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

["Andy Yan" <andyshrk@163.com>]

Hello，

At 2025-03-19 03:00:29, "Dmitry Baryshkov" <dmitry.baryshkov@oss.qualcomm.com> wrote:
>On Tue, Mar 18, 2025 at 04:51:19PM +0100, Maxime Ripard wrote:
>> On Fri, Mar 14, 2025 at 08:28:22PM +0200, Dmitry Baryshkov wrote:
>> > On Fri, Mar 14, 2025 at 06:40:24PM +0100, Maxime Ripard wrote:
>> > > 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?
>> > 
>> > Yes. But you can not get the connector by following bridge chain. Well,
>> > unless you include encoder into the chain. If that's what you have had
>> > in mind, then please excuse me, I didn't understand that from the
>> > beginning.
>> 
>> You can't include the encoder either, because the encoder doesn't have a
>> connector assigned yet at that time.
>> 
>> However, you can:
>> 
>>   - Store the bridge attach flags in drm_bridge
>> 
>>   - Create a hook that returns the connector a bridge creates, depending
>>     on the attach flags.
>> 
>>   - Create a helper that iterates over the next bridges until the
>>     previous hook returns !NULL. If it doesn't find anything, return
>>     NULL.
>> 
>> AFAIK, it solves all the problems being discussed here, while dealing
>> with legacy and new-style bridge drivers.
>
>I'm still fail to understand how does that solve the issue for new-style
>bridges. How do we find the created drm_bridge_connector for them?
>
>> 
>> > But frankly speaking, I think it might be easier to pass down the
>> > connector to the detect callback (as drm_connector_funcs.detect already
>> > gets the connecor) rather than making bridge drivers go through the
>> > chain to get the value that is already present in the caller function.
>> > 
>> > (For some other usecases I'd totally agree with you, especially if the
>> > connector isn't already available on the caller side).
>> 
>> Still, we've tried to converge to the same API for all entities, it
>> feels like a step backward to me.
>
>I'd argue here a bit. The drm_connector interface has connector here.
>drm_bridge is an extension/subpart of the drm_connector, so it would be
>logical to extend that interface.

Yes, since all the calls to drm_bridge_funcs.detect are initiated by the connecter, 
I think it is reasonable to pass this connecter to the corresponding bridge.
And this can prevent iterates failures caused by various complex  bridge connection logic changes.

>
>
>-- 
>With best wishes
>Dmitry