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media: atomisp: Add a notes.txt file

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Add a files documenting what I've learned about the driver while
working on various cleanups.

Link: https://lore.kernel.org/linux-media/20220615205037.16549-41-hdegoede@redhat.com
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
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Hans de Goede 2022-06-15 21:50:37 +01:00 committed by Mauro Carvalho Chehab
parent b74196e798
commit 9799a3bf8a
1 changed files with 30 additions and 0 deletions
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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).