Re: [PATCH 12/12] OMAPDSS: DPI: always use DSI PLL if available

[Tomi Valkeinen <tomba@iki.fi>]

On 2012-11-05 16:21, Rob Clark wrote:
> On 11/05/2012 02:55 AM, Tomi Valkeinen wrote:
>>>> But even then, choosing the manager is not easy, as whoever chooses the
>>>> >>manager needs to observe all the possible displays used at the same
>>>> >>time...
>>> >
>>> >Right. I was wondering if omapfb/omapdrm could understand the 'all
>>> >possible displays information' better compared to a panel's probe.
>>> >
>>> >Even omapdrm/omafb can't be perfect because we could insert a panel
>>> >driver module at any time, and omapfb/omapdrm may miss that out.
>> True, omapdrm/fb may have a better idea. It's still unclear though.
>> Currently we have quite strict order in the sequence the modules need to
>> be loaded, which is quite bad and causes issues. We should make things
>> more dynamic, so that the initialization of the drivers could happen
>> more freely.
>>
>> But that creates more problems: when booting up, omapfb starts. But
>> omapfb can't know if all the panel drivers have already been loaded.
>> omapfb may see that DVI is the default display, but what should it do if
>> DVI doesn't have a driver yet? It could wait, but perhaps the driver for
>> DVI will never even be loaded.
> 
> The encoder which is connected to the crtc (manager) is picked by
> combination of encoder->possible_crtcs bitmask and
> connector->best_encoder().  We could keep things limited so that the
> association of crtc to encoder (manager to output, roughly) never
> changes, but this isn't really the right thing to do.  It is better that
> the dssdev not rely on knowing the manager it is attached to at probe
> time, but instead grab resources more dynamically.
> 
> Also, at the moment we don't really have any notification to userspace
> about new encoders/connectors showing up (or conversely, being
> removed).  Only about existing connectors being plugged/unplugged.  The
> closest analogy is perhaps the USB display devices, but even there it is
> only the entire drm device that is plugged/unplugged.  And TBH I don't
> really see the point in supporting panel drivers being dynamically
> loaded.  It isn't like someone is dynamically soldering on a new display
> connector to some board that is running.  I think omapfb or omapdrm
> probe should trigger registering the compiled-in panel drivers, so that
> it can be sure that the dssdev's pop up before it goes and creates drm
> connector objects.  Currently we have to hack around this in omapdrm
> with late_initcall() to ensure the panel drivers are probed first, but
> that is an ugly hack that I'd like to get rid of.

We have panel devices and panel drivers, each of which can appear at any
time. Both are needed for the panel probe to happen. If we don't support
device hotplugging (dynamic creation of devices), we need to use
late_initcall for omapfb/drm. At least I don't see any other option.

You say that omapdrm should trigger registering of the drivers. How
would that work? Do you mean that the panel drivers would register
themselves to some common list, and omapdrm would go through this list
when drm is loaded, calling probe for the items in the list? I guess
that's doable, but... It's not how kernel drivers are supposed to work,
and so doesn't sound very clean approach to me.

I think we should support proper hotplugging of the panels. This would
fix the problem about init order, but it would also give us device
hotplug support. Obviously nobody is going to solder panel to a running
board, but I don't see any reason why panels, or, more likely, panels on
an add-on boards (like the capes being discussed in omap ml) would not
be hotpluggable using whatever connector is used on the particular use case.

And even if we don't support removing of the devices, things like the
add-on capes could cause the panel on the cape to be identified at some
late time (the panel is not described in the board file or DT data, but
found at runtime depending on the ID of the cape). This would add
another step to the init sequence that should be just right, if we don't
support hotplug.

Yes, I know it's not simple =). And I'm fine with simpler approach for
the time being, but I'd like full hotplug to be the future goal. At
least the common panel framework should not create restrictions about
this, even if drm wouldn't allow device hotplug.

 Tomi