Message-ID: <bed619c7-1a82-4328-825c-117c2ee3639d@ideasonboard.com>
Date: Fri, 27 Sep 2024 09:41:00 +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

On 27/09/2024 02:26, Dmitry Baryshkov wrote:
> On Thu, Sep 26, 2024 at 02:52:35PM GMT, Tomi Valkeinen wrote:
>> 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.
> 
> I think you have the NOT_DEVICE case as the DSS device corresponds to
> the parent of your OLDI nodes. There is no device which corresponds to
> the oldi@0 / oldi@1 device nodes (which contain corresponding port
> nodes).

Do you mean that I should already see FWNODE_FLAG_NOT_DEVICE set in the 
fwnode?

If I print information about the relevant fwnodes (from dss up to the 
oldi endpoints) in the DSS driver's probe, I see that none have 
FWNODE_FLAG_NOT_DEVICE set, all have FWNODE_FLAG_LINKS_ADDED set, and 
only the main DSS node has the fwnode->dev set (to 30200000.dss).

  Tomi

>>
>>> 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.
> 

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