drm/omap: Fix and improve crtc and overlay manager correlation

drm/omap: Fix and improve crtc and overlay manager correlation

The omapdrm driver currently takes a config/module arg to figure out the number
of crtcs it needs to create. We could create as many crtcs as there are overlay
managers in the DSS hardware, but we don't do that because each crtc eats up
one DSS overlay, and that reduces the number of planes we can attach to a single
crtc.

Since the number of crtcs may be lesser than the number of hardware overlay
managers, we need to figure out which overlay managers to use for our crtcs. The
current approach is to use pipe2chan(), which returns a higher numbered manager
for the crtc.

The problem with this approach is that it assumes that the overlay managers we
choose will connect to the encoders the platform's panels are going to use,
this isn't true, an overlay manager connects only to a few outputs/encoders, and
choosing any overlay manager for our crtc might lead to a situation where the
encoder cannot connect to any of the crtcs we have chosen. For example, an
omap5-panda board has just one hdmi output. If num_crtc is set to 1, with the
current approach, pipe2chan will pick up the LCD2 overlay manager, which cannot
connect to the hdmi encoder at all. The only manager that could have connected
to hdmi was the TV overlay manager.

Therefore, there is a need to choose our overlay managers keeping in mind the
panels we have on that platform. The new approach iterates through all the
available panels, creates encoders and connectors for them, and then tries to
get a suitable overlay manager to create a crtc which can connect to the
encoders.

We use the dispc_channel field in omap_dss_output to retrieve the desired
overlay manager's channel number, we then check whether the manager had already
been assigned to a crtc or not. If it was already assigned to a crtc, we assume
that out of all the encoders which intend use this crtc, only one will run at a
time. If the overlay manager wan't assigned to a crtc till then, we create a
new crtc and link it with the overlay manager.

This approach just looks for the best dispc_channel for each encoder. On DSS HW,
some encoders can connect to multiple overlay managers. Since we don't try
looking for alternate overlay managers, there is a greater possibility that 2
or more encoders end up asking for the same crtc, causing only one encoder to
run at a time.

Also, this approach isn't the most optimal one, it can do either good or bad
depending on the sequence in which the panels/outputs are parsed. The optimal
way would be some sort of back tracking approach, where we improve the set of
managers we use as we iterate through the list of panels/encoders. That's
something left for later.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>