linux-kernel - Re: Support for configurable PCIe endpoint

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Message-ID: <2795776.a946dnWVV6@wuerfel>
Date:	Wed, 03 Aug 2016 09:42:39 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.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>,
	Gabriele Paoloni <gabriele.paoloni@...wei.com>,
	David Daney <david.daney@...ium.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Tanmay Inamdar <tinamdar@....com>,
	Joao Pinto <jpinto@...opsys.com>,
	Murali Karicheri <m-karicheri2@...com>,
	Jason Cooper <jason@...edaemon.net>,
	Simon Horman <horms@...ge.net.au>,
	Mingkai Hu <mingkai.hu@...escale.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Richard Zhu <Richard.Zhu@...escale.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stanimir Varbanov <svarbanov@...sol.com>,
	Minghuan Lian <minghuan.Lian@...escale.com>,
	Zhou Wang <wangzhou1@...ilicon.com>,
	Ley Foon Tan <lftan@...era.com>,
	Roy Zang <tie-fei.zang@...escale.com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Jon Mason <jdmason@...zu.us>
Subject: Re: Support for configurable PCIe endpoint

On Wednesday, August 3, 2016 11:33:19 AM CEST 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.
> 
> 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.
> 
> 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.

(Adding Jon Mason)

We have the drivers/ntb framework, which in theory should be able to handle
this, but in practice is only used for a very small number of bridge
implementations, and is also limited in the way it can be configured
(compared to USB gadget)

> 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.

A lot of the other drivers in drivers/pci/host support endpoint mode too,
I don't think moving them all elsewhere is helpful or necessary here.

> *) Add library functions in pcie-designware.c specific to EP mode like
> initializing general ecam registers, BAR registers etc.. These functions should
> be invoked from a *function* driver (function driver should determine the use
> of a particular EP).
> 
> *) Add a sample *function* driver that can be used just for validation. This
> function driver will invoke the previously added functions in PCIe controller
> to initialize ecam, BAR etc.. This will invoke the PCIe controller functions
> through *ep-core* layer. That way the same function driver can be made to work
> with different PCIe controllers. (A PCIe driver corresponding to this function
> driver should also be implemented in RC side)
> 
> *) Add *ep-core* layer to bind the function driver to the controller driver and
> using which the function driver will invoke controller driver callbacks (to
> initialize ecam, BAR registers etc..)

I think we should first have a rough idea what the framework should look like,
and how it handles having multiple hardware implementation and multiple
protocol implementations, before we modify a particular driver.

So this step here should come a bit earlier than the others.

> *) Modify platform driver to support EP mode (in my case pci-dra7xx.c).
> 
> *) dt binding specific to EP mode should be created.

Right.

	Arnd