Date:   Mon, 27 Sep 2021 18:13:12 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Stephen Boyd <sboyd@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH] Revert "of: property: fw_devlink: Add support for remote-endpoint"

On Mon, Sep 27, 2021 at 5:56 PM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> [Adding Stephen and linux-arm-msm to the CC list, missed on the patch Cc
> list]
>
> On 28/09/2021 00:58, Saravana Kannan wrote:
> > On Mon, Sep 27, 2021 at 1:48 PM Dmitry Baryshkov
> > <dmitry.baryshkov@...aro.org> wrote:
> >>
> >> Since the commit f7514a663016 ("of: property: fw_devlink: Add support
> >> for remote-endpoint") Linux kernel started parsing and adding devlinks
> >> for the remote-endpoint properties. However this brings more harm than
> >> good.
> >>
> >> For all the remote-endpoints in the graph two links are created. Thus
> >> each and every remote-endpoint ends up in the cyclic graph (instead of
> >> the original intent of catching a cycle of graph + non-graph link):
> >
> > Yes, I'm well aware of this. I even called this out in the commit
> > text. This creating of cycles and then catching and relaxing it is
> > intentional.
> > https://lore.kernel.org/all/20210330185056.1022008-1-saravanak@google.com/
>
> What would be the reason two always create a cycle which gives no
> additional information? Maybe I'm just misunderstanding this piece of code.

It's basically a tiny bit of busy work. Ulf and I planned to fix it
and we know how to. Just haven't gotten around to it since it doesn't
really break anything.

> Regarding your commit message. Even if there is a non-remote-endpoint
> dependency, it will be hidden by the remote-endpoint cycle.

That's the point. Because there's no way to tell without the driver
involvement, we basically need to ignore all dependencies between
those two devices pointing at each other.

>
> And another consequence of remote-endpoint loops.
>
> Consider this part part of dmesg. One warning is correct (real cyclic
> dependency). Others are remote-endpoint spam. Can you spot, which ones?
>
> [    7.032225] platform 1d87000.phy: Fixing up cyclic dependency with
> 1d84000.ufshc
> [   21.760326] platform c440000.spmi:pmic@2:typec@...0: Fixing up cyclic
> dependency with c440000.spmi:pmic@2:pmic-tcpm
> [   21.944849] platform c440000.spmi:pmic@2:pdphy@...0: Fixing up cyclic
> dependency with c440000.spmi:pmic@2:pmic-tcpm
> [   23.541968] platform a600000.usb: Fixing up cyclic dependency with
> c440000.spmi:pmic@2:pmic-tcpm
> [   30.354170] i2c 5-002b: Fixing up cyclic dependency with hdmi-out

It's info, not warning if I'm not mistaken. If that's really a problem
we can make it a debug log. Not the end of the world.

>
>
> >>
> >> [    0.381057] OF: remote-endpoint linking /soc@...eniqup@...000/i2c@...000/hdmi-bridge@2b to /soc@...dss@...0000/dsi@...4000/ports/port@...ndpoint
> >> [    0.394421] OF: remote-endpoint linking /soc@...eniqup@...000/i2c@...000/hdmi-bridge@2b to /hdmi-out/port/endpoint
> >> [    0.407007] OF: remote-endpoint linking /soc@...hy@...9000 to /soc@...pmi@...0000/pmic@...mic-tcpm/connector/ports/port@...ndpoint@0
> >> [    0.419648] OF: remote-endpoint linking /soc@...sb@...8800/usb@...0000 to /soc@...pmi@...0000/pmic@...mic-tcpm/ports/port@...ndpoint@0
> >> [    0.432578] OF: remote-endpoint linking /soc@...ci@...f000/i2c-bus@...am1@c0 to /soc@...amss@...a000/ports/port@...ndpoint
> >> [    0.444450] OF: remote-endpoint linking /soc@...amss@...a000 to /soc@...ci@...f000/i2c-bus@...am1@...port/endpoint
> >> [    0.455292] OF: remote-endpoint linking /soc@...dss@...0000/mdp@...1000 to /soc@...dss@...0000/dsi@...4000/ports/port@...ndpoint
> >> [    0.467210] OF: remote-endpoint linking /soc@...dss@...0000/mdp@...1000 to /soc@...dss@...0000/dsi@...6000/ports/port@...ndpoint
> >> [    0.479239] OF: remote-endpoint linking /soc@...dss@...0000/dsi@...4000 to /soc@...dss@...0000/mdp@...1000/ports/port@...ndpoint
> >> [    0.491147] OF: remote-endpoint linking /soc@...dss@...0000/dsi@...4000 to /soc@...eniqup@...000/i2c@...000/hdmi-bridge@...ports/port@...ndpoint
> >> [    0.504979] OF: remote-endpoint linking /soc@...pmi@...0000/pmic@...ypec@...0 to /soc@...pmi@...0000/pmic@...mic-tcpm/ports/port@...ndpoint
> >> [    0.517958] OF: remote-endpoint linking /soc@...pmi@...0000/pmic@...dphy@...0 to /soc@...pmi@...0000/pmic@...mic-tcpm/ports/port@...ndpoint
> >> [    0.565326] OF: remote-endpoint linking /hdmi-out to /soc@...eniqup@...000/i2c@...000/hdmi-bridge@...ports/port@...ndpoint
> >>
> >> Under some conditions the device can become it's own supplier,
> >> preventing this device to be probed at all:
> >
> > I'm not sure this analysis is correct -- this shouldn't be happening.
> > If you go to the device link folder and cat "sync_state_only", I
> > expect it to be "1" in this case. Can you confirm that?
>
> It is "1".

