howto use one module's dev/functions in another module.

urup · 06-18-2008, 10:55 AM

Hi all, and thanks for helping.

The situation:
I have one module (ad7877), that creates entries under /sys, which I need to access using another module.

It's a simple gpio I need to toogle, but how do I do that?
I know that I can't access the filesystem (and why it's a bad idea), so instead I want to use a function in the first module to set the gpio value.

I know that I can export the function via export_symbol(...), but how do I pass arguments between the two (and how do I get the right ones)?

The function in the first module is:

Code:

static ssize_t ad7877_gpio1_store(struct device *dev,
                                     struct device_attribute *attr,
                                     const char *buf, size_t count)
{
        struct ad7877 *ts = dev_get_drvdata(dev);
        char *endp;
        int i;

        i = simple_strtoul(buf, &endp, 10);

        if (i) {
                ts->gpio1=1;
        } else {
                ts->gpio1=0;
        }

        ad7877_write(dev, AD7877_REG_EXTWRITE, AD7877_EXTW_GPIO_DATA | (ts->gpio3 << 5) | (ts->gpio2 << 6)| (ts->gpio1 << 7));

        return count;
}

where the "dev" and "attr" parameters are specific for the ad7877 device driver instance.

Since my system only has one ad7877 (touchscreen controller), I guess I could just export the dev and attr as well, but that doesn't seem like a good solution.

How is this best done?

pinniped · 06-19-2008, 04:57 AM

Export the functions so that other modules can call them. You can see that sort of thing throughout the driver code: EXPORT_SYMBOL

urup · 06-20-2008, 02:19 AM

pinniped, you're absolutely right!
That was also what I wanted to do, but I was missing some of "glue" to bind it all together. "Grepping" though some of the kernel code helped though.

For others that where just as confused as I was:
If you from module1 want to call a function in module2 (lets say "module2_test_function(u16 param)").

export the method you want to call in module1:
EXPORT_SYMBOL(module2_test_function);

In module2 just call the function:
u16 param;
module2_test_function(param);

Even though you don't specify the parameter list in the EXPORT_SYMBOL, it still works.

pinniped · 06-20-2008, 08:48 AM

Another trick you might want to look at is putting some module parameters into the sysfs tree so that you can change their values dynamically by simply "writing" to the parameter name (using 'echo' or any other mean); you can also query the current setting with a "read" (cat).

urup · 06-23-2008, 01:36 AM

As i've read, writing/reading directly to sysfs is a very bad idea.
But how would I go about reading and writing to the sysfs if I wanted to (to make it safe)?
using system utilities like 'echo' and 'cat' seams like a bad idea, and youcan't use open/popen/fstream since you're in kernelspace nad only allowed to include stuff from the kernel itself.

pinniped · 06-27-2008, 06:39 AM

I mean that module parameters (such as irq, io, and others) can be made read/write and often in such a way that you can change the parameters simply by 'writing' to its virtual directory entry. An example of such a parameter exposed for read/write is the forwarding parameter in the network iptables code. You can look at that code as an example, but also read the relevant chapters in Linux Device Drivers 3rd ed. by Rubini and others.