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

[Maxime Ripard <mripard@kernel.org>]

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On Thu, Mar 06, 2025 at 09:16:24AM +0800, Andy Yan wrote:
> 
> Hi Maxime and Dmitry:
> 
> At 2025-03-06 04:13:53, "Dmitry Baryshkov" <dmitry.baryshkov@linaro.org> 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@kernel.org> 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@linaro.org>
> >> > >Co-developed-by: Simona Vetter <simona.vetter@ffwll.ch>
> >> > >Signed-off-by: Maxime Ripard <mripard@kernel.org>
> >> > >---
> >> > > 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

I don't disagree on principle, but I think a better first step would be
to provide a detect_ctx hook to bridges.

Maxime

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