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

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

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@linux.intel.com> 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@linux.intel.com> 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