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

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Message-ID: <il3s26owmtyf3knniv7klkmw3uuz4dffrump7jo47dk6hxdsya@v5plmtjcbukf>
Date: Fri, 27 Sep 2024 02:26:00 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
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 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).

> 
> > 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.

-- 
With best wishes
Dmitry