Howto Write to the GPIO Pin of the CM108 Chip in Linux? - linux

Question

The CM108 from C-Media has 4 GPIO pin that you can access via a hid interface.
Using the generic write function in Windows I was able to write to the gpio pins.
However I'm trying to do the same thing in Linux without success.
The linux kernel detect the device as a hidraw device.
Note: I was able to read from the device, just not write. (I've run the app as root just to make sure it wasn't a permission issue).

Answer 1

I got this working, here's how.
I needed to create a new linux hid kernel mod. (it wasn't that hard)/*
/*
* Driver for the C-Media 108 chips
*
* Copyright (C) 2009 Steve Beaulac <steve#sagacity.ca>
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation, version 2.
*/
/*
* This driver is based on the cm109.c driver
*/
#include <linux/device.h>
#include <linux/hid.h>
#include <linux/module.h>
#define DRIVER_VERSION "20090526"
#define DRIVER_AUTHOR "Steve Beaulac"
#define DRIVER_DESC "C-Media 108 chip"
#define CM108_VENDOR_ID 0x0d8c
#define CM108_PRODUCT_ID 0x000c
#ifdef CONFIG_USB_DYNAMIC_MINORS
#define CM108_MINOR_BASE 0
#else
#define CM108_MINOR_BASE 96
#endif
/*
* Linux interface and usb initialisation
*/
static int cm108_hid_probe(struct hid_device *hdev, const struct hid_device_id *id)
{
int ret;
ret = hid_parse(hdev);
if (ret) {
dev_err(&hdev->dev, "parse failed\n");
goto error;
}
ret = hid_hw_start(hdev, HID_CONNECT_HIDRAW);
if (ret) {
dev_err(&hdev->dev, "hw start failed\n");
goto error;
}
return 0;
error:
return ret;
}
static struct hid_device_id cm108_device_table[] = {
{ HID_USB_DEVICE (CM108_VENDOR_ID, CM108_PRODUCT_ID) },
/* you can add more devices here with product ID 0x0008 - 0x000f */
{ }
};
MODULE_DEVICE_TABLE (hid, cm108_device_table);
static struct hid_driver hid_cm108_driver = {
.name = "cm108",
.id_table = cm108_device_table,
.probe = cm108_hid_probe,
};
static int hid_cm108_init(void)
{
return hid_register_driver(&hid_cm108_driver);
}
static void hid_cm108_exit(void)
{
hid_unregister_driver(&hid_cm108_driver);
}
module_init(hid_cm108_init);
module_exit(hid_cm108_exit);
MODULE_AUTHOR(DRIVER_AUTHOR);
MODULE_DESCRIPTION(DRIVER_DESC);
MODULE_LICENSE("GPL");
used This makefile
obj-m += cm108.o
and compile the module
make -C /lib/modules/`uname -r`/build/ M=`pwd` EXTRAVERSION="-generic" modules
sudo make -C /lib/modules/`uname -r`/build/ M=`pwd` EXTRAVERSION="-generic" modules_install
depmod -a
I had to modify the modules.order file so that my module would get queried before the generic hid linux module.
This modules make sure that the hidraw uses Interface 2.
Then I can use fopen to read and write to the GPIO pin of the CM108 chip.
BTW: when writing you need to write 5byte the 1st byte is used for the HID_OUTPUT_REPORT

Answer 2

Most hardware in Linux is accessible as a file. If the driver created a hardware node for it on the file-system, you're in luck. You will be able to write to it using regular file routines. Otherwise, you may need to do some assembly magic, which may require you to write a kernel module to do it.

Answer 3

Here is a complete example of how to write to the CM108/CM119 GPIO pins on Linux.
https://github.com/wb2osz/direwolf/blob/dev/cm108.c
You don't need to run as root or write your own device driver.
I have the opposite problem. I'm trying to figure out how to do the same thing on Windows.