linux-kernel - Re: [PATCH] PM: sleep: Don't wait for SYNC_STATE

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Message-ID: <CAJZ5v0gEeVraqck8q_Ave66RMLOSC=p8=33PaBCKsMBuHGLqKA@mail.gmail.com>
Date: Wed, 10 Sep 2025 15:09:00 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Pin-yen Lin <treapking@...omium.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>, Pavel Machek <pavel@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Danilo Krummrich <dakr@...nel.org>, 
	Saravana Kannan <saravanak@...gle.com>, Hsin-Te Yuan <yuanhsinte@...omium.org>, 
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, 
	Chen-Yu Tsai <wenst@...omium.org>
Subject: Re: [PATCH] PM: sleep: Don't wait for SYNC_STATE_ONLY device links

Hi,

On Wed, Sep 10, 2025 at 2:43 PM Pin-yen Lin <treapking@...omium.org> wrote:
>
> Hi Rafael,
>
> Thanks for the review.
>
> On Wed, Sep 10, 2025 at 7:56 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Tue, Sep 9, 2025 at 11:44 AM Pin-yen Lin <treapking@...omium.org> wrote:
> > >
> > > Device links with DL_FLAG_SYNC_STATE_ONLY should not affect suspend
> > > and resume, and functions like device_reorder_to_tail() and
> > > device_link_add() doesn't try to reorder the consumers with such flag.
> > >
> > > However, dpm_wait_for_consumers() and dpm_wait_for_suppliers() doesn't
> > > check this flag before triggering dpm_wait, leading to potential hang
> > > during suspend/resume.
> >
> > Have you seen this happen or is it just a theory?
>
> We initially see this with a downstream kernel, but I can reproduce
> this with the upstream kernel when I connect the usb host controller
> to a "usb-a-connector" node on MT8186 Corsola Chromebook. The
> devicetree looks like:
>
> usb-a-connector {
>         compatible = "usb-a-connector";
>         port {
>                 usb_a_con: endpoint {
>                         remote-endpoint = <&usb_hs>;
>                 };
>         };
> };
>
> usb_host {
>         compatible = "mediatek,mt8186-xhci", "mediatek,mtk-xhci";
>         port {
>                 usb_hs: endpoint {
>                         remote-endpoint = <&usb_a_con>;
>                 };
>         };
> };
>
> In this case, the two nodes form a cycle and I ended up seeing a
> SYNC_STATE_ONLY devlink from usb_host (supplier) to the
> usb-a-connector (consumer).

It would be good to add the above information to the patch changelog.

> I'm not very sure why we didn't see this issue before. Maybe it's
> related to the fact that the usb-a-connector has a compatible string
> (so a platform device is created) but no driver binds into it.
> >
> > > Add DL_FLAG_SYNC_STATE_ONLY in dpm_wait_for_consumers() and
> > > dpm_wait_for_suppliers() to fix this.
> >
> > The above sentence is incomplete AFAICS.
>
> Sorry, I meant "Add a check for DL_FLAG_SYNC_STATE_ONLY in ...". I'll
> update this in the next version.
> >
> > > Fixes: 05ef983e0d65a ("driver core: Add device link support for SYNC_STATE_ONLY flag")
> > > Signed-off-by: Pin-yen Lin <treapking@...omium.org>
> > > ---
> > >
> > >  drivers/base/power/main.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
> > > index 2ea6e05e6ec90..3271f4af2cb65 100644
> > > --- a/drivers/base/power/main.c
> > > +++ b/drivers/base/power/main.c
> > > @@ -282,7 +282,8 @@ static void dpm_wait_for_suppliers(struct device *dev, bool async)
> > >          * walking.
> > >          */
> > >         list_for_each_entry_rcu_locked(link, &dev->links.suppliers, c_node)
> > > -               if (READ_ONCE(link->status) != DL_STATE_DORMANT)
> > > +               if (READ_ONCE(link->status) != DL_STATE_DORMANT &&
> > > +                   !device_link_test(link, DL_FLAG_SYNC_STATE_ONLY))
> >
> > This should use a check like device_link_flag_is_sync_state_only(),
> > which is different from the above one, for consistency with
> > device_reorder_to_tail().
>
> Thanks. I'll fix this in the next version.

Thanks!