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Message-ID: <CAEXTbpdPGLtjai_3jxyEPcOaDM1D=bhw4M8Gz4U78k_BcobY6w@mail.gmail.com>
Date: Tue, 16 Sep 2025 17:26:15 +0800
From: Pin-yen Lin <treapking@...omium.org>
To: Saravana Kannan <saravanak@...gle.com>
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>,
linux-kernel@...r.kernel.org, Chen-Yu Tsai <wenst@...omium.org>,
Hsin-Te Yuan <yuanhsinte@...omium.org>, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3] PM: sleep: Don't wait for SYNC_STATE_ONLY device links
Hi Saravana,
Thanks for the review.
On Sat, Sep 13, 2025 at 2:30 AM Saravana Kannan <saravanak@...gle.com> wrote:
>
> On Fri, Sep 12, 2025 at 7:48 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.
> >
> > This can be reproduced on MT8186 Corsola Chromebook with devicetree 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>;
> > };
> > };
> > };
>
> We've had SYNC_STATE_ONLY devlink for years in the kernel. Also,
> SYNC_STATE_ONLY device links get deleted once the consumer device
> probes. You also can't suspend a device before it probes.
In this case, "usb-a-connector" is a descriptive-only node that has no
driver binding into it, and this is why the SYNC_STATE_ONLY link never
gets deleted.
Also, out of curiosity, does that mean a system might not suspend if I
add a device node but doesn't include its driver in the kernel?
>
> Why are you hitting this issue only now? Something doesn't sound
> right. I'm asking this because I'm wondering if some other recent
> change broke any of my statements/design decisions above.
For our team, we didn't describe the on-board USB topology until
Stephen's series for the on-board USB hub support[1]. So, on previous
platforms we worked on, we don't have a graph connection from the USB
hosts and the USB connectors.
One explanation is that there aren't many use cases where you want to
add a graph connection to a node without a driver binding into that.
However, the code has been here for so many years, so I'm not sure if
this explanation really makes sense either.
[1]: https://lore.kernel.org/all/20240223005823.3074029-3-swboyd@chromium.org/
>
> Not to fix this, but to also ensure better suspend/resume ordering in
> the future, you might want to add a "post-init-suppliers" to the real
> supplier node here and point it to the real consumer here. This will
> also help break the cycle here. Because even with this fix, all it
> does is avoid enforcing any ordering between these two devices. It is
> just working based on the order in DT, which you shouldn't count on
> for correctness.
In this case, there is no driver for "usb-a-connector", so we don't
need any dependency between them.
>
> Thanks,
> Saravana
Regards,
Pin-yen
>
> >
> > In this case, the two nodes form a cycle and a SYNC_STATE_ONLY devlink
> > between usb_host (supplier) and usb-a-connector (consumer) is created.
> >
> > Export device_link_flag_is_sync_state_only() and use it to check this in
> > dpm_wait_for_consumers() and dpm_wait_for_suppliers() to fix this.
> >
> > Fixes: 05ef983e0d65a ("driver core: Add device link support for SYNC_STATE_ONLY flag")
> > Signed-off-by: Pin-yen Lin <treapking@...omium.org>
> > ---
> >
> > Changes in v3:
> > - Squash to one patch and fix the export approach
> >
> > Changes in v2:
> > - Update commit message
> > - Use device_link_flag_is_sync_state_only()
> >
> > drivers/base/base.h | 1 +
> > drivers/base/core.c | 2 +-
> > drivers/base/power/main.c | 6 ++++--
> > 3 files changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/base/base.h b/drivers/base/base.h
> > index 123031a757d9..80415b140ce7 100644
> > --- a/drivers/base/base.h
> > +++ b/drivers/base/base.h
> > @@ -248,6 +248,7 @@ void device_links_driver_cleanup(struct device *dev);
> > void device_links_no_driver(struct device *dev);
> > bool device_links_busy(struct device *dev);
> > void device_links_unbind_consumers(struct device *dev);
> > +bool device_link_flag_is_sync_state_only(u32 flags);
> > void fw_devlink_drivers_done(void);
> > void fw_devlink_probing_done(void);
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index d22d6b23e758..741aa0571fc7 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -287,7 +287,7 @@ static bool device_is_ancestor(struct device *dev, struct device *target)
> > #define DL_MARKER_FLAGS (DL_FLAG_INFERRED | \
> > DL_FLAG_CYCLE | \
> > DL_FLAG_MANAGED)
> > -static inline bool device_link_flag_is_sync_state_only(u32 flags)
> > +inline bool device_link_flag_is_sync_state_only(u32 flags)
> > {
> > return (flags & ~DL_MARKER_FLAGS) == DL_FLAG_SYNC_STATE_ONLY;
> > }
> > diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
> > index 2ea6e05e6ec9..73a1916170ae 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_flag_is_sync_state_only(link->flags))
> > dpm_wait(link->supplier, async);
> >
> > device_links_read_unlock(idx);
> > @@ -339,7 +340,8 @@ static void dpm_wait_for_consumers(struct device *dev, bool async)
> > * unregistration).
> > */
> > list_for_each_entry_rcu_locked(link, &dev->links.consumers, s_node)
> > - if (READ_ONCE(link->status) != DL_STATE_DORMANT)
> > + if (READ_ONCE(link->status) != DL_STATE_DORMANT &&
> > + !device_link_flag_is_sync_state_only(link->flags))
> > dpm_wait(link->consumer, async);
> >
> > device_links_read_unlock(idx);
> > --
> > 2.51.0.384.g4c02a37b29-goog
> >
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