[PATCH v4 00/11] drm: add support for Atmel HLCDC Display Controller

Boris BREZILLON boris.brezillon at free-electrons.com
Thu Aug 21 08:30:40 PDT 2014

On Thu, 21 Aug 2014 17:04:34 +0200
Andrzej Hajda <a.hajda at samsung.com> wrote:

> On 08/21/2014 03:21 PM, Thierry Reding wrote:
> > On Thu, Aug 21, 2014 at 12:32:43PM +0200, Andrzej Hajda wrote:
> >> On 08/21/2014 11:52 AM, Thierry Reding wrote:
> >>> On Thu, Aug 21, 2014 at 11:41:59AM +0200, Boris BREZILLON wrote:
> >>>> On Thu, 21 Aug 2014 11:04:07 +0200
> >>>> Thierry Reding <thierry.reding at gmail.com> wrote:
> >>>>
> >>>>> On Thu, Aug 21, 2014 at 10:37:06AM +0200, Boris BREZILLON wrote:
> >>>>>> Hi Ludovic,
> >>>>>>
> >>>>>> On Thu, 21 Aug 2014 10:16:19 +0200
> >>>>>> Ludovic Desroches <ludovic.desroches at atmel.com> wrote:
> >>>>>>
> >>>>>>> Hi Boris,
> >>>>>>>
> >>>>>>> You can add
> >>>>>>>
> >>>>>>> Tested-by: Ludovic Desroches <ludovic.desroches at atmel.com>
> >>>>>> Thanks for testing this driver.
> >>>>>>
> >>>>>>> Only one issue but not related to your patches, you can't display
> >>>>>>> quickly the bootup logo since the panel detection takes too much
> >>>>>>> time.
> >>>>>> Yes, actually this is related to the device probe order: the
> >>>>>> hlcdc-display-controller device is probed before the simple-panel, thus
> >>>>>> nothing is detected on the RGB connector (I use of_drm_find_panel to
> >>>>>> check for panel availability) when the display controller is
> >>>>>> instantiated. I rely on the default polling infrastructure provided by
> >>>>>> the DRM/KMS framework which polls for a new connector every 10s, and
> >>>>>> this is far more than you kernel boot time.
> >>>>>>
> >>>>>> Do anyone see a solution to reduce this delay (without changing the
> >>>>>> polling interval). I thought we could add a notifier infrastructure to
> >>>>>> the DRM panel framework, but I'm not sure this is how you want things
> >>>>>> done...
> >>>>> Other drivers return -EPROBE_DEFER when a panel hasn't been registered
> >>>>> yet. This will automatically take care of ordering things in a way that
> >>>>> DRM/KMS will only be initialized after the panel has been probed.
> >>>> Actually I'd like to avoid doing this with a deferred probe, because,
> >>>> AFAIU, the remote endpoint is not tightly linked with the display
> >>>> controller driver (I mean the display controller can still be
> >>>> initialized without having a display connected on it).
> >>>> Moreover the atmel dev kit I'm using has an HDMI bridge connected on
> >>>> the same RGB connector and I'd like to use it in a near future.
> >>>> Returning -EPROBE_DEFER in case of several devices connected on the
> >>>> same connector implies that I'll have to wait for all the remote
> >>>> end-points to be available before my display controller could be
> >>>> instantiated.
> >>>>
> >>>> While this could be acceptable when all drivers are statically linked
> >>>> in the kernel, it might be problematic when you're using modules,
> >>>> meaning that you won't be able to display anything on your LCD panel
> >>>> until your HDMI bridge module has been loaded.
> >>> No. HDMI should be using proper hotplugging anyway, hence it should be
> >>> always be loaded anyway. You're in for a world of pain if you think you
> >>> can run DRM with a driver that's composed of separate kernel modules.
> >>>
> >>> Also if you don't want to use deferred probe, then you're in for the
> >>> full hotplugging panel dance and that implies that you need to fix a
> >>> bunch of things in DRM (one being the framebuffer console instantiation
> >>> that I referred to in the other thread). You also can't be using the
> >>> current device tree bindings because they all assume a dependency from
> >>> the display controller/output to the panel. For hotplugging you'd need
> >>> the dependency the other way around (the panel needs to refer to the
> >>> output by phandle).
> >> I have tested panel as a module in exynos-dsi + panel-s6e8aa0
> >> configuration, everything works. There is a workaround for fb console
> >> not being reconfigurable, but it does not make thing worse than before.
> >> And I do not see a problem with phandles, ie in DT they point both ways,
> >> according to binding advices at the time, but in the code it is display
> >> controller/encoder which is looking for the panel.
> > That works because it's DSI. And we have attach/detach callbacks for
> > DSI. We don't have those for regular panels, so we'd need to find a way
> > to add that.
> Maybe I have misread your answer, but you showed it as very
> difficult/painful
> process: "hotplugging panel dance", "fix a bunch of things in DRM". In fact
> we are missing here only good notifications about panel appearance.
> >
> > The way that this currently works is that an encoder/connector driver
> > looks up the panel and attaches it to itself. If you allow panels to be
> > hotpluggable, then they have no knowledge about what they are connected
> > to, so there needs to be a way to inject that knowledge so that they can
> > attach to a connector.
> I do not understand that. Currently it is the connector who looks for
> the panel
> and attaches it.
> So the scenario, after adding panel tracking, could be:
>  - encoder parses its phandle to panel, and start tracking appearance of
> the panel
> identified by this phandle,
>  - when panel appears encoder callback is called, and encoder attaches
> the panel,
>  - when panel wants to disappear encoder callback is called, encoder
> detaches the panel.
> All this I have already presented together with generic interface
> tracker [1].

Well, your attempt at doing a generic tracker framework sounds
interesting, but given the answer you've got from greg-kh and Russel,
I'd say this patch series is in a dead-end (unless there are other
versions I haven't seen yet).

How about implementing a specific notifier interface for the drm_panel
framework first, and move to your generic implementation if it gets

These are the two proposal I sent to Thierry:

http://code.bulix.org/scq4g3-86804 (v1)


http://code.bulix.org/7urh8v-86806 (v2)

Feel free to propose any alternative to those implementations.

Best Regards,


Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering

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