I am trying to run short from Linux Device Drivers book, a driver which uses by default the parallel interface of a pc at io address base 0x378. I am specifically using the /dev/short0 device. Quoting from the book
/dev/short0 writes to and reads from the 8-bit port located at the I/O address base (0x378 unless changed at load time).
The write operation (on default behavior) essentially does that
while (count--) {
outb(*(ptr++), port);
wmb( );
}
The ptr variable holds a pointer to the data the user has requested to be written to the device. Only the last byte of course survives, as preceding bytes get overwritten. The read operation works similarly, by using inb instead of outb.
Quoting also from the book
If you choose to read from an output port, you most likely get back the last value written to the port (this applies to the parallel interface and to most other digital I/O circuits in common use)
So when i do
$ echo -n "a" > /dev/short0
$ dd if=/dev/short0 bs=1 count=1 | od -t x1
as suggested in the book, i expect to get back the ascii code for 'a' in hex, but what i get is 0xff:
1+0 records in
1+0 records out
1 byte (1 B) copied, 0,000155485 s, 6,4 kB/s
0000000 ff
0000001
I have verified, adding some printks and using dmesg, that the relevant code of the driver actually gets executed and beyond that, i' m stuck. What are some possible reasons for this not working? Or where should i look next to find out why it is not working?
For what it matters, the io address range 0x378-0x37a is initially allocated from the parport module, so i rmmod it along with a few other modules that use parport before i load the short module. Finally, on my system uname -a gives
Linux Crete 3.13.0-24-generic #47-Ubuntu SMP Fri May 2 23:31:42 UTC 2014 i686 i686 i686 GNU/Linux