media: atomisp: Add a notes.txt file

Some notes about the working of the atomisp drivers (learned while working

on cleaning it up).

The atomisp seems to be a generic DSP(ISP) like processor without a fixed

pipeline. It does not have its own memory, but instead uses main memory.

The ISP has its own address-space and main memory needs to be mapped into

its address space through the ISP's MMU.

Memory is allocated by the hmm code. hmm_alloc() returns an ISP virtual

address. The hmm code keeps a list of all allocations and when necessary

the hmm code finds the backing hmm-buffer-object (hmm_bo) by looking

up the hmm_bo based on the ISP virtual address.

The actual processing pipeline is made by loading one or more programs,

called binaries. The shisp_240??0_v21.bin firmware file contains many

different binaries. Binaries are picked by filling a ia_css_binary_descr

struct with various input and output parameters and then calling

ia_css_binary_find(). Some binaries support creating multiple outputs

(preview + video frame?) at the same time.

For example for the /dev/video0 preview node load_preview_binaries()

from atomisp/pci/sh_css.c is called and then loads a preview and

optionally a scalar binary. Note when digital zoom is disabled

(it is enabled by default) only the preview binary is loaded.

So in this case a single binary handles the entire pipeline.

Since getting a picture requires multiple processing steps,

this means that unlike in fixed pipelines the soft pipelines

on the ISP can do multiple processing steps in a single pipeline

element (in a single binary).