Implementing Polled I/O in Console Frame Buffer Drivers

The polled I/O interfaces are implemented as functions in the driver and are called directly by the kernel terminal emulator. The driver passes the address of its polled I/O entry points to the terminal emulator during the execution of the VIS_DEVINIT ioctl command. The VIS_DEVINIT command is initiated by the terminal emulator.

The vis_polledio structure is shown in the following code.

typedef void * vis_opaque_arg_t;

struct vis_polledio {
      struct vis_polledio_arg *arg;
      void    (*display)(vis_opaque_arg_t, struct vis_consdisplay *);
      void    (*copy)(vis_opaque_arg_t, struct vis_conscopy *);
      void    (*cursor)(vis_opaque_arg_t, struct vis_conscursor *);
};

The polled I/O interfaces provide the same functionality as the VIS_CONSDISPLAY, VIS_CONSCOPY, and VIS_CONSCURSOR ioctl interfaces. The polled I/O interfaces should follow the same steps that are described above for the respective ioctl commands. The polled I/O interfaces must very strictly adhere to the additional restrictions that are described in the remainder of this section.

The polled I/O interfaces are called only when the operating system is quiesced and in standalone mode. The system enters standalone mode whenever the user enters OpenBoot PROM or enters the kmdb debugger, or when the system panics. Only one CPU and one thread are active. All other CPUs and threads are stopped. Timesharing, DDI interrupts, and system services are turned off.

Standalone mode severely restricts driver functionality but simplifies driver synchronization requirements. For example, a user application cannot access the console frame buffer driver by way of the driver's memory mappings from within a polled I/O routine.

In standalone mode, the console frame buffer driver must not perform any of the following actions:

• Wait for interrupts

• Wait for mutexes

• Allocate memory

• Use DDI or LDI interfaces

• Use system services

These restrictions are not difficult to obey since the polled I/O functions are relatively simple operations. For example, when working with the rendering engine, the console frame buffer driver can poll a bit in the device rather than wait for an interrupt. The driver can use pre-allocated memory to render blit data. DDI or LDI interfaces should not be needed.