| ### Sysmem |
| |
| ## Sysmem Driver Environment |
| |
| Sysmem initially will run in the platform bus driver's devhost process, because |
| driver to driver communication is currently easiest when a driver with incoming |
| connections runs there. Later on, it'll run in its own devhost. |
| |
| Sysmem code must not fail the whole process, only failing/closing the specific |
| context that failed, because multiple clients in general will be connected to |
| the process that sysmem is running in (among other reasons). |
| |
| ## Connecting to sysmem |
| |
| A child driver of the platform bus driver can connect to sysmem by the client |
| being instantiated with the TBD_SYSMEM protocol, which loads the sysmem client |
| proxy driver into the client driver's devhost process. Or, the child driver may |
| also be loaded directly into the platform bus driver's devhost process. Either |
| way, the client driver requests the TBD_SYSMEM protocol to get a limited C ABI |
| protocol that allows sending sysmem the server end of a |
| fuchsia.sysmem.Allocator channel. The client driver can then make FIDL |
| requests using the client end of that channel. |
| |
| A zircon non-driver (aka "user mode") process (such as virtcon) can connect to |
| sysmem by requesting fuchsia.sysmem.Allocator service (from among zircon |
| services), and make requests using the client end of that channel (like a normal |
| service request). This is achieved via a sysmem zircon service that brokers the |
| FIDL protocol request through to the sysmem driver. |
| |
| A garnet (etc) non-driver program can connect to sysmem by requesting |
| fuchsia.sysmem.Allocator service (from among garnet services), and make |
| requests using the client end of that channel. This is achieved by having |
| svcmgr garnet code broker the sysmem protocol request through to the zircon |
| sysmem service which in turn sends the request to the sysmem zircon driver. |
| |
| This way, all of the following categories of clients (potential participants) |
| can connect to sysmem: |
| * Zircon drivers (must be child of platform bus driver for now) |
| * Garnet+ drivers (must be child of platform bus driver for now) |
| * Zircon processes (non-driver zircon processes) |
| * Garnet processes (non-driver garnet processes) |
| |
| Regardless of how a client connects, the client ends up with a |
| fuchsia.sysmem.Allocator client channel, on which FIDL requests can be made. |
| |
| ## Serving FIDL in sysmem driver |
| |
| Fidl is served from the sysmem driver, including async request completion, by |
| using SimpleBinding<>, which together with FIDL C generated code acts a lot like |
| Binding<> does when used with FIDL C++ generated code. |
| |
| ## Sysmem protocol description |
| |
| See comments in allocator.fidl and related files. |
