Memory Use in User-Level Drivers

Memory Use in User-Level Drivers

When you control a device from a user process, your code executes entirely in user process space, and has no direct access to any of the other spaces described in this chapter.

Depending on the device and other considerations, you may use the mmap() function to map device registers into the address space of your process (see the mmap(2) reference page). When the kernel maps a device address into process space, it does it using the TLB mechanism. From mmap() you receive a valid address in process space. This address is mapped through a TLB entry to an address in segment that accesses uncached physical memory. When your program refers to this address, the reference is directed to the system bus and the device.

Portions of kernel virtual memory (kseg0 or xkseg) can be accessed from a user process. Access is based on the use of device special files (see the mem(7) reference page). Access is done using two models, a device model and a memory map model.

Access Using a Device Model

The device special file /dev/mem represents physical memory. A process that can open this device can use lseek() and read() to copy physical memory into process virtual memory. If the process can open the device for output, it can use write() to patch physical memory.

The device special file /dev/kmem represents kernel virtual memory (kseg0 or xkseg). It can be opened, read and written similarly to /dev/mem. Clearly both of these devices should have file permissions that restrict their use even for input.

Access Using mmap()

The mmap() function allows a user process to map an open file into the process address space (see the mmap(2) reference page). When the file that is mapped is /dev/mem, the process can map a specified segment of physical memory. The effect of mmap() is to set up a page table entry and TLB entry so that access to a range of virtual addresses in user space is redirected to the mapped physical addresses in cached physical memory (kseg0 or the equivalent segment of xkphys).

The /dev/kmem device, representing kernel virtual memory, cannot be used with mmap(). However, a third device special, /dev/mmem (note the double "m"), represents access to only those addresses that are configured in the file /var/sysgen/master.d/mem. As distributed, this file is configured to allow access to the free-running timer device and, in some systems, to graphics hardware.

For an example of mapped access to physical memory, see the example code in the syssgi(2) reference page related to the SGI_QUERY_CYCLECNTR option. In this operation, the address of the timer (a device register) is mapped into the process's address space using a TLB entry. When the user process accesses the mapped address, the TLB entry converts it to an address in kseg1/xkphys, which then bypasses the cache.

Mapped Access Provided by a Device Driver

A kernel-level device driver can provide mapped access to device registers or to memory allocated in kernel virtual space. An example of such a driver is shown in Part III, "Kernel-Level Drivers."