linux-kernel - Re: [PATCH v2 2/2] of: property: Improve cycle detection when one of the devices is never added

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Date:   Wed, 17 Jun 2020 16:12:21 -0600
From:   Rob Herring <robh@...nel.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Frank Rowand <frowand.list@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, kernel-team@...roid.com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH v2 2/2] of: property: Improve cycle detection when one of
 the devices is never added

On Tue, 09 Jun 2020 18:19:34 -0700, Saravana Kannan 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 the
> following 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@...gle.com>
> ---
>  drivers/of/property.c | 62 ++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 56 insertions(+), 6 deletions(-)
> 

Both patches applied.

Rob