linux-kernel - Re: [PATCH 10/14] PCI: tegra: Move PCIe driver to drivers/pci/host

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Date:	Tue, 15 Jan 2013 22:14:41 +0100
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Andrew Murray <andrew.murray@....com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Stephen Warren <swarren@...dotorg.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH 10/14] PCI: tegra: Move PCIe driver to drivers/pci/host

On Tue, Jan 15, 2013 at 03:40:38PM +0000, Andrew Murray wrote:
> On Tue, Jan 15, 2013 at 12:44:12PM +0000, Arnd Bergmann wrote:
> > On Tuesday 15 January 2013, Thierry Reding wrote:
> > > I'm not sure I follow you're reasoning here. Is it possible to use MSIs
> > > without PCI? If not then I think there's little sense in keeping the
> > > implementations separate.
> > 
> > Conceptually, you can use MSI for any device, but the Linux interfaces
> > for MSI are tied to PCI. If you use an MSI controller for a non-PCI
> > device, it would probably just appear as a regular interrupt controller.
> > 
> > > Furthermore, if MSI controller and PCI host bridge are separate entities
> > > how do you look up the MSI controller given a PCI device?
> > 
> > The host bridge can contain a pointer ot the MSI controller. You can
> > have multiple host bridges sharing a single MSI controller or you
> > can have separate ones for each host.
> 
> Yes and I hoped this relationship would be described by a device tree phandle
> as is done for relating devices to their interrupt-parent (where device trees
> are used). This would provide (arguably unnecessarily) greater flexibility,
> e.g. if you have two PCI/MSI controller pairs, the MSIs only offer limited MSIs
> and you only use one PCI fabric - you could service different parts of the
> fabric by different MSI controllers (assuming you relate MSI controllers to
> part of the fabric and that you'd want to). Perhaps there would be benefits for
> virtualisation as well?

Is there actually hardware that supports this? I assumed that the MSI
controller would have to be tightly coupled to the PCI host bridge in
order to raise an interrupt when an MSI is received via PCI.

Thierry

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