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

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Message-ID: <55611d3e028b6ea418cba1ef9d94fe7bf1e1b1fd.camel@suse.de>
Date:   Thu, 16 Apr 2020 13:37:48 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Saravana Kannan <saravanak@...gle.com>
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, 2020-04-15 at 11:30 -0700, Saravana Kannan wrote:
> 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.

I'll point out to the example that triggered this issue on my reply to patch
#4.

> 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.

Agree, I'll update the patch.

Regards,
Nicolas


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