linux-kernel - [RFD] drivers: base: A driver's ->sync

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Date:   Tue, 31 Aug 2021 15:55:47 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Saravana Kannan <saravanak@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     Rob Herring <robh@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>
Subject: [RFD] drivers: base: A driver's ->sync_state() callback don't get called

Hi Saravana, Rafael, Stephen,

I have observed an issue with a driver's  ->sync_state() callback that
doesn't get called, even if consumers/supplier device links conditions
should have been satisfied. I have narrowed down the problem, but I am
not sure what is the best solution to fix it, hence I am turning to
you for some discussion/advice.

I have two test platform drivers, one that matches on the
"test,pm-domains-test" compatible string (this driver also has the
->sync_state() callback assigned) and another driver that matches on
"test,runtime-pm-test".

This is the relevant part in my DTS file:

       pm_domain_test {
                compatible = "test,pm-domains-test";

                pdParent: power-domain-parent {
                        #power-domain-cells = <0>;
                };

                pdChild: power-domain-child {
                        #power-domain-cells = <0>;
                        power-domains = <&pdParent>;
                };
        };

        soctest: soctest {
                compatible = "simple-bus";

                rpmtest0 {
                        compatible = "test,runtime-pm-test";
                        power-domains = <&pdParent>;
                };
        };

During boot the fw_devlinks are being created and their corresponding
links. With some debug enabled this shows some of the interesting
parts that are being printed to the log:

[    0.041539] device: 'pm_domain_test': device_add
[    0.041629] OF: Not linking pm_domain_test to pm_domain_test - is descendant
[    0.041718] device: 'soctest': device_add
[    0.041803] OF: Linking rpmtest0 (consumer) to pm_domain_test (supplier)
[    0.041829] device: 'platform:pm_domain_test--platform:soctest': device_add
[    0.041892] platform soctest: Linked as a sync state only consumer
to pm_domain_test
[    0.041957] OF:    create child: /soctest/rpmtest0
[    0.041995] device: 'soctest:rpmtest0': device_add
[    0.042072] device:
'platform:pm_domain_test--platform:soctest:rpmtest0': device_add
[    0.042132] devices_kset: Moving soctest:rpmtest0 to end of list
[    0.042141] platform soctest:rpmtest0: Linked as a consumer to pm_domain_test

The interesting thing here is the "sync state only" link that gets
created. I assume there are good reasons for creating this link, even
if I fail to understand exactly why.

In any case, the sync state only link never gets dropped, which I
assume is because there is no driver getting bound for the "soctest"
device (it has only the "simple-bus" compatible).

In other words, it doesn't matter that both the rpmtest0 and the
pm_domain_test devices are probed, thus satisfying the
supplier/consumer conditions, the ->sync_state() callback doesn't get
called anyway.

Can you perhaps help to point me in a direction of how to best fix this problem?

Kind regards
Ulf Hansson