How do I make my device driver respond to lseek?

[Konstantin Belousov]

On Sun, Dec 03, 2017 at 12:13:16PM -0500, Lee D wrote:
> 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?

You did not say which driver you implement.

It could be devfs cdev, with only supported d_read/d_write methods. In
this case, io request parameters are packed into struct uio, including
the offset where io starts.

Or you might implement it as proper block-oriented device by providing
d_strategy. Then struct bio contains the block number.

Or your driver might be geom disk, see geom/geom_disk.h, in which case
disk_strategy method takes struct bio as well.

Last reasonable variant is to have driver implementing CAM SIM, then
sim_action processes ccb's, also holding all needed information about
io request parameters.