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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