On Tue, Feb 07, 2017 at 02:06:56PM -0600, Bjorn Helgaas wrote: > 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. I should have also mentioned Documentation/PCI/pci.txt. That's a pretty good place to start.
