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| 1 | +.. SPDX-License-Identifier: GPL-2.0 |
| 2 | +
|
| 3 | +========================================================= |
| 4 | +Raspberry Pi PiSP Back End Memory-to-Memory ISP (pisp-be) |
| 5 | +========================================================= |
| 6 | + |
| 7 | +The PiSP Back End |
| 8 | +================= |
| 9 | + |
| 10 | +The PiSP Back End is a memory-to-memory Image Signal Processor (ISP) which reads |
| 11 | +image data from DRAM memory and performs image processing as specified by the |
| 12 | +application through the parameters in a configuration buffer, before writing |
| 13 | +pixel data back to memory through two distinct output channels. |
| 14 | + |
| 15 | +The ISP registers and programming model are documented in the `Raspberry Pi |
| 16 | +Image Signal Processor (PiSP) Specification document`_ |
| 17 | + |
| 18 | +The PiSP Back End ISP processes images in tiles. The handling of image |
| 19 | +tessellation and the computation of low-level configuration parameters is |
| 20 | +realized by a free software library called `libpisp |
| 21 | +<https://github.com/raspberrypi/libpisp>`_. |
| 22 | + |
| 23 | +The full image processing pipeline, which involves capturing RAW Bayer data from |
| 24 | +an image sensor through a MIPI CSI-2 compatible capture interface, storing them |
| 25 | +in DRAM memory and processing them in the PiSP Back End to obtain images usable |
| 26 | +by an application is implemented in `libcamera <https://libcamera.org>`_ as |
| 27 | +part of the Raspberry Pi platform support. |
| 28 | + |
| 29 | +The pisp-be driver |
| 30 | +================== |
| 31 | + |
| 32 | +The Raspberry Pi PiSP Back End (pisp-be) driver is located under |
| 33 | +drivers/media/platform/raspberrypi/pisp-be. It uses the `V4L2 API` to register |
| 34 | +a number of video capture and output devices, the `V4L2 subdev API` to register |
| 35 | +a subdevice for the ISP that connects the video devices in a single media graph |
| 36 | +realized using the `Media Controller (MC) API`. |
| 37 | + |
| 38 | +The media topology registered by the `pisp-be` driver is represented below: |
| 39 | + |
| 40 | +.. _pips-be-topology: |
| 41 | + |
| 42 | +.. kernel-figure:: raspberrypi-pisp-be.dot |
| 43 | + :alt: Diagram of the default media pipeline topology |
| 44 | + :align: center |
| 45 | + |
| 46 | + |
| 47 | +The media graph registers the following video device nodes: |
| 48 | + |
| 49 | +- pispbe-input: output device for images to be submitted to the ISP for |
| 50 | + processing. |
| 51 | +- pispbe-tdn_input: output device for temporal denoise. |
| 52 | +- pispbe-stitch_input: output device for image stitching (HDR). |
| 53 | +- pispbe-output0: first capture device for processed images. |
| 54 | +- pispbe-output1: second capture device for processed images. |
| 55 | +- pispbe-tdn_output: capture device for temporal denoise. |
| 56 | +- pispbe-stitch_output: capture device for image stitching (HDR). |
| 57 | +- pispbe-config: output device for ISP configuration parameters. |
| 58 | + |
| 59 | +pispbe-input |
| 60 | +------------ |
| 61 | + |
| 62 | +Images to be processed by the ISP are queued to the `pispbe-input` output device |
| 63 | +node. For a list of image formats supported as input to the ISP refer to the |
| 64 | +`Raspberry Pi Image Signal Processor (PiSP) Specification document`_. |
| 65 | + |
| 66 | +pispbe-tdn_input, pispbe-tdn_output |
| 67 | +----------------------------------- |
| 68 | + |
| 69 | +The `pispbe-tdn_input` output video device receives images to be processed by |
| 70 | +the temporal denoise block which are captured from the `pispbe-tdn_output` |
| 71 | +capture video device. Userspace is responsible for maintaining queues on both |
| 72 | +devices, and ensuring that buffers completed on the output are queued to the |
| 73 | +input. |
| 74 | + |
| 75 | +pispbe-stitch_input, pispbe-stitch_output |
| 76 | +----------------------------------------- |
| 77 | + |
| 78 | +To realize HDR (high dynamic range) image processing the image stitching and |
| 79 | +tonemapping blocks are used. The `pispbe-stitch_output` writes images to memory |
| 80 | +and the `pispbe-stitch_input` receives the previously written frame to process |
| 81 | +it along with the current input image. Userspace is responsible for maintaining |
| 82 | +queues on both devices, and ensuring that buffers completed on the output are |
| 83 | +queued to the input. |
| 84 | + |
| 85 | +pispbe-output0, pispbe-output1 |
| 86 | +------------------------------ |
| 87 | + |
| 88 | +The two capture devices write to memory the pixel data as processed by the ISP. |
| 89 | + |
| 90 | +pispbe-config |
| 91 | +------------- |
| 92 | + |
| 93 | +The `pispbe-config` output video devices receives a buffer of configuration |
| 94 | +parameters that define the desired image processing to be performed by the ISP. |
| 95 | + |
| 96 | +The format of the ISP configuration parameter is defined by |
| 97 | +:c:type:`pisp_be_tiles_config` C structure and the meaning of each parameter is |
| 98 | +described in the `Raspberry Pi Image Signal Processor (PiSP) Specification |
| 99 | +document`_. |
| 100 | + |
| 101 | +ISP configuration |
| 102 | +================= |
| 103 | + |
| 104 | +The ISP configuration is described solely by the content of the parameters |
| 105 | +buffer. The only parameter that userspace needs to configure using the V4L2 API |
| 106 | +is the image format on the output and capture video devices for validation of |
| 107 | +the content of the parameters buffer. |
| 108 | + |
| 109 | +.. _Raspberry Pi Image Signal Processor (PiSP) Specification document: https://datasheets.raspberrypi.com/camera/raspberry-pi-image-signal-processor-specification.pdf |
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