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

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Message-ID: <CAA8EJprDdst-mcwMsWs=0AHGCNa_5Ng90tubSJ7VAHamx2T93g@mail.gmail.com>
Date: Fri, 27 Sep 2024 10:35:25 +0200
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 Fri, 27 Sept 2024 at 08:41, Tomi Valkeinen
<tomi.valkeinen@...asonboard.com> wrote:
>
> 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?

No, I think you should set it for you DSS links. If I understand
correctly, this should prevent fwdevlink from waiting on the OLDI to
materialize as a device.
Note: my understanding is based on a quick roaming through the code
some time ago.

>
> 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.
> >
>


-- 
With best wishes
Dmitry