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    SubjectRe: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge.
    On Fri, 23 Jun 2017 09:22:15 +0200
    Andrzej Hajda <a.hajda@samsung.com> wrote:

    > On 22.06.2017 15:34, Boris Brezillon wrote:
    > > On Thu, 22 Jun 2017 15:16:47 +0200
    > > Andrzej Hajda <a.hajda@samsung.com> wrote:
    > >
    > >> On 22.06.2017 14:41, Boris Brezillon wrote:
    > >>> On Thu, 22 Jun 2017 14:29:07 +0200
    > >>> Andrzej Hajda <a.hajda@samsung.com> wrote:
    > >>>
    > >>>> On 22.06.2017 11:23, Boris Brezillon wrote:
    > >>>>> On Thu, 22 Jun 2017 13:47:43 +0530
    > >>>>> Archit Taneja <architt@codeaurora.org> wrote:
    > >>>>>
    > >>>>>> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote:
    > >>>>>>> 2017-06-20 19:31 GMT+02:00 Eric Anholt <eric@anholt.net>:
    > >>>>>>>> Archit Taneja <architt@codeaurora.org> writes:
    > >>>>>>>>
    > >>>>>>>>> On 06/16/2017 08:13 PM, Eric Anholt wrote:
    > >>>>>>>>>> Archit Taneja <architt@codeaurora.org> writes:
    > >>>>>>>>>>
    > >>>>>>>>>>> On 06/16/2017 02:11 AM, Eric Anholt wrote:
    > >>>>>>>>>>>> If the panel-bridge is being set up after the drm_mode_config_reset(),
    > >>>>>>>>>>>> then the connector's state would never get initialized, and we'd
    > >>>>>>>>>>>> dereference the NULL in the hotplug path. We also need to register
    > >>>>>>>>>>>> the connector, so that userspace can get at it.
    > >>>>>>>>>>>>
    > >>>>>>>>>>> Shouldn't the KMS driver make sure the panel-bridge is set up before
    > >>>>>>>>>>> drm_mode_config_reset? Is it the case when we're inserting the
    > >>>>>>>>>>> panel-bridge driver as a module?
    > >>>>>>>>>>>
    > >>>>>>>>>>>
    > >>>>>>>>>>> All the connectors that have been added are registered automatically
    > >>>>>>>>>>> when drm_dev_register() is called by the KMS driver. Registering a
    > >>>>>>>>>>> connector in the middle of setting up our driver is prone to race
    > >>>>>>>>>>> conditions if the userspace decides to use them immediately.
    > >>>>>>>>>> Yeah, this is fixing initializing panel_bridge at DSI host_attach time,
    > >>>>>>>>>> which in the case of a panel module that creates the DSI device
    > >>>>>>>>>> (adv7533-style, like you said I should use as a reference) will be after
    > >>>>>>>>>> drm_mode_config_reset() and drm_dev_register().
    > >>>>>>>>> Okay. In the case of the msm kms driver, we defer probe until the
    > >>>>>>>>> adv7533 module is inserted, only then we proceed to drm_mode_config_reset()
    > >>>>>>>>> and drm_dev_register(). I assumed this was the general practice followed by
    > >>>>>>>>> most kms drivers. I.,e the kms driver defers probe until all connector
    > >>>>>>>>> related modules are inserted, and only then proceed to create a drm device.
    > >>>>>>>> The problem, though, is the panel driver needs the MIPI DSI host to
    > >>>>>>>> exist to call mipi_dsi_device_register_full() during the probe process.
    > >>>>>>>> The adv7533 driver gets around this by registering the DSI device in the
    > >>>>>>>> bridge attach step, but drm_panel doesn't have an attach step.
    > >>>>>> I'm not sure how we can get around this. We had discussion about this on irc
    > >>>>>> recently, but couldn't come up with a good conclusion. We could come up with a
    > >>>>>> panel_attach() callback to make it similar to bridges, but that's just us avoiding
    > >>>>>> the real issue.
    > >>>>> How about making DSI dev registration fully asynchronous, that is, DSI
    > >>>>> devs declared in the DT under the DSI host node will be
    > >>>>> registered/attached at probe time, and devs using another control bus
    > >>>>> (like the adv7533 controller over i2c) will be registered afterwards.
    > >>>>>
    > >>>>> That implies moving the drm_brige registration logic outside of the DSI
