VM: Bizzare MM tricks...

From: jmcmullan@linuxcare.com
Date: Wed Apr 26 2000 - 15:51:57 EST

        I'm working on a port of a driver from NT to Linux, and was
wondering how to best go about mapping kernel memory to
user space and vice versa. mmap(2) is a possibility, but would
require a vast rework of the code, and break the ability of
the customer to use their existing code base in NT/95.

        Is this possible under the current (2.2.14) VM scheme?
If so, any hints as to how to go about it? Any tricky issues?

ie:

----------------------- EXAMPLE CODE --------------------
void *device_buffer=ioremap(DEV_MEM_ADDR,DEV_MEM_SIZE);
void *user_buffer=NULL;

... ioctl(...)
{
        switch (cmd) {
                MAP_DEVICE_BUFFER:
                        /* MYTHICAL: vm_kernel_to_user(addr,size)
                         */
                        *(void **)arg=vm_kernel_to_user(device_buffer,DEV_MEM_SIZE);
                        break;
                MAP_USER_BUFFER:
                        /* MYTHICAL: vm_drop_mapping(addr)
                         * MYTHICAL: vm_user_to_kernel(addr,size)
                         */
                        if (user_buffer != NULL)
                                vm_drop_mapping(user_buffer);
                        user_buffer=vm_user_to_kernel((void *)arg,USER_MEM_SIZE);
                        break;
                ...
        }
}

/* Now we need to make sure to remove any mappings in the kernel
 * when the process closes the device
 */
... release(...)
{
        ...
        if (user_buffer != NULL) {
                /* MYTHICAL: vm_drop_mapping(addr)
                 */
                vm_drop_mapping(user_buffer);
                user_buffer=NULL;
        }
} 

-- 
Jason McMullan, Senior Linux Consultant, Linuxcare, Inc.
412.422.8077 tel, 412.656.3519 cell, 415.701.0792 fax
jmcmullan@linuxcare.com, http://www.linuxcare.com/
Linuxcare. Support for the revolution.

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