sorry for how long my
> > > > > > > responses have been taking, but my plate seems to be finally
> > > > > > > clearing up for the foreseeable future.
> > > > > > > 
> > > > > > > That being said - it sounds like with this we still aren't
> > > > > > > actually clear on where the topology refcount leak is
> > > > > > > happening - only when it's happening, which says to me that's
> > > > > > > the issue we really need to be figuring out the cause of as
> > > > > > > opposed to trying to workaround it.
> > > > > > > 
> > > > > > > Actually - refcount leaks is an issue I've ran into a number
> > > > > > > of times before in the past, so a while back I actually added
> > > > > > > some nice debugging features to assist with debugging leaks.
> > > > > > > If you enable the following options in your kernel config:
> > > > > > > 
> > > > > > > CONFIG_EXPERT=y # This must be set first before the next
> > > > > > > option CONFIG_DRM_DEBUG_DP_MST_TOPOLOGY_REFS=y
> > > > > > > 
> > > > > > > Unfortunately, I'm suddenly realizing after typing this that
> > > > > > > apparently I never bothered adding a way for us to debug the
> > > > 
> > > > > > > refcounts of ports/mstbs that haven't been released yet - only
> > > > > > > the ones for ones that have. This shouldn't be difficult at
> > > > > > > all for me to add, so I'll send you a patch either today or at
> > > > > > > the start of next week to try debugging with using this, and
> > > > > > > then we can figure out where this leak is really coming from.
> > > > > > 
> > > > > > Thanks Lyude!
> > > > > > 
> > > > > > Sorry to bother you, but I would like to clarify this again.  So
> > > > > > it sounds
> > > > > 
> > > > > It's no problem! It's my job and I'm happy to help :).
> > > > 
> > > > Thanks!
> > > > I would like to learn more from you as below : p
> > > > > 
> > > > > > like you also agree that we should destroy associated connector
> > > > > 
> > > > > Not quite. I think a better way of explaining this might be to
> > > > > point out that the lifetime of an MST port and its connector isn't
> > > > > supposed to be determined by whether or not it has something
> > > > > plugged into it - its lifetime is supposed to depend on whether
> > > > > there's a valid path from us down the MST topology to the port
> > > > > we're trying to reach. So an MSTB with ports that is unplugged
> > > > > would destroy all of its ports - but an unplugged port should just
> > > > > be the same as a disconnected DRM connector - even if the port
> > > > > itself is just hosting a branching device.
> > > > 
> > > > This is the part a bit difficult to me. I treat DRM connector as the
> > > > place where we associate with a stream sink. So if the statement is
> > > > "All DP mst output ports are places we connect with stream sink", I
> > > > would say false to this since I can find the negative example when
> > > > output port is connected with mst branch device. Thus, looks like we
> > > > could only determine whether to create a connector for an output
> > > > port when the peer device type is known?
> > > > > 
> > > > > Additionally - we don't want to try "delaying" connector creation
> > > > > either.
> > > > > In the modern world hotplugging is almost always reliable in
> > > > > normal situations, but even so there's still use cases for wanting
> > > > > force probing for analog devices on DP converters and just in
> > > > > general as it's a feature commonly used by developers or users
> > > > > working around monitors with problematic HPD issues or EDID issues.
> > > > 
> > > > I think I understand that why we want to create connectors for all
> > > > output ports here. But under these mentioned use cases, aren't we
> > > > still capable to force connector to enable stream? MST hub with
> > > > muti-functon capability, it will enumerate connected virtual DP peer
> > > > device.
> > > > For problematic HPD issues or EDID issues, their connection status
> > > > is also connected.
> > > > 
> > > > My understanding of output port is it is an internal node to help
> > > > construct an end-to-end virtual channel between a stream source
> > > > device and a stream sink device. Creating connectors for internal
> > > > nodes within a virtual channel is a bit hard for me to get the idea.
> > > > Please correct me if I misunderstand anything here. Thanks Lyude!
> > > > > 
> > > > > > when we unplug sst monitor from a mst hub in the case that I
> > > > > > described? In the case I described (unplug sst monitor), we only
> > > > > > receive CSN from the hub that notifying us the connection status
> > > > > > of one of its downstream output ports is changed to
> > > > > > disconnected. There is no topology refcount needed to be
> > > > > > decreased on this disconnected port but the malloc refcount.
