| # Modifying board drivers |
| |
| Caution: This page may contain information that is specific to the legacy |
| version of the driver framework (DFv1). Also the workflows documented on |
| this page may only be specific to the Fuchsia source checkout |
| (`fuchsia.git`) environment. |
| |
| 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](/docs/glossary/README.md#board-driver) |
| to expose hardware resources that your display driver can use. See |
| [amlogic-display.cc][amlogic-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 `AddComposite` 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. |
| |
| [This change][vim3-cl] is a complete example of board driver modifications for the VIM3. |
| |
| <!-- xrefs --> |
| [bti]: /docs/reference/kernel_objects/bus_transaction_initiator.md |
| [sysmem]: https://fuchsia.dev/reference/fidl/fuchsia.sysmem |
| [cda]: https://fuchsia.googlesource.com/fuchsia/+/7a5659579e2340a50cbd5063ee7925b46d4fcf6f/src/devices/board/drivers/astro/astro-display.cc#125 |
| [amlogic-display]: /src/graphics/display/drivers/amlogic-display/amlogic-display.cc |
| [vim3-cl]: https://fxrev.dev/479034 |
