[v1,2/2] of: property: Improve cycle detection when one of the devices is never added

Consider this example where -> means LHS device is a consumer of RHS
device and indentation represents "child of" of the previous device.

Device A -> Device C

Device B -> Device A
	Device C

Without this commit:
1. Device A is added.
2. Device A is added to waiting for supplier list (Device C)
3. Device B is added
4. Device B is linked as a consumer to Device A
5. Device A doesn't probe because it's waiting for Device C to be added.
6. Device B doesn't probe because Device A hasn't probed.
7. Device C will never be added because it's parent hasn't probed.

So, Device A, B and C will be in a probe/add deadlock.

This commit detects this scenario and stops trying to create a device
link between Device A and Device C since doing so would create a cycle:
Device A -> Devic C -(parent)-> Device B -> Device A.

With this commit:
1. Device A is added.
3. Device B is added
4. Device B is linked as a consumer to Device A
5. Device A probes.
6. Device B probes because Device A has probed.
7. Device C is added and probed.

Signed-off-by: Saravana Kannan <saravanak@google.com>
---
 drivers/of/property.c | 44 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 38 insertions(+), 6 deletions(-)

On Fri, Jun 5, 2020 at 5:36 PM Saravana Kannan <saravanak@google.com> wrote:
>
> Consider this example where -> means LHS device is a consumer of RHS
> device and indentation represents "child of" of the previous device.
>
> Device A -> Device C
>
> Device B -> Device A
>         Device C
>
> Without this commit:
> 1. Device A is added.
> 2. Device A is added to waiting for supplier list (Device C)
> 3. Device B is added
> 4. Device B is linked as a consumer to Device A
> 5. Device A doesn't probe because it's waiting for Device C to be added.
> 6. Device B doesn't probe because Device A hasn't probed.
> 7. Device C will never be added because it's parent hasn't probed.
>
> So, Device A, B and C will be in a probe/add deadlock.
>
> This commit detects this scenario and stops trying to create a device
> link between Device A and Device C since doing so would create a cycle:
> Device A -> Devic C -(parent)-> Device B -> Device A.
>
> With this commit:
> 1. Device A is added.
> 3. Device B is added
> 4. Device B is linked as a consumer to Device A
> 5. Device A probes.
> 6. Device B probes because Device A has probed.
> 7. Device C is added and probed.
>
> Signed-off-by: Saravana Kannan <saravanak@google.com>
> ---
>  drivers/of/property.c | 44 +++++++++++++++++++++++++++++++++++++------
>  1 file changed, 38 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/of/property.c b/drivers/of/property.c
> index 1f2086f4e7ce..7eebe21274a4 100644
> --- a/drivers/of/property.c
> +++ b/drivers/of/property.c
> @@ -1014,6 +1014,20 @@ static bool of_is_ancestor_of(struct device_node *test_ancestor,
>         return false;
>  }
>
> +static struct device *of_get_next_parent_dev(struct device_node *np)
> +{
> +       struct device *dev = NULL;
> +
> +       of_node_get(np);
> +       do {
> +               np = of_get_next_parent(np);
> +               if (np)
> +                       dev = get_dev_from_fwnode(&np->fwnode);
> +       } while (np && !dev);
> +       of_node_put(np);
> +       return dev;
> +}
> +
>  /**
>   * of_link_to_phandle - Add device link to supplier from supplier phandle
>   * @dev: consumer device
> @@ -1035,10 +1049,9 @@ static bool of_is_ancestor_of(struct device_node *test_ancestor,
>  static int of_link_to_phandle(struct device *dev, struct device_node *sup_np,
>                               u32 dl_flags)
>  {
> -       struct device *sup_dev;
> +       struct device *sup_dev, *sup_par_dev;
>         int ret = 0;
>         struct device_node *tmp_np = sup_np;
> -       int is_populated;
>
>         of_node_get(sup_np);
>         /*
> @@ -1075,16 +1088,35 @@ static int of_link_to_phandle(struct device *dev, struct device_node *sup_np,
>                 return -EINVAL;
>         }
>         sup_dev = get_dev_from_fwnode(&sup_np->fwnode);
> -       is_populated = of_node_check_flag(sup_np, OF_POPULATED);
> -       of_node_put(sup_np);
> -       if (!sup_dev && is_populated) {
> +       if (!sup_dev && of_node_check_flag(sup_np, OF_POPULATED)) {
>                 /* Early device without struct device. */
>                 dev_dbg(dev, "Not linking to %pOFP - No struct device\n",
>                         sup_np);
> +               of_node_put(sup_np);
>                 return -ENODEV;
>         } else if (!sup_dev) {
> -               return -EAGAIN;
> +               sup_par_dev = of_get_next_parent_dev(sup_np);
> +               of_node_put(sup_np);
> +
> +               /*
> +                * DL_FLAG_SYNC_STATE_ONLY doesn't block probing, so cycle
> +                * detection isn't necessary and shouldn't be done.
> +                */
> +               if (dl_flags & DL_FLAG_SYNC_STATE_ONLY)
> +                       return -EAGAIN;

I need to put_device(sup_par_dev) before I return here and at other places
below. I'll send a v2 later to fix this.


-Saravana

> +
> +               /*
> +                * If devices haven't been created for any of the ancestors, we
> +                * can't check for cycles. So let's try again later.
> +                */
> +               if (!sup_par_dev)
> +                       return -EAGAIN;
> +
> +               /* Cyclic dependency detected, don't try to link */
> +               if (device_is_dependent(dev, sup_par_dev))
> +                       return -EINVAL;
>         }
> +       of_node_put(sup_np);
>         if (!device_link_add(dev, sup_dev, dl_flags))
>                 ret = -EINVAL;
>         put_device(sup_dev);
> --
> 2.27.0.278.ge193c7cf3a9-goog
>