> > > > > > Since the output port is still declared by
> > > > > 
> > > > > Apologies - I misunderstood your original mail as implying that
> > > > > topology refcounts were being leaked - but it sounds like it's
> > > > > actually malloc refcounts being leaked instead? In any case - that
> > > > > means we're still tracing down a leak, just a malloc ref leak.
> > > > > 
> > > > > But, this still doesn't totally make sense to me. Malloc refs only
> > > > > keep the actual drm_dp_mst_port/drm_dp_mst_branch struct alive in
> > > > > memory.
> > > > > Nothing else is kept around, meaning the DRM connector (and I
> > > > > assume by proxy, the dc_sink) should both be getting dropped still
> > > > > and the only thing that should be leaked is a memory allocation.
> > > > > These things should instead be dropped once there's no longer any
> > > > > topology references around. So, are we _sure_ that the problem
> > > > > here is a missing
> > > > > drm_dp_mst_port_put_malloc() or drm_dp_mst_mstb_put_malloc()?
> > > > 
> > > > Just my two cents, I don't think it's leak of malloc ref neither. As
> > > > you said, malloc ref is dealing with the last step to free port/mstb.
> > > > If there is still topology refcount on port/mstb in my case, we
> > > > won't free port/mstb.
> > > > > 
> > > > > If we are unfortunately we don't have equivalent tools for
> > > > > malloc() tracing. I'm totally fine with trying to add some if we
> > > > > have trouble figuring out this issue, but I'm a bit suspicious of
> > > > > the commits you mentioned that introduced this problem. If the
> > > > > problem doesn't happen until those two commits, then it's
> > > > > something in the code changes there that are causing this problem.
> > > > 
> > > > I think we probably also have the problem before these commits, but
> > > > we didn't notice this before. Just when we change to clean up all
> > > > things in dm_dp_mst_connector_destroy(), I start to try to figure
> > > > out all these things out.
> > > > > 
> > > > > The main thing I'm suspicious of just from looking at changes in
> > > > > 09b974e8983a4b163d4a406b46d50bf869da3073 is that the call to
> > > > > amdgpu_dm_update_freesync_caps() that was previously in
> > > > > dm_dp_destroy_mst_connector() appears to be dropped and not
> > > > > re-added in (oh dear, this is a /very/ confusingly similar
> > > > > function
> > > > 
> > > > Lol. I also have hard time on this..
> > > > > name!!!) dm_dp_mst_connector_destroy(). I don't remember if this
> > > > > was intentional on my part, but does adding a call back to
> > > > > amdgpu_dm_update_freesync_caps() into
> > > > > dm_dp_destroy_mst_connector() right before the
> > > > > dc_link_remove_remote_sink() call fix anything?
> > > > > 
> > > > > As well, I'm far less suspicious of this one but does re-adding
> > > > > this
> > > > > hunk:
> > > > > 
> > > > >       aconnector->dc_sink = NULL;
> > > > >       aconnector->dc_link->cur_link_settings.lane_count = 0;
> > > > > 
> > > > > After dc_sink_release() fix anything either?
> > > > 
> > > > So the main problem is we don't have chance to call
> > > > dc_link_remove_remote_sink() in the unplugging SST case. We only
> > > > have chance to remove the remote sink of a link when unplug a mstb.
> > > > > 
> > > > > > the mst hub,  I think we shouldn't destroy the port. Actually,
> > > > > > no ports nor mst branch devices should get destroyed in this
> > > > > > case I think.
> > > > > > The result of LINK_ADDRESS is still the same before/after
> > > > > > removing the sst monitor except the
> > > > > > DisplayPort_Device_Plug_Status/ Legacy_Device_Plug_Status.
> > > > > > 
> > > > > > Hence, if you agree that we should put refcount of the connector
> > > > > > of the disconnected port within the unplugging sst monitor case
> > > > > > to release the allocated resource, it means we don't want to
> > > > > > create connectors for those disconnected ports. Which conflicts
> > > > > > current flow to create connectors for all declared output ports.
> > > > > > 
> > > > > > Thanks again for your time Lyude!
> > > > > 
> > > > > --
> > > > > Cheers,
> > > > >  Lyude Paul (she/her)
> > > > >  Software Engineer at Red Hat
> > > > --
> > > > Regards,
> > > > Wayne
> > > > 
> > > 
> > 
> > --
> > Cheers,
> >  Lyude Paul (she/her)
> >  Software Engineer at Red Hat
> --
> Regards,
> Wayne Lin
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat