linux-kernel - Re: Support for configurable PCIe endpoint

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Date:	Wed, 03 Aug 2016 18:03:54 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Kishon Vijay Abraham I <kishon@...com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Jingoo Han <jingoohan1@...il.com>,
	Pratyush Anand <pratyush.anand@...il.com>,
	Ley Foon Tan <lftan@...era.com>,
	Rob Herring <robh@...nel.org>,
	Tanmay Inamdar <tinamdar@....com>,
	Roy Zang <tie-fei.zang@...escale.com>,
	Mingkai Hu <mingkai.hu@...escale.com>,
	Minghuan Lian <minghuan.Lian@...escale.com>,
	Richard Zhu <Richard.Zhu@...escale.com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Murali Karicheri <m-karicheri2@...com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Jason Cooper <jason@...edaemon.net>,
	Thierry Reding <thierry.reding@...il.com>,
	Simon Horman <horms@...ge.net.au>,
	Joao Pinto <jpinto@...opsys.com>,
	Zhou Wang <wangzhou1@...ilicon.com>,
	Gabriele Paoloni <gabriele.paoloni@...wei.com>,
	Stanimir Varbanov <svarbanov@...sol.com>,
	David Daney <david.daney@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	jdmason@...zu.us
Subject: Re: Support for configurable PCIe endpoint

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