Thanks for confirming.

>
> > Which means it won't block probing. Yes, the link itself is useless
> > and it'll get auto deleted once mdss probes and it's easy to not
> > create it in the first place. But this is definitely not your issue.
> >
> >> $ ls -l /sys/bus/platform/devices/ae00000.mdss/
> >> lrwxrwxrwx    1 root     root             0 Aug  4 15:13 consumer:platform:ae00000.mdss -> ../../../virtual/devlink/platform:ae00000.mdss--platform:ae00000.mdss
> >>
> >> I think that until of_link can be tought to handle bi-directional links
> >> on its own, we should not parse remote-endpoint properties. Thus the
> >> aforementioned commit should be reverted.
> >
> > Nak. remote-endpoint parsing is working as intended. I don't think the
> > analysis is correct.
> >
> > Can you please enable the logs in all these functions and attach the
> > log so we can see why it's not probing mdss?
> > device_link_add
> > device_links_check_suppliers
> > func fw_devlink_relax_link
> > fw_devlink_create_devlink
>
> After doing the analysis, I can confirm that I was too quick regarding
> the mdss links preventing it from being probed. Sorry about that.
>
> It all went up to the DP phy having a link with usb-c-connector. I was
> running the kernel 5.15-rc1, so your tcpm fix is already present.
> However my colleague has disabled the tcpm device (which I did not
> notice). So the driver did not call fw_devlink_purge_absent_suppliers().
> The devlink still exists:

Let me take a closer look at this before the end of this week. Can you
point me to the exact DT changes that were made that's causing this
issue? It should help me debug the issue. I have a guess on what the
issue might be.

>
> [   53.426446] platform 88e9000.phy: probe deferral - wait for supplier
> connector
>
> However it is not present in the sysfs:

Right, because it's not a device link yet. It's waiting for the device
to show up to create the device link (it has to for the grand scheme
of things to work correctly).

>
> root@...m-armv8a:~# ls -l /sys/bus/platform/devices/88e9000.phy/
> lrwxrwxrwx    1 root     root             0 Aug  4 15:13
> consumer:platform:a600000.usb ->
> ../../../virtual/devlink/platform:88e9000.phy--platform:a600000.usb
> lrwxrwxrwx    1 root     root             0 Aug  4 15:13
> consumer:platform:af00000.clock-controller ->
> ../../../virtual/devlink/platform:88e9000.phy--platform:af00000.clock-controller
> -rw-r--r--    1 root     root          4096 Aug  4 15:13 driver_override
> -r--r--r--    1 root     root          4096 Aug  4 15:13 modalias
> lrwxrwxrwx    1 root     root             0 Aug  4 15:13 of_node ->
> ../../../../firmware/devicetree/base/soc@...hy@...9000
> drwxr-xr-x    2 root     root             0 Aug  4 15:13 power
> lrwxrwxrwx    1 root     root             0 Aug  4 15:10 subsystem ->
> ../../../../bus/platform
> lrwxrwxrwx    1 root     root             0 Aug  4 15:13
> supplier:platform:100000.clock-controller ->
> ../../../virtual/devlink/platform:100000.clock-controller--platform:88e9000.phy
> lrwxrwxrwx    1 root     root             0 Aug  4 15:13
> supplier:platform:18200000.rsc:clock-controller ->
> ../../../virtual/devlink/platform:18200000.rsc:clock-controller--platform:88e9000.phy
> lrwxrwxrwx    1 root     root             0 Aug  4 15:13
> supplier:platform:18200000.rsc:pm8150-rpmh-regulators ->
> ../../../virtual/devlink/platform:18200000.rsc:pm8150-rpmh-regulators--platform:88e9000.phy
> -rw-r--r--    1 root     root          4096 Aug  4 15:10 uevent
> -r--r--r--    1 root     root          4096 Aug  4 15:13
> waiting_for_supplier
>
> Thus it is not possible to spot this device link without
> CONFIG_DEBUG_DRIVER=y (or any similar debugging technique).

I sent out some patches to make this easier. But doesn't look like
it'll land in 5.15.
https://lore.kernel.org/lkml/20210915172808.620546-1-saravanak@google.com/

> If I re-enabled tcpm device or if I reverted remote-endpoint parsing, DP
> PHY probing would go fine. The DP PHY does not really depend on the
> connector (or TCPM) being present in the system. The driver will
> continue working w/o it. However it does not have a change to declare that.
>
> Furthermore I went back to the original case that caused you to add
> remote-endpoint support. The DSI-eDP bridge and eDP panel using the GPIO
> provided by that bridge. I think the proper fix for the original problem
> was implemented by the commit bf73537f411b ("drm/bridge: ti-sn65dsi86:
> Break GPIO and MIPI-to-eDP bridge into sub-drivers"). It split the
> DSI-eDP bridge driver into functional parts (devices), so that GPIO part
> and eDP parts are independent, thus breaking this cyclic dependency in a
> functional way. The remote-endpoint parsing is no longer necessary in
> this case (Stephen, please correct me if I'm wrong).

Even if the original case doesn't need remote-endpoint to work
correctly and the cycle has been broken, that doesn't remove the need
for parsing remote-endpoint. There could be other cases like the
original case.

> I still think that remote endpoint parsing does more harm and noise than
> good and thus should be reverted.

I'll agree to disagree. I'm sure your issue can be fixed without
removing support for remote-endpoint parsing -- let's work on that
(I've asked for more details above).

-Saravana

Powered by blists - more mailing lists