My understanding is BIOS or EFI detects the hardware during bootup and determines interrupt number, then passes it to Linux once kernel is up and running. And based on my research the lower the interrupt number the higher its priority.
My question is how does BIOS/EFI decide which hardware should have high priority over another? Is it something that is configurable or is hardcoded by BIOS/EFI?
Kind of.
When using the legacy 8259A PIC chip, one of the priority modes is based on the IRQ number - with lower IRQs having more priority.
However with the IO APIC and the MSI(X) technology the IRQ priority is handled in the LAPIC and it is configurable by the OS.
For the legacy scenario, these devices have fixed IRQs (not configurable).
The priority was assigned so that important/frequent tasks could interrupt less important/frequent ones.
Today those devices are emulated and their IRQ can be reassigned (in same case, it depends on the chipset/superio/embedded controller) if needed but that could cause some compatibility issue.
So every device that impersonate a legacy one (e.g. an HDD) is usually assigned its legacy IRQ number.
A different topic is the PCI interrupts (PCIe deprecated the INTx# lines in favour of MSI) for non legacy devices (e.g. a NIC).
Those were (are) the real programmable IRQs, each PCI-to-PCI bridge remap its four PIRQA-PIRQD input pins to its four INTA#-INTD# output pins (that are connected to the bridge's parent PIRQA-PIRQD pins in a tangled fashion).
The Host-to-PCI-bridge INTA#-INTD# connects (conceptually) to the 8259A and the IO-APIC.
The mapping is configurable with some chipset registers (e.g. see Chapter 29 of the Intel Series 200 PCH datasheet Volume 2).
So the firmware is free to remap at least the PCI interrupts for non legacy devices. I think the algorithm used is simply to assign the lower free IRQ to the most "important" device.
However, as said above, as soon as the OS switch away from the 8259A mode these priorities stop to matter.