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Message-ID: <k2md4q2tgwsdynaeljieqlzjdds677wqqduimfyrgmvyfi732a@vh3dbg7kdaop>
Date: Fri, 14 Mar 2025 07:52:35 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Andy Yan <andyshrk@....com>
Cc: Maxime Ripard <mripard@...nel.org>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Thomas Zimmermann <tzimmermann@...e.de>,
David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
Andrzej Hajda <andrzej.hajda@...el.com>,
Neil Armstrong <neil.armstrong@...aro.org>,
Robert Foss <rfoss@...nel.org>,
Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
Jonas Karlman <jonas@...boo.se>,
Jernej Skrabec <jernej.skrabec@...il.com>,
Douglas Anderson <dianders@...omium.org>,
Herve Codina <herve.codina@...tlin.com>,
dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
Simona Vetter <simona.vetter@...ll.ch>, lumag@...nel.org
Subject: Re: [PATCH v5 04/16] drm/atomic: Introduce helper to lookup
connector by encoder
On Fri, Mar 14, 2025 at 08:50:29AM +0800, Andy Yan wrote:
>
> Hi Maxime and Simona,
>
>
> At 2025-03-13 19:55:33, "Maxime Ripard" <mripard@...nel.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@....com> wrote:
> >> >At 2025-03-04 19:10:47, "Maxime Ripard" <mripard@...nel.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@...aro.org>
> >> >>Co-developed-by: Simona Vetter <simona.vetter@...ll.ch>
> >> >>Signed-off-by: Maxime Ripard <mripard@...nel.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.
--
With best wishes
Dmitry
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