linux-kernel - Re: [PATCH v2 08/17] driver core: Add fwnode link support

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Date:   Mon, 7 Dec 2020 11:25:03 -0800
From:   Saravana Kannan <saravanak@...gle.com>
To:     Leon Romanovsky <leon@...nel.org>
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>,
        Android Kernel Team <kernel-team@...roid.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 08/17] driver core: Add fwnode link support

On Sat, Dec 5, 2020 at 11:48 PM Leon Romanovsky <leon@...nel.org> wrote:
>
> On Fri, Nov 20, 2020 at 06:02:23PM -0800, Saravana Kannan wrote:
> > Add support for creating supplier-consumer links between fwnodes.  It is
> > intended for internal use the driver core and generic firmware support
> > code (eg. Device Tree, ACPI), so it is simple by design and the API
> > provided is limited.
> >
> > Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> > ---
> >  drivers/base/core.c    | 98 ++++++++++++++++++++++++++++++++++++++++++
> >  drivers/of/dynamic.c   |  1 +
> >  include/linux/fwnode.h | 14 ++++++
> >  3 files changed, 113 insertions(+)
> >
> > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > index 401fa7e3505c..e2b246a44d1a 100644
> > --- a/drivers/base/core.c
> > +++ b/drivers/base/core.c
> > @@ -50,6 +50,104 @@ static LIST_HEAD(wait_for_suppliers);
> >  static DEFINE_MUTEX(wfs_lock);
> >  static LIST_HEAD(deferred_sync);
> >  static unsigned int defer_sync_state_count = 1;
> > +static DEFINE_MUTEX(fwnode_link_lock);
> > +
> > +/**
> > + * fwnode_link_add - Create a link between two fwnode_handles.
> > + * @con: Consumer end of the link.
> > + * @sup: Supplier end of the link.
> > + *
> > + * Create a fwnode link between fwnode handles @con and @sup. The fwnode link
> > + * represents the detail that the firmware lists @sup fwnode as supplying a
> > + * resource to @con.
> > + *
> > + * The driver core will use the fwnode link to create a device link between the
> > + * two device objects corresponding to @con and @sup when they are created. The
> > + * driver core will automatically delete the fwnode link between @con and @sup
> > + * after doing that.
> > + *
> > + * Attempts to create duplicate links between the same pair of fwnode handles
> > + * are ignored and there is no reference counting.
>
> Sorry to ask, but why is that?
> Isn't this a programmer error?

No, not a programmer error.

One firmware node can point to the same supplier many times. For
example, the consumer can be using multiple clocks from the same
supplier clock controller. In the context of fw_devlink, there's no
reason to keep track of each clock dependency separately because we'll
be creating only one device link from fwnode link. So multiple fwnode
link attempts between the same two devices are just treated as one
instance of dependency. I hope that clarifies things.

-Saravana