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parport_pc driver parallel port detection with manually provided io ports.

Link to this post 02 Apr 13

Hi,

I've been recently trying to use my cheap pci parallel port card based on ch353 chip. Unfortunately parport_pc driver works in a following way:

insmod parport_pc

It recognizes my build-in parallel port (port 0x378), but does not detect the card.
When I try (0xa800 is the pci card port):

insmod parport_pc io=0x378,0xa800

Then my pci card works well but the driver cannot recognize built-in parallel port, dmesg
gives something like this:

parport 0x378 (WARNING): CTR: wrote 0x0c, read 0xff
parport 0x378 (WARNING): DATA: wrote 0xaa, read 0xff
parport 0x378: You gave this address, but there is probably no parallel port there!

Now, when I look at the driver (linux/drivers/parport/parport_pc.c) the init function looks like this:


3611 static int __init parport_pc_init(void)
3612 {
...
3622 if (io[0]) {
3623 int i;
3624 /* Only probe the ports we were given. */
3625 user_specified = 1;
3626 for (i = 0; i < PARPORT_PC_MAX_PORTS; i++) {
3627 if (!io)
3628 break;
3629 if (io_hi == PARPORT_IOHI_AUTO)
3630 io_hi = 0x400 + io;
3631 parport_pc_probe_port(io, io_hi,
3632 irqval, dmaval, NULL, 0);
3633 }
3634 } else
3635 parport_pc_find_ports(irqval[0], dmaval[0]);
3636
3637 return 0;
3638 }

The reason for such behaviour seems to be obvious. The driver checks standard
ports only if no io= ports has been specified (and io= doesn't seem to work with
standard ports).

Now if I do something like this (move autodetection out of if-else statement):


3611 static int __init parport_pc_init(void)
3612 {
...
3620 parport_pc_find_ports(irqval[0], dmaval[0]);
3621
3622 if (io[0]) {
3623 int i;
3624 /* Only probe the ports we were given. */
3625 user_specified = 1;
3626 for (i = 0; i < PARPORT_PC_MAX_PORTS; i++) {
3627 if (!io)
3628 break;
3629 if (io_hi == PARPORT_IOHI_AUTO)
3630 io_hi = 0x400 + io;
3631 parport_pc_probe_port(io, io_hi,
3632 irqval, dmaval, NULL, 0);
3633 }
3634 }
3635
3636 return 0;
3637 }

Now autodetection seems to work and I can use both parallel ports.

Does anybody know what is the reason for parport_pc driver to work this way?
Is it a bug or am I missing something?

Regards

Link to this post 03 Apr 13

The first part in the script seems as if you were adding the driver to both the onboard parallel port and the pci port. The onboard port my have been using another driver that allowed the kernel to access it. I would leave that be.

That last part of the script indicates that the kernel auto-detects what io ports )io_hi are on available and assigns the appropriate driver. You told the kernel what parport to detect rather then letting it do it itself. Seems like either would've worked. From observation ;-)

Link to this post 09 Apr 13


"
The first part in the script seems as if you were adding the driver to both the onboard parallel port and the pci port. The onboard port my have been using another driver that allowed the kernel to access it. I would leave that be.
"

I think the only driver that uses parallel port on my test system (tinycore linux) is parport_pc. As I said, if I load the driver without arguments everything is fine with the standard port.


"
That last part of the script indicates that the kernel auto-detects what io ports )io_hi are on available and assigns the appropriate driver. You told the kernel what parport to detect rather then letting it do it itself. Seems like either would've worked. From observation ;-)
"

That was exactly my expectation, unfortunately parport_pc does not autodetect my card (ch535 chip) so there is no other choice but to give the port for this as a parameter and if you do so you have to provide the built-in parallel port io port as well because (what is apparent from the code) if you give any parameters, the driver won't perform autodetection.

I've been sniffing around in the driver code and so far it seems that It does exactly the same thing regardless of how the built-in parallel port io port is provided which is even more baffling.

I will try to find out what is going on when I have some more time (tried to add the chip
to the autodetection list but so far without success, I think I have to remind myself
how the pci works :c).

Thanks for help!

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