difference between core.c and pci.c in Linux nvme driver

Question

I want to learn how nvme driver works in Linux,

So I look into nvme driver source code here

what confuses me is that there are two source file containing "module_init()"

core.c

module_init(nvme_core_init);

and pci.c

module_init(nvme_init);

I know the module_init() function is the entry of the driver

but how come there are two entry in nvme driver ?

Answer 1

module_init() is the entry point of a module, and with abstraction we layer the modules to logically separate functionality, improve code reuse, etc...

• pci.c provides the PCI / PCIe interface/abstraction layer for using NVMe devices over a PCI bus - it has much more to do with managing tangible devices:
  • nvme_id_table
  • nvme_probe()
  • nvme_irq()
• core.c provides the core driver for generically dealing with an NVMe devices connected to the system (it doesn't care how). This module cares more about providing the standardised block device access, etc...
  • nvme_fops
  • nvme_submit_io()

This is a common idiom throughout the kernel, and is done so that if an NVMe device became accessible via another bus, then core.c would be reused with no / minimal changes, and new_bus.c would be written to interface between the two.

---

If you're using NVMe over PCIe, then the following chain will hopefully help things make sense:

1. pci.c implements nvme_pci_reg_read32()
2. pci.c registers nvme_pci_reg_read32() in the nvme_ctrl_ops structure, named nvme_pci_ctrl_ops
3. core.c implements nvme_init_ctrl(), which is called with a pointer to one of these structures
4. core.c keeps a reference to the structure
5. core.c implements nvme_init_identify(), which needs assistance of the lower-level - pci.c
6. core.c calls pci.c's nvme_pci_reg_read32() via the reference retained above

If we were to develop a new bus that could support an NVMe device, then we could swap out pci.c for new_bus.c with no changes to core.c (as mentioned above).

---

It's also worth checking out the Kconfig files as they can hint at things like this - though there is a certain amount of mental gymnastics to tie the source files to the menu options via the Makefiles.