The simple pipeline handler has grown over time, and isn't that simple
anymore that it can easily be understood by an unfamiliar reader.
Document the design to explicitly state the expectations of the pipeline
handler, and to explain how it operates.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Phi-Bang Nguyen <pnguyen@baylibre.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
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