Support for configurable PCIe endpoint

[hch@infradead.org (Christoph Hellwig)]

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.

> Validation or demo is itself a valid use case in my opinion (consider something
> similar to gadget zero for USB). There can be other use cases as well. The RC
> can use the SoC with EP mode support as an accelerator to accomplish specific
> task. Here RC gives data to the EP. The EP processes the data. The processing
> can be done either in ARM itself or it can use other hardware accelerators
> (like DSP, IVA-HD etc..) present in the EP system. If HW accelerator is used,
> the linux kernel running in ARM can be used to accomplish other tasks. Once EP
> mode support is added, I think more use cases will be added.

That sounds useful as well.

> >From the high level this should look _similar_ to the gadget framework of USB.
> One difference from USB would be this should allow HW components (like DSP, PRU
> etc.. and maybe even some peripheral) in the EP system to be used by RC system.

Indeed.

> So these are the high-level steps that I thought would be needed to add EP
> support in linux.
> *) move pcie-designware.c out of drivers/pci/host (maybe create a
> drivers/pci/designware/ folder?). All users of pcie-designware.c should be
> moved here.
> This is in preparation for adding EP mode support to designware.

I'd use a new drivers/pci/controller.  Or maybe just skip the rename
for now and see how this evolves.

The rest of the plan sounds fine to me.