CODEWITHSUNDEEP
clinuxlinux-kernellinux-device-driverprocfs
QnA
QnA

Reputation: 1103

How procfs outputs /proc/iomem?

I have looked into similar questions on this site (listed at the end) but still feel like missing a couple points, hopefully someone can help here:

  1. Is there a hook into the proc file system that connects the /proc/iomem inode to a function that dumps the information? I wasn't able to find where in proc fs this function lives. I did a grep under the linux source tree fs/proc for iomem, got nothing. So maybe it is a more of a procfs question... The answer to this question might help me to dig up the answer to the next question..

  2. The /proc/iomem has more entries than the BIOS E820 information I extracted from either dmesg or /sys/firmware/memmap (these two are actually consistent with each other). For example, /sys/firmware/memmap does not seem to have pci memory mapped regions. Drivers' init code calls the request_mem_region() and add more info to the map, so somewhere there should be a global variable (root of all resources ?) that remembers this graph?

The questions on stackoverflow I have looked into:

Upvotes: 1

Views: 1833

Answers (1)

seldak
seldak

Reputation: 291

  1. struct resource iomem_resource is what you're looking for, and it is defined and initialized in kernel/resource.c (via proc_create_seq_data()). In the same file, the instance struct seq_operations resource_op defines what happens when you, for example cat the file from userland.
  2. iomem_resource is a globally exported symbol, and is used throughout the kernel, drivers included, to request resources. You can find instances scattered across the kernel of devm_/request_resource() which take either iomem_resource or its sibling ioport_resource based on either fixed settings, or based on configurations. Examples of methods that take configurations are a) device trees which is prevalent in embedded settings, and b) E820 or UEFI, which can be found more on x86.

Starting with b) which was asked in the question, the file arch/x86/kernel/e820.c shows examples of how reserved memory gets inserted into /proc/iomem via insert_resource(). This excellent link has more details on the dynamics of requesting memory map details from the BIOS.

Another alternative sequence (which relies on CONFIG_OF) for how a device driver requests the needed resources is:

  1. The Open Firmware API is traversing the device tree, and finds a matching driver. For example via a struct of_device_id.
  2. The driver defines a struct platform_device which contains both the struct of_device_id and a probe function. This probing function is thus called.
  3. Inside the probe function, a call to platform_get_resource() is made which reads the reg property from the device tree. This property defines the physical memory map for a specific device.
  4. A call to devm_request_mem_region() is made (which is just a call to request_region()) to actually allocate the resources and add it to /proc/iomem.

Upvotes: 2

Related Questions