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

[Dmitry Baryshkov]

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().

-- 
With best wishes
Dmitry