Date:   Tue, 28 Jun 2022 18:55:00 +0300
From:   Abel Vesa <abel.vesa@...aro.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        kernel-team@...roid.com, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        devicetree@...r.kernel.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v2 13/17] driver core: Use device's fwnode to check if it
 is waiting for suppliers

On 22-06-27 15:30:25, Saravana Kannan wrote:
> On Mon, Jun 27, 2022 at 4:42 AM Abel Vesa <abel.vesa@...aro.org> wrote:
> >
> > On 20-11-20 18:02:28, Saravana Kannan wrote:
> > > To check if a device is still waiting for its supplier devices to be
> > > added, we used to check if the devices is in a global
> > > waiting_for_suppliers list. Since the global list will be deleted in
> > > subsequent patches, this patch stops using this check.
> > >
> > > Instead, this patch uses a more device specific check. It checks if the
> > > device's fwnode has any fwnode links that haven't been converted to
> > > device links yet.
> > >
> > > Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> > > ---
> > >  drivers/base/core.c | 18 ++++++++----------
> > >  1 file changed, 8 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > index 395dece1c83a..1873cecb0cc4 100644
> > > --- a/drivers/base/core.c
> > > +++ b/drivers/base/core.c
> > > @@ -51,6 +51,7 @@ static DEFINE_MUTEX(wfs_lock);
> > >  static LIST_HEAD(deferred_sync);
> > >  static unsigned int defer_sync_state_count = 1;
> > >  static DEFINE_MUTEX(fwnode_link_lock);
> > > +static bool fw_devlink_is_permissive(void);
> > >
> > >  /**
> > >   * fwnode_link_add - Create a link between two fwnode_handles.
> > > @@ -995,13 +996,13 @@ int device_links_check_suppliers(struct device *dev)
> > >        * Device waiting for supplier to become available is not allowed to
> > >        * probe.
> > >        */
> > > -     mutex_lock(&wfs_lock);
> > > -     if (!list_empty(&dev->links.needs_suppliers) &&
> > > -         dev->links.need_for_probe) {
> > > -             mutex_unlock(&wfs_lock);
> > > +     mutex_lock(&fwnode_link_lock);
> > > +     if (dev->fwnode && !list_empty(&dev->fwnode->suppliers) &&
> > > +         !fw_devlink_is_permissive()) {
> > > +             mutex_unlock(&fwnode_link_lock);
> >
> > Hi Saravana,
> >
> > First of, sorry for going back to this.
>
> No worries at all. If there's an issue with fw_devlink, I want to have it fixed.
>
> > There is a scenario where this check will not work and probably should
> > work. It goes like this:
> >
> > A clock controller is not allowed to probe because it uses a clock from a child device of a
> > consumer, like so:
> >
> >         dispcc: clock-controller@...0000 {
> >                 clocks = <&dsi0_phy 0>;
> >         };
> >
> >         mdss: mdss@...0000 {
> >                 clocks = <&dispcc DISP_CC_MDSS_MDP_CLK>;
> >
> >                 dsi0_phy: dsi-phy@...4400 {
> >                         clocks = <&dispcc DISP_CC_MDSS_AHB_CLK>,
> >                 };
> >         };
> >
> > This is a real scenario actually, but I stripped it down to the essentials.
>
> I'm well aware of this scenario and explicitly wrote code to address this :)
>

Actually, the problem seems to be when you have two dsi phys.
Like so:

         dispcc: clock-controller@...0000 {
                 clocks = <&dsi0_phy 0>;
                 clocks = <&dsi1_phy 0>;
         };

         mdss: mdss@...0000 {
                 clocks = <&dispcc DISP_CC_MDSS_MDP_CLK>;

                 dsi0_phy: dsi-phy@...4400 {
                         clocks = <&dispcc DISP_CC_MDSS_AHB_CLK>,
                 };

		 dsi1_phy: dsi-phy@...4400 {
                         clocks = <&dispcc DISP_CC_MDSS_AHB_CLK>,
                 };
         };

And from what I've seen happening so far is that the device_is_dependent
check for the parent of the supplier (if it also a consumer) seems to return
false on second pass of the same link due to the DL_FLAG_SYNC_STATE_ONLY
being set this time around.

