Re:Re: [PATCH v5 04/16] drm/atomic: Introduce helper to lookup connector by encoder

[Andy Yan]

Hi Maxime and Dmitry:

At 2025-03-06 04:13:53, "Dmitry Baryshkov" <dmitry.baryshkov@xxxxxxxxxx> wrote:
>On Wed, Mar 05, 2025 at 02:19:36PM +0100, Maxime Ripard wrote:
>> Hi Andy,
>> 
>> On Wed, Mar 05, 2025 at 07:55:19PM +0800, Andy Yan wrote:
>> > At 2025-03-04 19:10:47, "Maxime Ripard" <mripard@xxxxxxxxxx> wrote:
>> > >With the bridges switching over to drm_bridge_connector, the direct
>> > >association between a bridge driver and its connector was lost.
>> > >
>> > >This is mitigated for atomic bridge drivers by the fact you can access
>> > >the encoder, and then call drm_atomic_get_old_connector_for_encoder() or
>> > >drm_atomic_get_new_connector_for_encoder() with drm_atomic_state.
>> > >
>> > >This was also made easier by providing drm_atomic_state directly to all
>> > >atomic hooks bridges can implement.
>> > >
>> > >However, bridge drivers don't have a way to access drm_atomic_state
>> > >outside of the modeset path, like from the hotplug interrupt path or any
>> > >interrupt handler.
>> > >
>> > >Let's introduce a function to retrieve the connector currently assigned
>> > >to an encoder, without using drm_atomic_state, to make these drivers'
>> > >life easier.
>> > >
>> > >Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
>> > >Co-developed-by: Simona Vetter <simona.vetter@xxxxxxxx>
>> > >Signed-off-by: Maxime Ripard <mripard@xxxxxxxxxx>
>> > >---
>> > > drivers/gpu/drm/drm_atomic.c | 45 ++++++++++++++++++++++++++++++++++++++++++++
>> > > include/drm/drm_atomic.h     |  3 +++
>> > > 2 files changed, 48 insertions(+)
>> > >
>> > >diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
>> > >index 9ea2611770f43ce7ccba410406d5f2c528aab022..b926b132590e78f8d41d48eb4da4bccf170ee236 100644
>> > >--- a/drivers/gpu/drm/drm_atomic.c
>> > >+++ b/drivers/gpu/drm/drm_atomic.c
>> > >@@ -985,10 +985,55 @@ drm_atomic_get_new_connector_for_encoder(const struct drm_atomic_state *state,
>> > > 
>> > > 	return NULL;
>> > > }
>> > > EXPORT_SYMBOL(drm_atomic_get_new_connector_for_encoder);
>> > > 
>> > >+/**
>> > >+ * drm_atomic_get_connector_for_encoder - Get connector currently assigned to an encoder
>> > >+ * @encoder: The encoder to find the connector of
>> > >+ * @ctx: Modeset locking context
>> > >+ *
>> > >+ * This function finds and returns the connector currently assigned to
>> > >+ * an @encoder.
>> > >+ *
>> > >+ * Returns:
>> > >+ * The connector connected to @encoder, or an error pointer otherwise.
>> > >+ * When the error is EDEADLK, a deadlock has been detected and the
>> > >+ * sequence must be restarted.
>> > >+ */
>> > >+struct drm_connector *
>> > >+drm_atomic_get_connector_for_encoder(const struct drm_encoder *encoder,
>> > >+				     struct drm_modeset_acquire_ctx *ctx)
>> > >+{
>> > >+	struct drm_connector_list_iter conn_iter;
>> > >+	struct drm_connector *out_connector = ERR_PTR(-EINVAL);
>> > >+	struct drm_connector *connector;
>> > >+	struct drm_device *dev = encoder->dev;
>> > >+	int ret;
>> > >+
>> > >+	ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
>> > >+	if (ret)
>> > >+		return ERR_PTR(ret);
>> > 
>> > It seems that this will cause a deadlock when called from a hotplug
>> > handling path, I have a WIP DP diver[0], which suggested by Dmitry to
>> > use this API from a &drm_bridge_funcs.detect callback to get the
>> > connector, as detect is called by drm_helper_probe_detect, which will
>> > hold connection_mutex first, so the deaklock happens:
>> >
>> > drm_helper_probe_detect(struct drm_connector *connector,
>> >                         struct drm_modeset_acquire_ctx *ctx,
>> >                         bool force)
>> > {
>> >         const struct drm_connector_helper_funcs *funcs = connector->helper_private;
>> >         struct drm_device *dev = connector->dev;
>> >         int ret;
>> > 
>> >         if (!ctx)
>> >                 return drm_helper_probe_detect_ctx(connector, force);
>> > 
>> >         ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
>> >         if (ret)
>> >                 return ret;
>> > 
>> >         if (funcs->detect_ctx)
>> >                 ret = funcs->detect_ctx(connector, ctx, force);
>> >         else if (connector->funcs->detect)
>> >                 ret = connector->funcs->detect(connector, force);
>> >         else
>> >                 ret = connector_status_connected;
>> > 
>> >         if (ret != connector->status)
>> >                 connector->epoch_counter += 1;
>> > 
>> > So I wonder can we let drm_bridge_funcs.detect pass a connector for
>> > this case ?
>> 
>> Do you actually see a deadlock occurring? AFAIK, drm_modeset_lock is
>> fine with reentrancy from the same context, so it should work just fine.
>
>Andy, that probably means that you should use .detect_ctx() and pass the
>context to drm_atomic_get_connector_for_encoder().

Unfortunately, the drm_bridge_funcs does not have a .detect_ctx()  version .
The call chain is:
 drm_helper_probe_detect 
 --> drm_bridge_connector_detect(struct drm_connector *connector, bool force)
--> drm_bridge_funcs.detect(bridge)
The ctx got dropped when drm_helper_probe_detect call  drm_bridge_connector_detect
The connector got dropped  when connector call it's bridege.detect

So I think the simplest solution is to have drm_bridge_funcs.detect directly pass the connector

>
>-- 
>With best wishes
>Dmitry