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

Andrzej Hajda a.hajda at samsung.com
Thu Aug 21 03:32:43 PDT 2014

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
>>> will still cause some delay before everything gets set up, but hopefully
>>> less than what you're seeing now. There's also another thread where this
>>> is being discussed because deferred probing is causing "unacceptable"
>>> delays as well.
>> Could you point this thread out to me please ?
> I Cc'ed you on it.
> Thierry
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