Re: [RFC PATCH v1 01/12] Revert "drm/sysfs: Link DRM connectors to corresponding Type-C connectors"

[Dmitry Baryshkov]

On 06/09/2023 16:38, Heikki Krogerus wrote:
> On Wed, Sep 06, 2023 at 03:48:35PM +0300, Dmitry Baryshkov wrote:
> > On Wed, 6 Sept 2023 at 15:44, Heikki Krogerus
> > <heikki.krogerus@xxxxxxxxxxxxxxx> wrote:
> > > On Tue, Sep 05, 2023 at 01:56:59PM +0300, Dmitry Baryshkov wrote:
> > > > Hi Heikki,
> > > >
> > > > On Tue, 5 Sept 2023 at 11:50, Heikki Krogerus
> > > > <heikki.krogerus@xxxxxxxxxxxxxxx> wrote:
> > > > > Hi Dmitry,
> > > > >
> > > > > On Mon, Sep 04, 2023 at 12:41:39AM +0300, Dmitry Baryshkov wrote:
> > > > > > The kdev->fwnode pointer is never set in drm_sysfs_connector_add(), so
> > > > > > dev_fwnode() checks never succeed, making the respective commit NOP.
> > > > >
> > > > > That's not true. The dev->fwnode is assigned when the device is
> > > > > created on ACPI platforms automatically. If the drm_connector fwnode
> > > > > member is assigned before the device is registered, then that fwnode
> > > > > is assigned also to the device - see drm_connector_acpi_find_companion().
> > > > >
> > > > > But please note that even if drm_connector does not have anything in
> > > > > its fwnode member, the device may still be assigned fwnode, just based
> > > > > on some other logic (maybe in drivers/acpi/acpi_video.c?).
> > > > >
> > > > > > And if drm_sysfs_connector_add() is modified to set kdev->fwnode, it
> > > > > > breaks drivers already using components (as it was pointed at [1]),
> > > > > > resulting in a deadlock. Lockdep trace is provided below.
> > > > > >
> > > > > > Granted these two issues, it seems impractical to fix this commit in any
> > > > > > sane way. Revert it instead.
> > > > >
> > > > > I think there is already user space stuff that relies on these links,
> > > > > so I'm not sure you can just remove them like that. If the component
> > > > > framework is not the correct tool here, then I think you need to
> > > > > suggest some other way of creating them.
> > > >
> > > > The issue (that was pointed out during review) is that having a
> > > > component code in the framework code can lead to lockups. With the
> > > > patch #2 in place (which is the only logical way to set kdev->fwnode
> > > > for non-ACPI systems) probing of drivers which use components and set
> > > > drm_connector::fwnode breaks immediately.
> > > >
> > > > Can we move the component part to the respective drivers? With the
> > > > patch 2 in place, connector->fwnode will be copied to the created
> > > > kdev's fwnode pointer.
> > > >
> > > > Another option might be to make this drm_sysfs component registration optional.
> > >
> > > You don't need to use the component framework at all if there is
> > > a better way of determining the connection between the DP and its
> > > Type-C connector (I'm assuming that that's what this series is about).
> > > You just need the symlinks, not the component.
> >
> > The problem is that right now this component registration has become
> > mandatory. And if I set the kdev->fwnode manually (like in the patch
> > 2), the kernel hangs inside the component code.
> > That's why I proposed to move the components to the place where they
> > are really necessary, e.g. i915 and amd drivers.
>
> So why can't we replace the component with the method you are
> proposing in this series of finding out the Type-C port also with
> i915, AMD, or whatever driver and platform (that's the only thing that
> component is used for)?

The drm/msm driver uses drm_bridge for the pipeline (including the last 
DP entry) and the drm_bridge_connector to create the connector. I think 
that enabling i915 and AMD drivers to use drm_bridge fells out of scope 
for this series.


> Determining the connection between a DP and its Type-C connector is
> starting to get really important, so ideally we have a common solution
> for that.

Yes. This is what we have been discussing with Simon for quite some time 
on #dri-devel.

Unfortunately I think the solution that got merged was pretty much 
hastened in instead of being well-thought. For example, it is also not 
always possible to provide the drm_connector / typec_connector links (as 
you can see from the patch7. Sometimes we can only express that this is 
a Type-C DP connector, but we can not easily point it to the particular 
USB-C port.

So, I'm not sure, how can we proceed here. Currently merged patch breaks 
drm/msm if we even try to use it by setting kdef->fwnode to 
drm_connector->fwnode. The pointed out `drivers/usb/typec/port-mapper.c` 
is an ACPI-only thing, which is not expected to work in a non-ACPI cases.

--
With best wishes
Dmitry