> See this comment in fw_devlink_create_devlink()
>
>        /*
>          * If we can't find the supplier device from its fwnode, it might be
>          * due to a cyclic dependency between fwnodes. Some of these cycles can
>          * be broken by applying logic. Check for these types of cycles and
>          * break them so that devices in the cycle probe properly.
>          *
>          * If the supplier's parent is dependent on the consumer, then the
>          * consumer and supplier have a cyclic dependency. Since fw_devlink
>          * can't tell which of the inferred dependencies are incorrect, don't
>          * enforce probe ordering between any of the devices in this cyclic
>          * dependency. Do this by relaxing all the fw_devlink device links in
>          * this cycle and by treating the fwnode link between the consumer and
>          * the supplier as an invalid dependency.
>          */
>

So when this thing you mentioned above is happening for the second dsi
phy (order doesn't matter), since the dsi phy itself cannot be found,
the device_is_dependent is run for the same link: dispcc -> mdss
(supplier -> consumer), but again, since it has the
DL_FLAG_SYNC_STATE_ONLY this time around, it will skip that specific
link.

> Applying this comment to your example, dispcc is the "consumer",
> dsi0_phy is the "supplier" and mdss is the "supplier's parent".
>
> And because we can't guarantee the order of addition of these top
> level devices is why I also have this piece of recursive call inside
> __fw_devlink_link_to_suppliers():
>
>                 /*
>                  * If a device link was successfully created to a supplier, we
>                  * now need to try and link the supplier to all its suppliers.
>                  *
>                  * This is needed to detect and delete false dependencies in
>                  * fwnode links that haven't been converted to a device link
>                  * yet. See comments in fw_devlink_create_devlink() for more
>                  * details on the false dependency.
>                  *
>                  * Without deleting these false dependencies, some devices will
>                  * never probe because they'll keep waiting for their false
>                  * dependency fwnode links to be converted to device links.
>                  */
>                 sup_dev = get_dev_from_fwnode(sup);
>                 __fw_devlink_link_to_suppliers(sup_dev, sup_dev->fwnode);
>                 put_device(sup_dev);
>
> So when mdss gets added, we'll link it to dispcc and then check if
> dispcc has any suppliers it needs to link to. And that's when the
> logic will catch the cycle and fix it.
>
> Can you tell me why this wouldn't unblock the probing of dispcc? Are
> you actually hitting this on a device? If so, can you please check why
> this logic isn't sufficient to catch and undo the cycle?
>

This is happening on Qualcomm SDM845 with Linus's tree.

> Thanks,
> Saravana
>
> > So, the dsi0_phy will be "device_add'ed" (through of_platform_populate) by the mdss probe.
> > The mdss will probe defer waiting for the DISP_CC_MDSS_MDP_CLK, while
> > the dispcc will probe defer waiting for the dsi0_phy (supplier).
> >
> > Basically, this 'supplier availability check' does not work when a supplier might
> > be populated by a consumer of the device that is currently trying to probe.
> >
> >
> > Abel
> >
> >
> > >               return -EPROBE_DEFER;
> > >       }
> > > -     mutex_unlock(&wfs_lock);
> > > +     mutex_unlock(&fwnode_link_lock);
> > >
> > >       device_links_write_lock();
> > >
> > > @@ -1167,10 +1168,7 @@ static ssize_t waiting_for_supplier_show(struct device *dev,
> > >       bool val;
> > >
> > >       device_lock(dev);
> > > -     mutex_lock(&wfs_lock);
> > > -     val = !list_empty(&dev->links.needs_suppliers)
> > > -           && dev->links.need_for_probe;
> > > -     mutex_unlock(&wfs_lock);
> > > +     val = !list_empty(&dev->fwnode->suppliers);
> > >       device_unlock(dev);
> > >       return sysfs_emit(buf, "%u\n", val);
> > >  }
> > > @@ -2202,7 +2200,7 @@ static int device_add_attrs(struct device *dev)
> > >                       goto err_remove_dev_groups;
> > >       }
> > >
> > > -     if (fw_devlink_flags && !fw_devlink_is_permissive()) {
> > > +     if (fw_devlink_flags && !fw_devlink_is_permissive() && dev->fwnode) {
> > >               error = device_create_file(dev, &dev_attr_waiting_for_supplier);
> > >               if (error)
> > >                       goto err_remove_dev_online;
> > > --
> > > 2.29.2.454.gaff20da3a2-goog
> > >
> > >
>

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