Re: [PATCH 00/21] OMAPDSS: DISPC changes for writeback pipeline

On Thu, 2012-09-13 at 17:44 +0530, Archit Taneja wrote:

DSS HW on OMAP4 onwards supports a new pipeline called writeback. Unlike other
pipelines(called overlays in OMAPDSS), writeback takes pixel data from an
overlay output or a overlay manager output and writes it back into a specified
address in memory.

writeback pipeline allows us to take benefit of the hardware processing
available inside the DISPC like color space conversion, rescaling, compositing
etc and do either a) perform memory-to-memory transfer with data processing,
b) capture a displayed frame. The former is known as memory to memory mode of
the writeback pipeline, and the latter is known as capture mode. More details
about writeback can be found in the Display Subsystem section of the OMAP4/5 TRMs.

witeback has properties of both overlays and overlay managers. It is like an
overlay as it has programmable base addresses and contains blocks like scalar,

You consistently use the term "scalar" in the patches, but I believe the
correct term is "scaler".

color conversion unit, truncation unit, DISPC DMA FIFO. It is like a manager as
enabling it immediately starts transfer to the memory, and it has a GO bit to use
a new writeback configuration.

This series prepares the low level DISPC driver(dispc.c) to configure writeback
registers. The aim is to reuse most of the code as most of its registers are
like overlay or manager registers, and are configured in the same way in most
cases. The first few patches rename dispc_ovl_* functions to dispc_plane_*

I'm not sure if the renaming causes more confusion than clarity... It
kinda creates a mishmash of ovl/plane names, and the term "plane"
doesn't really sound like it's a base for both overlays and wb. Could we
consider the wb as a special case, and keep the ovl name for most of the
things and have "wb" used for wb specific things?

functions. The next few patches change how overlay caps are used within the
dispc functions, this helps reusing more functions between overlays and

I dislike this a bit, I think dispc driver should know what HW it has,
you shouldn't need to pass caps to it. So I'd prefer the dispc driver to
to have this information in dispc_features. I believe all OVL_CAPS
should be there, and then exported to other drivers via some means. I
guess this means could for now be just initializing ovl->caps with data
from dispc.c.

 Tomi