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Message-ID: <4df0c7d6.5baa.19583f0f14d.Coremail.andyshrk@163.com>
Date: Tue, 11 Mar 2025 14:42:40 +0800 (CST)
From: "Andy Yan" <andyshrk@....com>
To: "Simona Vetter" <simona.vetter@...ll.ch>
Cc: "Maxime Ripard" <mripard@...nel.org>,
"Dmitry Baryshkov" <dmitry.baryshkov@...aro.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
Subject: Re:Re: Re:Re: [PATCH v5 04/16] drm/atomic: Introduce helper to
lookup connector by encoder
Hi Simona and Laurent,
At 2025-03-07 21:25:02, "Simona Vetter" <simona.vetter@...ll.ch> wrote:
>On Fri, Mar 07, 2025 at 03:30:41PM +0800, Andy Yan wrote:
>>
>> Hi All,
>> At 2025-03-07 09:08:48, "Andy Yan" <andyshrk@....com> wrote:
>> >Hi All,
>> >
>> >At 2025-03-06 23:41:24, "Simona Vetter" <simona.vetter@...ll.ch> wrote:
>> >>On Thu, Mar 06, 2025 at 08:10:16AM +0100, Maxime Ripard wrote:
>> >>> On Thu, Mar 06, 2025 at 09:16:24AM +0800, Andy Yan wrote:
>> >>> >
>> >>> > Hi Maxime and Dmitry:
>> >>> >
>> >>> > At 2025-03-06 04:13:53, "Dmitry Baryshkov" <dmitry.baryshkov@...aro.org> wrote:
>> >>> > >On Wed, Mar 05, 2025 at 02:19:36PM +0100, Maxime Ripard wrote:
>> >>> > >> Hi Andy,
>> >>> > >>
>> >>> > >> On Wed, Mar 05, 2025 at 07:55:19PM +0800, Andy Yan 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 ?
>> >>> > >>
>> >>> > >> Do you actually see a deadlock occurring? AFAIK, drm_modeset_lock is
>> >>> > >> fine with reentrancy from the same context, so it should work just fine.
>> >>> > >
>> >>> > >Andy, that probably means that you should use .detect_ctx() and pass the
>> >>> > >context to drm_atomic_get_connector_for_encoder().
>> >>> >
>> >>> > Unfortunately, the drm_bridge_funcs does not have a .detect_ctx() version .
>> >>> > The call chain is:
>> >>> > drm_helper_probe_detect
>> >>> > --> drm_bridge_connector_detect(struct drm_connector *connector, bool force)
>> >>> > --> drm_bridge_funcs.detect(bridge)
>> >>> > The ctx got dropped when drm_helper_probe_detect call drm_bridge_connector_detect
>> >>> > The connector got dropped when connector call it's bridege.detect
>> >>> >
>> >>> > So I think the simplest solution is to have drm_bridge_funcs.detect
>> >>> > directly pass the connector
>> >>>
>> >>> I don't disagree on principle, but I think a better first step would be
>> >>> to provide a detect_ctx hook to bridges.
>> >>
>> >>Yup. There's other reasons you really want to get at the locking context
>> >>in detect callbacks, doing this special case by passing something for
>> >>everyone doesn't sound like the right approach to me.
>> >
>> >Ok, I will add a detect_ctx hook for bridge. Thanks for your advice.
>> >
>> >Just confirm that can I send this add detect_ctx hook patch alone first?
>> >I think this patch will be easy to merge, so it can help my WIP DP driver stay light on dependencies。
>
>Yeah I think sending this prep work as a standalone thing is ok,
>especially since it looks like there's going to be more work involved.
>
>> When I try to add the detect_ctx hook to bridge, I found that there is still a case that there is no ctx to
>> pass to detect_ctx:
>>
>> int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
>> uint32_t maxX, uint32_t maxY)
>> {
>> ...............
>> struct drm_modeset_acquire_ctx ctx;
>>
>> WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
>>
>> drm_modeset_acquire_init(&ctx, 0);
>>
>> drm_dbg_kms(dev, "[CONNECTOR:%d:%s]\n", connector->base.id,
>> connector->name);
>>
>> retry:
>> ret = drm_modeset_lock(&dev->mode_config.connection_mutex, &ctx);
>> ...................
>> ret = drm_helper_probe_detect(connector, &ctx, true);
>> ......................................
>>
>> count = drm_helper_probe_get_modes(connector);
>>
>> Then in drm_bridge_connector_get_modes:
>>
>> static int drm_bridge_connector_get_modes(struct drm_connector *connector)
>> {
>> struct drm_bridge_connector *bridge_connector =
>> to_drm_bridge_connector(connector);
>> struct drm_bridge *bridge;
>>
>> ...........................................
>> /*
>> * If display exposes EDID, then we parse that in the normal way to
>> * build table of supported modes.
>> */
>> bridge = bridge_connector->bridge_edid;
>> if (bridge)
>> return drm_bridge_connector_get_modes_edid(connector, bridge);
>>
>> static int drm_bridge_connector_get_modes_edid(struct drm_connector *connector,
>> struct drm_bridge *bridge)
>> {
>> enum drm_connector_status status;
>> const struct drm_edid *drm_edid;
>> int n;
>>
>> status = drm_bridge_connector_detect(connector, false);
>>
>> ......................
>>
>> -drm_bridge_connector_detect(struct drm_connector *connector, bool force)
>> +drm_bridge_connector_detect(struct drm_connector *connector,
>> + struct drm_modeset_acquire_ctx *ctx,
>> + bool force)
>> {
>> struct drm_bridge_connector *bridge_connector =
>> to_drm_bridge_connector(connector);
>> @@ -186,7 +188,7 @@ drm_bridge_connector_detect(struct drm_connector *connector, bool force)
>> enum drm_connector_status status;
>>
>> if (detect) {
>> status = detect->funcs->detect(detect, ctx);
>>
>> There is still no ctx in this call chain.
>>
>> So there will be deadlock if I use drm_atomic_get_new_connector_for_encoder to find connector in
>> my bridge detect_ctx hook.
>
>Hm yeah looks like bridge callchain isn't split up into ->detec and
>->get_modes in exactly the same way as connectors. I think the clean
>solution would be to pull drm_bridge_connector_detect() out from
>drm_bridge_connector_get_modes(), but that might unravel a huge amount of
>work. But wiring the ctx through all the get_mode functions might also not
>work out great, hence why I'd try moving _detect() first.
Since drm_connector_helper_funcs.get_modes is called by drm_helper_probe_single_connector_modes,
which already called drm_helper_probe_detect to make sure the connector is at connected state, so
Can we directly remove drm_bridge_connector_detect within drm_bridge_connector_get_modes_edid?
>
>Once you have bridge_connector_detect at the same split-up like any other
>connectors, it should be possible to add the contect to
>drm_bridge_connector_detect and have it everywhere. In general I think
>calling drm_bridge_connector_detect with a NULL ctx should be a bug, and
>we should catch that with a WARN_ON and bail out. Otherwise the locking is
>going to be a complete mess.
>
>And yeah definitely do this prep work as a separate series, I think the
>need is clearly established, so we don't need a user right away to justify
>this rework.
>
>Cheers, Sima
>>
>>
>>
>> >
>> >
>> >>-Sima
>> >>--
>> >>Simona Vetter
>> >>Software Engineer, Intel Corporation
>> >>http://blog.ffwll.ch
>
>--
>Simona Vetter
>Software Engineer, Intel Corporation
>http://blog.ffwll.ch
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