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

From: Maxime Ripard
Date: Thu Mar 13 2025 - 07:55:41 EST

Hi,

On Thu, Mar 13, 2025 at 04:09:54PM +0800, Andy Yan wrote:
> At 2025-03-05 19:55:19, "Andy Yan" <andyshrk@xxxxxxx> 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 ?
> >
> >
> >
> >[0]https://lore.kernel.org/linux-rockchip/047EECFC-7E55-44EC-896F-13FE04333E4D@xxxxxxxxx/T/#m25bc53b79f5cc7bddfcb7aae5656f68df396f094
> >>+
> >>+ drm_connector_list_iter_begin(dev, &conn_iter);
> >>+ drm_for_each_connector_iter(connector, &conn_iter) {
> >>+ if (!connector->state)
> >>+ continue;
> >>+
> >>+ if (encoder == connector->state->best_encoder) {
> >>+ out_connector = connector;
>
>
> When try to use this patch in my bridge driver, I found that the connector->state->best_encoder
> maybe NULL when drm_bridge_funcs.detect or drm_bridge_funcs.detect_ctx is called:
>
> [ 52.713030] Invalid return value -22 for connector detection
> [ 52.713539] WARNING: CPU: 7 PID: 288 at drivers/gpu/drm/drm_probe_helper.c:602 drm_helper_probe_single_connector_modes+0x5e0/
> 0x63c
> [ 52.714568] Modules linked in:
>
> [ 52.724546] Call trace:
> [ 52.724762] drm_helper_probe_single_connector_modes+0x5e0/0x63c (P)
> [ 52.725319] drm_mode_getconnector+0x2a4/0x488
> [ 52.725711] drm_ioctl_kernel+0xb4/0x11c
> [ 52.726057] drm_ioctl+0x22c/0x544
> [ 52.726358] __arm64_sys_ioctl+0xac/0xe0
> [ 52.726706] invoke_syscall+0x44/0x100
> [ 52.727039] el0_svc_common.constprop.0+0x3c/0xd4
>
> This is because best_encoder is set by set_best_encoder, which is called from
> drm_atomic_helper_check_modeset. When we call drm_mode_getconnector
> for the first time, the functions mentioned above have not been called yet,
> then we can't match the encoder from connector->state->best_encoder for this case.

As far as I'm concerned, it's by design. Encoders and connectors have
1:N relationship, and only once a connector has been enabled it has an
encoder.

If the connector is disabled, there's no associated encoder.

Maxime

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