linux-kernel - Re: fw_devlinks preventing a panel driver from probing

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Message-ID: <5a540bdb-e3ca-494a-b68d-8f81f4d1cc1a@ideasonboard.com>
Date: Thu, 26 Sep 2024 14:52:35 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Linux Kernel List <linux-kernel@...r.kernel.org>,
 Saravana Kannan <saravanak@...gle.com>,
 Aradhya Bhatia <aradhya.bhatia@...ux.dev>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 Devarsh Thakkar <devarsht@...com>
Subject: Re: fw_devlinks preventing a panel driver from probing

Hi,

On 21/09/2024 23:15, Dmitry Baryshkov wrote:
> On Mon, Sep 16, 2024 at 02:51:57PM GMT, Tomi Valkeinen wrote:
>> Hi,
>>
>> We have an issue where two devices have dependencies to each other,
>> according to drivers/base/core.c's fw_devlinks, and this prevents them from
>> probing. I've been adding debugging to the core.c, but so far I don't quite
>> grasp the issue, so I thought to ask. Maybe someone can instantly say that
>> this just won't work...
> 
> Well, just 2c from my side. I consider that fw_devlink adds devlinks for
> of-graph nodes to be a bug. It doesn't know about the actual direction
> of dependencies between corresponding devices or about the actual
> relationship between drivers. It results in a loop which is then broken
> in some way. Sometimes this works. Sometimes it doesn't. Sometimes this
> hides actual dependencies between devices. I tried reverting offending
> parts of devlink, but this attempt failed.

I was also wondering about this. The of-graphs are always two-way links, 
so the system must always mark them as a cycle. But perhaps there are 
other benefits in the devlinks than dependency handling?

>> If I understand the fw_devlink code correctly, in a normal case the links
>> formed with media graphs are marked as a cycle (FWLINK_FLAG_CYCLE), and then
>> ignored as far as probing goes.
>>
>> What we see here is that when using a single-link OLDI panel, the panel
>> driver's probe never gets called, as it depends on the OLDI, and the link
>> between the panel and the OLDI is not a cycle.
> 
> I think in your case you should be able to fix the issue by using the
> FWNODE_FLAG_NOT_DEVICE, which is intented to be used in such cases. You

How would I go using FWNODE_FLAG_NOT_DEVICE? Won't this only make a 
difference if the flag is there at early stage when the linux devices 
are being created? I think it's too late if I set the flag when the dss 
driver is being probed.

> have a dependency on DT node which doesn't have backing device.

Well, there is a backing device, the DSS. But if you mean that the 
system at the moment cannot figure out that there is a backing device, 
then true.

  Tomi