Still struggling with Xilinx GPIO...

John Bonesio john.bonesio at
Thu May 29 02:09:34 EST 2008

Hi Guillaume,

gpio_ioctl.chan = x;

That's what determines which gpio device gets used. Since using 1 at
least doesn't give you a core dump, I would guess that 1 is the correct
value to use there.

So the next thing to check is to make sure that the ioctl command codes
being used are right.

The only way I know how to do this is to look in the linux kernel source
for the gpio device. Try looking here:

Then you find the routine that is handling the ioctl commands. It'll
probably be called xgpio_ioctl. In this routine you'll see something
    switch(cmd) {
    case XGPIO_IN:

The value for XGPIO_IN is defined in a header file (probably). The
header file might be called xgpio_ioctl.h.

So here's the key:
    Make sure the value for XGPIO_IN used in the user application is the
same as the one used in the kernel.

At one point I had see a case where the values in the kernel had
changed, but the user application was still trying to use the old

Hope this helps,

- John

On Wed, 2008-05-28 at 11:51 +0200, Guillaume Dargaud wrote:
> Hi John,
> > Try using channel 1. It may be set up where the first channel is 1 and not 
> > 0.
> Not sure what you mean...
> I'm actually not clear how many devices I should have:
> $ grep "GPIO.*BASEADDR" 
> arch/ppc/platforms/4xx/xparameters/xparameters_ml405.h
> But depending on the examples I look at, they define either:
> crw-rw-rw-    1 root     root      10, 185 May 26 13:49 xgpio
> or:
> crw-rw-rw-    1 root     root      10, 185 May 26 13:18 xgpio0
> crw-rw-rw-    1 root     root      10, 186 May 26 13:18 xgpio1
> crw-rw-rw-    1 root     root      10, 187 May 26 13:18 xgpio2
> If that is the former, I see the code uses:
>  gpio_ioctl.chan=0;
> And I guess that's what you are refering to, right ?
> Putting 1 gives me a working program, but no LED lights up and the buttons 
> are always shown at 0.
> Putting 0 gives a nice core dump as in my previous msg.
> Putting 2 locks up the system.
> And... I don't see how the code manages to read/wite all 3 GPIOs with only 
> two IOCTL calls. Shouldn't they be 3 different calls using different 
> addresses ?!? Argh, this complete lack of documentation in maddening. Not 
> everything can be understood from reading the source.
> Thanks.


John Bonesio
Commercial Linux Solutions
john.bonesio at
(408) 879-5569

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