Writing Portable Device Drivers
Unlike on most typical embedded systems, accessing I/O memory on Linux cannot be done directly. This is due to the wide range of different memory types and maps present on the wide range of processors on which Linux runs. To access I/O memory in a portable manner, you must call ioremap() to gain access to a memory region and iounmap() to release access.
ioremap() is defined as:
void * ioremap (unsigned long offset, unsigned long size);
You pass in a starting offset of the region you wish to access and the size of the region in bytes. You cannot just use the return value as a memory location to read and write from directly, but rather it is a token that must be passed to different functions to read and write data.
The functions to read and write data using memory mapped by ioremap() are:
u8 readb (unsigned long token); /* read 8 bits */ u16 readw (unsigned long token); /* read 16 bits */ u32 readl (unsigned long token); /* read 32 bits */ void writeb (u8 value, unsigned long token); /* write 8 bits */ void writew (u16 value, unsigned long token); /* write 16 bits */ void writel (u32 value, unsigned long token); /* write 32 bits */
After you are finished accessing memory, you must call iounmap() to free up the memory so that others can use it if they want to.
The code example in Listing 4 from the Compaq PCI Hot Plug driver in drivers/hotplug/cpqphp_core.c shows how to access a PCI device's resource memory properly.
To access the PCI memory of a device, you again must use some general functions and not try to access the memory directly. This is due to the different ways the PCI bus can be accessed, depending on the type of hardware you have. If you use the general functions, then your PCI driver will be able to work on any type of Linux system that has a PCI bus.
To read data from the PCI bus use the following functions:
int pci_read_config_byte(struct pci_dev *dev, int where, u8 *val); int pci_read_config_word(struct pci_dev *dev, int where, u16 *val); int pci_read_config_dword(struct pci_dev *dev, int where, u32 *val);
and to write data, use these functions:
int pci_write_config_byte(struct pci_dev *dev, int where, u8 val); int pci_write_config_word(struct pci_dev *dev, int where, u16 val); int pci_write_config_dword(struct pci_dev *dev, int where, u32 val);
These functions allow you to write 8, 16 or 32 bits to a specific location that is assigned to a specific PCI device. If you wish to access the memory location of a specific PCI device that has not been initialized by the Linux PCI core yet, you can use the following functions that are present in the pci_hotplug core code:
int pci_read_config_byte_nodev(struct pci_ops *ops, u8 bus, u8 device, u8 function, int where, u8 *val); int pci_read_config_word_nodev(struct pci_ops *ops, u8 bus, u8 device, u8 function, int where, u16 *val); int pci_read_config_dword_nodev(struct pci_ops *ops, u8 bus, u8 device, u8 function, int where, u32 *val); int pci_write_config_byte_nodev(struct pci_ops *ops, u8 bus, u8 device, u8 function, int where, u8 val); int pci_write_config_word_nodev(struct pci_ops *ops, u8 bus, u8 device, u8 function, int where, u16 val); int pci_write_config_dword_nodev(struct pci_ops *ops, u8 bus, u8 device, u8 function, int where, u32 val);
An example of reading and writing to PCI memory by a driver can be seen in the USB OHCI driver at drivers/usb/usb-ohci.c (see Listing 5).
If you follow these different rules when creating a new Linux kernel device driver, or when modifying an existing one, the resulting code will run successfully on a wide range of processors. These rules are also good to remember when debugging a driver that only works on one platform (remember those endian issues).
The most important resource to remember is to look at existing kernel drivers that are known to work on different platforms. One of Linux's strengths is the open access of its code, which provides a powerful learning tool for aspiring driver authors.
Greg Kroah-Hartman is currently the Linux USB and PCI Hot Plug kernel maintainer. He works for IBM, doing various Linux kernel-related things and can be reached at firstname.lastname@example.org.
Practical Task Scheduling Deployment
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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- My +1 Sword of Productivity
- Non-Linux FOSS: Caffeine!
- Managing Linux Using Puppet
- SUSE LLC's SUSE Manager
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- Doing for User Space What We Did for Kernel Space
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