| # Modifying board drivers |
| |
| Most system on chip (SoC) board rely on explicitly enumerated hardware |
| resources, rather than discovery mechanisms like ACPI that are common on |
| workstations, servers, or laptops. To add support for an SoC's display |
| controller, you will need to modify the board driver to expose hardware |
| resources that your display driver can use. See |
| [vim2/vim-display.cc][vim-display] for an example. Most devices need some |
| combination of the following: |
| |
| * MMIO regions |
| * Interrupts for vsync, hotplug events (typically through GPIO), etc. For example, |
| the AMLogic display controller has both an RDMA engine and a capture |
| interface with their own interrupts. |
| * [BTIs][bti], Zircon's primitive for pinning virtual memory to physical addresses for |
| hardware interaction. |
| * GPIO pins, e.g. to turn on/off hardwired LCD panels. |
| * Power resources, to turn on power to the controller and select operating |
| voltages. |
| * I2C bridges, e.g. for controlling DSI-to-HDMI output converters. |
| * DSI access, because DSI support is typically a separate driver offered by the |
| kernel. |
| * [Sysmem] heaps, so that clients can allocate memory that is accessible to the |
| device. |
| |
| After all these resources have been enumerated, call `CompositeDeviceAdd` to |
| [expose them][cda] as a unit. The device manager will look for drivers that can |
| bind to this composite device and load the appropriate display driver. |
| |
| <!-- xrefs --> |
| [bti]: /docs/reference/kernel_objects/bus_transaction_initiator.md |
| [sysmem]: https://fuchsia.dev/reference/fidl/fuchsia.sysmem |
| [cda]: https://fuchsia.googlesource.com/fuchsia/+/a7e9836345caf947df79d72882ac864eceae7eb8/src/devices/board/drivers/vim2/vim-display.cc#126 |
| [vim-display]: /src/devices/board/drivers/vim2/vim-display.cc |
