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

[Maxime Ripard <mripard@kernel.org>]

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On Fri, Mar 14, 2025 at 09:59:36AM +0200, Dmitry Baryshkov wrote:
> On Fri, Mar 14, 2025 at 08:45:17AM +0100, Maxime Ripard wrote:
> > On Fri, Mar 14, 2025 at 07:52:35AM +0200, Dmitry Baryshkov wrote:
> > > On Fri, Mar 14, 2025 at 08:50:29AM +0800, Andy Yan wrote:
> > > > At 2025-03-13 19:55:33, "Maxime Ripard" <mripard@kernel.org> wrote:
> > > > >Hi,
> > > > >
> > > > >On Thu, Mar 13, 2025 at 04:09:54PM +0800, Andy Yan wrote:
> > > > >> At 2025-03-05 19:55:19, "Andy Yan" <andyshrk@163.com> 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 ?
> > > > >> >
> > > > >> >
> > > > >> >
> > > > >> >[0]https://lore.kernel.org/linux-rockchip/047EECFC-7E55-44EC-896F-13FE04333E4D@gmail.com/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.
> > > > 
> > > > Does this prove that this API is not suitable for my application scenario: 
> > > > Get the connector in the bridge's .detect callback, so this means that I may
> > > > still need to modify the bridge's connector callback so that it can pass the connector ?
> > > 
> > > I'd say, yes, please.
> > 
> > And I'd say no :)
> 
> Fair enough :-)
> 
> > There's no reason to deviate from the API other entities have here. It's
> > just that the switch to DRM_BRIDGE_ATTACH_NO_CONNECTOR hasn't been
> > completely thought through and it's one of the part where it shows.
> > 
> > We have two alternative solutions: Either the driver creates the
> > connector itself, since it doesn't seem to use any downstream bridge
> > anyway, or we need a new bridge helper to find the connector on a bridge
> > chain.
> > 
> > We have the iterator already, we just need a new accessor to retrieve
> > the (optional) connector of a bridge, and if there's none, go to the
> > next bridge and try again.
> 
> The problem is that there is no guarantee that the the created connector
> is created for or linked to any bridge. For example, for msm driver I'm
> waiting for several series to go in, but after that I plan to work on
> moving connector creation to the generic code within the msm driver.
> 
> In other words, with DRM_BRIDGE_ATTACH_NO_CONNECTOR in place it is
> perfectly legit not to have a bridge which has "connector of a bridge".
> It is possible to create drm_bridge_connector on the drm_encoder's side
> after the drm_bridge_attach() succeeds.

Sure, but then I'd expect detect and get_modes to only be called *after*
that connector has been created, right?

Returning NULL in the case where we don't have a connector (yet?) in
such a case would make total sense to me, just like we return NULL if
the connector is disabled and doesn't have an encoder here.

Maxime

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