On Tue, Feb 07, 2017 at 04:55:28PM +0100, Mason wrote: > On 06/02/2017 16:54, Mason wrote: > > > My platform ( arch/arm/mach-tango ) provides a PCIe controller from PLDA. > > > > I do have access to a driver that works for a few PCIe boards, but it's > > an ancient out-of-tree driver (targeting v3.4). Also, I'm not sure it > > follows the best-practice guidelines (e.g. it hard-codes a few > > work-arounds directly in drivers/pci/probe.c) Indeed, having to change drivers/pci/probe.c doesn't sound like a best practice :) If you can share details of what changes you need, people could probably suggest other ways to do it within the generic PCI framework. > > If I understand correctly, while PCIe is a standard, it is not codified > > as much as USB (XHCI) or SATA (AHCI), which means the register layout > > (and possibly other things) are left up to the integrator? Which means > > there cannot be some kind of "generic" driver that works for a random > > PCIe controller? There is no standardization of registers in MMIO or I/O port space. The size, location, and type of those registers is configurable in a generic way via the BARs, of course. But that tells you nothing about the *functionality* of those MMIO or I/O port registers. For config space, the first 64 bytes (the PCIe r3.0 spec calls it the "PCI 3.0 Compatible Configuration Space Header") are completely standardized. This is where device IDs, BARs, and other generic registers are. PCIe devices can implement up to 4096 bytes of config space. The space after the 64-byte header can contain a list of standardized PCI/PCIe Capabilities, non-standardized device-specific registers, or both. There is a way to include device-specific registers in a "Vendor Defined" Capability -- that way drivers can use generic mechanisms to find registers so they don't have to hard-code register layout assumptions. > > I guess my main question is: do you have pointers on getting started > > writing a PCIe driver good enough for upstream? > > > > Is Documentation/PCI/PCIEBUS-HOWTO.txt still a relevant resource? PCIEBUS-HOWTO.txt is still relevant, but only to drivers for services of PCIe ports, e.g., hotplug, error reporting, etc. It's not relevant to PCIe endpoint drivers. > > Are there good resources to get up to speed on the PCIe terminology? > > I'm also wondering: in many Linux subsystems, there are often > several drivers of different quality; some are obsolescent and > have not been updated in a while, while some are recent and > follow all the latest best-practice guidelines (and the rest > is somewhere in the middle). > > Are there 1 or 2 very good PCIe drivers to take as examples > as good starting points? (Perhaps a simple driver, and a > more complex driver.) I'm not really the best person to ask about this because I deal more with the PCI core than with the drivers that use the core. If I were looking, I might start with drivers/nvme/host/pci.c or drivers/usb/host/xhci-pci.c. Bjorn
