Support for configurable PCIe endpoint

[arnd@arndb.de (Arnd Bergmann)]

On Wednesday, August 3, 2016 2:47:47 AM CEST Christoph Hellwig wrote:
> On Wed, Aug 03, 2016 at 11:33:19AM +0530, Kishon Vijay Abraham I wrote:
> > Hi,
> > 
> > The PCIe controller present in TI's DRA7 SoC is capable of operating either in
> > Root Complex mode or Endpoint mode. (It uses Synopsys Designware Core).I'd
> > assume most of the PCIe controllers on other platforms that use Designware core
> > should also be capable to operate in endpoint mode. But linux kernel right now
> > supports only RC mode.
> > 
> > PCIe endpoint support discussion came up briefly before [1] but it was felt the
> > practical use case will find firmware more suitable and endpoint support in
> > kernel can be used only for validation or demo.
> 
> I disagree.  It's highly useful for rapid prototyping of hardware
> interfaces, and I've been looking into PCIe EP drivers for exactly
> that reason recently.  Going a little offtopic: any good DRA7 eval
> boards you'd recommend to try for this purpose?
> 
> We already have a EP driver in the tree:
> 
> drivers/misc/spear13xx_pcie_gadget.c
> 
> but as far as I can tell it doesn't really work at the moment.
> 

This is indeed for the designware pcie hardware, but it never worked
in a mainline kernel, and I marked it 'depends on BROKEN' after it
had not correctly compiled for a long time. It's also lacking because
it neither abstracts the hardware nor the protocol, and I think we
want both.

drivers/ntb seems like a reasonable start, while an alternative
approach that we have discussed in the past would be based on top
of virtio, so we could use the existing front-end drivers (net, block,
v9fs, console, ...).

	Arnd