linux-kernel - Re: Adding depends-on DT binding to break cyclic dependencies

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Date:   Tue, 27 Aug 2019 13:18:43 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Frank Rowand <frowand.list@...il.com>
Cc:     "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: Adding depends-on DT binding to break cyclic dependencies

On Wed, Aug 21, 2019 at 11:54 PM Saravana Kannan <saravanak@...gle.com> wrote:
>
> Hi Rob,
>
> Frank, Greg and I got together during ELC and had an extensive and
> very productive discussion about my "postboot supplier state cleanup"
> patch series [1]. The three of us are on the same page now -- the
> series as it stands is the direction we want to go in, with some minor
> refactoring, documentation and naming changes.
>
> However, one of the things Frank is concerned about (and Greg and I
> agree) in the current patch series is that the "cyclic dependency
> breaking" logic has been pushed off to individual drivers using the
> edit_links() callback.
>
> The concern being, there are going to be multiple device specific ad
> hoc implementations to break a cyclic dependency. Also, if a device
> can be part of a cyclic dependency, the driver for that device has to
> check for specific system/products in which the device is part of a
> cyclic dependency (because it might not always be part of a cycle),
> and then potentially have cycle/product specific code to break the
> cycle (since the cycle can be different on each system/product).
>
> One way to avoid all of the device/driver specific code and simplify
> my patch series by a non-trivial amount would be by adding a
> "depends-on" DT binding that can ONLY be used to break cycles. We can
> document it as such and reject any attempts to use it for other
> purposes. When a depends-on property is present in a device node, that
> specific device's supplier list will be parsed ONLY from the
> depends-on property and the other properties won't be parsed for
> deriving dependency information for that device.
>
> Frank, Greg and I like this usage model for a new depends-on DT
> binding. Is this something you'd be willing to accept?
>
> Thanks,
> Saravana
>
> [1] - https://lore.kernel.org/lkml/20190731221721.187713-1-saravanak@google.com/

Friendly reminder.

-Saravana