[PATCH v4 3/7] drm/tilcdc: Add support for external tda998x encoder

[Russell King - ARM Linux]

On Wed, Apr 01, 2015 at 11:49:27AM +0300, Jyri Sarha wrote:
> Add support for an external compontised DRM encoder. The external
> encoder can be connected to tilcdc trough device tree graph binding.
> The binding document for tilcdc has been updated. The current
> implementation supports only tda998x encoder.
> 
> To be able to filter out the unsupported video modes the tilcdc driver
> needs to hijack the external connectors helper functions. The tilcdc
> installes new helper functions that are otherwise identical to
> orignals, but the mode_valid() call-back check the mode first localy,
> before calling the original call-back. The tilcdc dirver restores the
> original helper functions before it is unbound from the external
> device.

This is where the DRM model is weak - we don't really have a way to
say "this is the set of CRTCs which /can/ be associated with this
connector, can any of the CRTCs accept this mode?" and eliminate
modes which fail that check.

This problem seems to be one which recurrs, so I wonder if it's
something which ought to be solved properly.

It's made slightly more difficult because we don't really know which
connectors could be associated with which CRTCs - that information is
stored at the encoder level (with the encoders possible_crtcs), and
I'm not sure we have a way for generic DRM code to know which encoders
could be associated with which connectors.

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