How do I make my device driver respond to lseek?

[Lee D]

Hi,

I've been trying to figure out how to make my device driver respond to
lseek(). There doesn't seem to be an appropriate entry in the cdevsw
structure (in src/sys/sys/conf.h).

Obviously I can make an ioctl() call for this (and I have, in the
interim), but I'd like to do it it the right way.

I have a feeling like I am misunderstanding some critical abstraction
layer...  But at some point the device driver must be told what
position to start reading from/writing to, right?

FWIW, this is a device driver interface to a SPI flash in my custom
ARM embedded system. I need to be able to locate to a point in the
flash to read and write my app config info, without disturbing my boot
loader.

I want to be able to write code like this:

    int fd = open ("/dev/my_spi_flash0", O_RDWR);
    lseek(fd, 0x10000, SEEK_SET);
    write(fd, buf, 100);
    close(fd);

Does anyone know the proper way to implement lseek?