    > >>>>> host ->probe() path. The idea would be to check if all devs connected
    > >>>>> to the DSI bus are ready at dsi_host->attach() time. If they are, we
    > >>>>> can finally register the XXX -> DSI bridge. If they're not (because
    > >>>>> some devs connected to the DSI bus have not been probed yet), then we
    > >>>>> do not register the drm_bridge and wait for the next dsi_host->attach()
    > >>>>> event.
    > >>>> I guess you assumes that dsi-host knows all devs connected to it, thanks to:
    > >>>> - subnodes of the host - ie. devices controlled via dsi bus,
    > >>>> - graph links from host ports/endpoints - ie. devices controlled by
    > >>>> other buses, for example adv7533.
    > >>> Yep, but I think that's already a requirement when populating devices
    > >>> with the OF graph method (if one of the DSI output endpoint does not
    > >>> have a drm_bridge/panel attached to it, the DSI host driver returns
    > >>> -EPROBE_DEFER).
    > >>>
    > >>>> I would separate both abstractions to make it more clear:
    > >>>> 1. MIPI bus should be registered early - to allow create/bind devices on it,
    > >>> Exactly.
    > >>>
    > >>>> 2. drm_bridge should be registered only if all required sinks
    > >>>> (bridges/panels) are registered.
    > >>> That's true, until we find a solution to support add DRM bridge hotplug.
    > >>>
    > >>>> First point seems OK, I am not sure about the 2nd one - if used
    > >>>> consistently, it would require building pipeline from sink to source.
    > >>> Yes.
    > >> Since drm_bridge_attach requires encoder to be not null pipeline
    > >> creation would be painful:
    > >> 1. Every driver must call drm_of_find_panel_or_bridge on sink(s) before
    > >> registering bridge and cache the result for later use.
    > > Shouldn't be hard to do since dsi_host->attach() is called each time a
    > > sink is added (and ready to use). All you need to do is retrieve the
    > > bridge pointer and put it in a list embedded in the DSI host priv
    > > struct. Once you have all sinks added to this list (can be checked by
    > > counting the number of endpoints and DSI devs at probe time), you can
    > > register the DSI bridge and wait for someone to call ->attach() on it.
    > >
    > > In the ->attach() hook of the DSI bridge, you'll have to attach all
    > > sinks stored in the list to the DSI bridge. Note that right now you have
    > > a 1:1 relationship, which prevents you from having one DSI bridge that
    > > can attach to different bridges available on the DSI bus (e.g. DSI ->
    > > HDMI bridge on channel 0 and DSI -> LVDS bridge on channel 1).
    > >
    > >> 2. After encoder finds directly connected bridge, it can attach it.
    > > I don't get that one.
    >
    > If you have pipeline:
    >
    > crtc -> encoder -> bridge1 -> bridge2 -> panel
    >
    > encoder knows only about bridge1, and must wait till it is registered,
    > before attaching it,
    > and assuming bridge must wait for its sinks before registration the
    > whole pipeline construction will look like:
    >
    > 0. encoder waits for bridge1, bridge1 waits for bridge2, bridge2 waits
    > for panel, usually by deferring.
    > 1. panel is registered.
    > 2. bridge2 finds panel and is registered.
    > 3. bridge1 finds bridge2 and is registered.
    > 4. encoder finds bridge1 and attach it to encoder,
    > 4a. bridge1->attach() attach bridge2 to encoder after bridge1
    > 4b. bridge2->attach() attach panel to bridge2
    >
    > This is why it seems for me quite complicated.

    But that's already what happens in most drivers today (probably because
    things were designed before connector hotplug was supported). I agree,
    it's far from ideal, but until we get full-hotplug support, we'll have
    to rely on such hacks.

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