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

[Andy Yan]

Hi Maxime,

At 2025-03-05 19:55:19, "Andy Yan" <andyshrk@xxxxxxx> wrote:
>
>
>Hi Maxime,
>
>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.



>>+			break;
>>+		}
>>+	}
>>+	drm_connector_list_iter_end(&conn_iter);
>>+	drm_modeset_unlock(&dev->mode_config.connection_mutex);
>>+
>>+	return out_connector;
>>+}
>>+EXPORT_SYMBOL(drm_atomic_get_connector_for_encoder);
>>+
>>+
>> /**