64-bit resources

"Resource" is the term used within the Linux kernel for a specific set of I/O-related hardware resources - I/O memory and ports, in particular. Device drivers allocate specific resources with functions like request_region(), but, underneath that layer, Linux has a set of generic resource allocation utilities. And at the core of that code is struct resource, which tracks individual resource allocations. A read of /proc/iomem or /proc/ioports is really just dumping out one of those resource data structures.

Since the resource management code was added by Linus at the beginning of the 2.3 development cycle, the unsigned long type has been used to track actual resource values. That worked at the time, but it can be problematic on 32-bit machines which have I/O memory resources at high addresses. If a memory region is located out of the 32-bit range, the resource management code can no longer deal with it.

The solution, of course, is to start using 64-bit numbers to track resource allocations. Vivek Goyal (along with others) has been working for some time on a set of patches which makes this change. Those patches have been fixed up by Greg Kroah-Hartman and, by all appearances, are set for merging once the 2.6.18 development cycle starts.

Introducing new typedefs into the kernel is generally frowned upon, but this patch creates resource_size_t anyway. Early in the patch series, this type is just unsigned long; only when the type has propagated through the source is it changed to a 64-bit value. There is a configuration option controlling whether 64-bit resources are used; interestingly, 64-bit is the default, and the old 32-bit mode is marked "experimental."

The result of the change is that the prototypes for some commonly-used functions change:

    struct resource *request_region(resource_size_t start,
                                    resource_size_t n,
				    const char *name);
    void release_region(resource_size_t start, resource_size_t n);

    struct resource *request_mem_region(resource_size_t start,
                                        resource_size_t n,
					const char *name);
    void release_mem_region(resource_size_t start, resource_size_t n);

Most driver code will be unaffected by these changes; simple constant resource locations will still work, and, in many cases, the bus layer handles the details of resource allocation anyway. But, in cases where a driver is directly storing or working with resource locations, the new type will have to be used.

Index entries for this article
Kernel	I/O memory
Kernel	Resources

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