Re: [PATCH] drm/msm: Initialize mode_config earlier

[Johan Hovold <johan@kernel.org>]

On Thu, Mar 02, 2023 at 03:17:04PM -0800, Bjorn Andersson wrote:
> On Wed, Mar 01, 2023 at 02:58:50PM +0100, Johan Hovold wrote:

> > So after debugging this issue a third time, I can conclude that it is
> > still very much present in 6.2.
> > 
> > It appears you looked at the linux-next tree when you concluded that
> > this patch was not needed. In 6.2 the bridge->hpd_cb callback is set
> > before mode_config.funcs is initialised as part of
> > kms->funcs->hw_init(kms).
> > 
> > The hpd DRM changes heading into 6.3 do appear to avoid the NULL-pointer
> > dereference by moving the bridge->hpd_cb initialisation to
> > drm_kms_helper_poll_init() as you mention above.

I can confirm that as expected my reproducer no longer triggers with
6.3-rc1.
 
> > The PMIC GLINK altmode driver still happily forwards notifications
> > regardless of the DRM driver state though, which can lead to missed
> > hotplug events. It seems you need to implement the
> > hpd_enable()/disable() callbacks and either cache or not enable events
> > in fw until the DRM driver is ready.
> > 
> 
> It's not clear to me what the expectation from the DRM framework is on
> this point. We register a drm_bridge which is only capable of signaling
> HPD events (DRM_BRIDGE_OP_HPD), not querying HPD state (DRM_BRIDGE_OP_DETECT).

I think the assumption is that any bridge that can generate hotplug
events also has a way of detecting whether it is connected (i.e.
DRM_BRIDGE_OP_HPD => DRM_BRIDGE_OP_DETECT).

The pmic_glink_altmode driver appears to be the only driver that sets
DRM_BRIDGE_OP_HPD but not DRM_BRIDGE_OP_DETECT.

> Does this imply that any such bridge must ensure that hpd events are
> re-delivered once hpd_enable() has been invoked (we can't invoke it from
> hpd_enable...)?
> 
> Is it reasonable to do this retriggering in the altmode driver? Or is it
> the job of the TCPM (it seems reasonable to not send the PAN_EN message
> until we get hpd_enable()...)?

Are you sure there is no way to query the firmware about the connected
state?

Otherwise, enabling the notification messages when hpd_enable() is
called looks like it should work as the fw currently appears to always
send a disconnected event followed by a connect event if connected.

But that's not going to be enough unless you can also disable events in
fw on hpd_disable() so that the state can again be updated on the next
hpd_enable().

If that's not possible, it seems you need to cache the state in the
driver and hope you get a notification after a suspend cycle if the
state has changed.

But in any case, the DRM documentation is pretty clear on that a bridge
driver should not be calling drm_bridge_hpd_notify() until hpd_enable()
is called (and also not after hpd_disable()) as the pmic_glink_altmode
driver currently do.

	hpd_enable

	Enable hot plug detection. From now on the bridge shall call
	drm_bridge_hpd_notify() each time a change is detected in the
	output connection status, until hot plug detection gets disabled
	with hpd_disable.

	This callback is optional and shall only be implemented by
	bridges that support hot-plug notification without polling.
	Bridges that implement it shall also implement the hpd_disable
	callback and set the DRM_BRIDGE_OP_HPD flag in their
	drm_bridge->ops.

	https://dri.freedesktop.org/docs/drm/gpu/drm-kms-helpers.html#c.drm_bridge_funcs

Johan