linux-kernel - Re: [PATCH 2/4] of: property: Do not link to disabled devices

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Date:   Wed, 15 Apr 2020 11:30:25 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] of: property: Do not link to disabled devices

On Wed, Apr 15, 2020 at 8:06 AM Nicolas Saenz Julienne
<nsaenzjulienne@...e.de> wrote:
>
> When creating a consumer/supplier relationship between two devices, make
> sure the supplier node is actually active. Otherwise this will create a
> device link that will never be fulfilled. This, in the worst case
> scenario, will hang the system during boot.
>
> Note that, in practice, the fact that a device-tree represented
> consumer/supplier relationship isn't fulfilled will not prevent devices
> from successfully probing.
>
> Fixes: a3e1d1a7f5fc ("of: property: Add functional dependency link from DT bindings")
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> ---
>  drivers/of/property.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/drivers/of/property.c b/drivers/of/property.c
> index a8c2b13521b27..487685ff8bb19 100644
> --- a/drivers/of/property.c
> +++ b/drivers/of/property.c
> @@ -1052,6 +1052,13 @@ static int of_link_to_phandle(struct device *dev, struct device_node *sup_np,
>                 return -ENODEV;
>         }
>
> +       /* Don't allow linking a device node as consumer of a disabled node */
> +       if (!of_device_is_available(sup_np)) {
> +               dev_dbg(dev, "Not linking to %pOFP - Not available\n", sup_np);
> +               of_node_put(sup_np);
> +               return -ENODEV;
> +       }
> +

Again, surprised I haven't hit this situation with the number of
disabled devices I have.

The idea is right, but the implementation can be better. I think this
check needs to be the first check after the of_node_get(sup_np) --
before we do any of the "walk up to find the device" part.

Otherwise, you could have a supplier device (the one with compatible
prop) that's available with a child node that's disabled. And the
phandle could be pointing to that disabled child node. If you don't do
this as the first check, you might still try to form a pointless
device link. It won't affect probing (because the actual struct device
will probe) but it's still a pointless device link and a pointless
delay in probing, etc.

-Saravana