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

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Message-ID: <20130129133140.GA24193@arm.com>
Date:	Tue, 29 Jan 2013 13:31:40 +0000
From:	Andrew Murray <andrew.murray@....com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	Thierry Reding <thierry.reding@...onic-design.de>,
	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>,
	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 22, 2013 at 07:29:01PM +0000, Jason Gunthorpe wrote:
> On Thu, Jan 17, 2013 at 04:22:18PM +0000, Andrew Murray wrote:
> 
> > > In either of those cases, does it make sense to use the MSI support
> > > outside the scope of the PCI infrastructure? That is, would devices
> > > other than PCI devices be able to generate an MSI?
> > 
> > I've come around to your way of thinking. Your approach sounds good for
> > registration of MSI ops - let the RC host driver do it (it probably has its
> > own), or use a helper for following a phandle to get ops that are not part
> > of the driver. MSIs won't be used outside of PCI devices.
> 
> Here is a bit of additional info on some MSI stuff..
> 
> This can be pretty complex. For instance on hyper transport systems
> the PCI to HT bridge has an MSI controller that maps between PCI and
> HT MSI formats, that mapping is configurable, so technically each
> brige could be considered a MSI controller. Typically the mapping
> controllers are all setup the same so there is not much problem with
> this. However *native* HT devices can (which are super rare) can use a
> different MSI format than PCI devices. From a linux perspective HT is
> just a variant of PCI.
>  
> On x86 the MSI is delivered to the CPU APIC complex which converts it
> into a vectored interrupt - part of the value of MSI is that the MSI
> data can vector the interrupt to a specific CPU, or group of CPUs or
> whatever.
> 
> Presumably SMP ARMs will evolve similar MSI based interrupt vectoring
> capabilities, and presumably on-chip, non-PCI peripherals will evolve
> options to use MSI as well (ie multi-queue ethernet). So it might be
> worth giving some thought to how things could migrate in that
> direction someday.
> 
> I have a bit hacky MSI driver for Kirkwood, this work you have to
> generalize the interface could let me actually upstream it :) The MSI
> is built using the Host2CPU doorbell registers, so it is entirely
> unrelated to the PCI-E RC driver.
> 
> However, my use of the MSI driver on kirkwood is to assign MSIs to a
> PCI-E device via non-standard registers, more like an on chip
> peripheral. This is because the Host2CPU doorbell doesn't fit 100%
> perfectly with the standard PCI MSI stuff, and the hardware has funny
> needs.. So an 'allocate a MSI interrupt' API would be snazzy too :)

Thanks for this. I believe Thierry may be working on improving the MSI
API - so perhaps we can see where that takes us.

Andrew Murray

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