linux-kernel - Re: Removal of bus->msi assignment breaks MSI with stacked domains

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Message-ID: <alpine.DEB.2.11.1411211142051.6439@nanos>
Date:	Fri, 21 Nov 2014 11:49:05 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Marc Zyngier <marc.zyngier@....com>
cc:	Yijing Wang <wangyijing@...wei.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	Will Deacon <Will.Deacon@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: Removal of bus->msi assignment breaks MSI with stacked domains

Marc,

On Fri, 21 Nov 2014, Marc Zyngier wrote:
> On 21/11/14 01:46, Thomas Gleixner wrote:
> > So the real question is:
> > 
> >    What is the association level requirement to properly identify the
> >    irqdomain for a specific device on any given architecture with and
> >    without IOMMU, interrupt redirection etc.
> > 
> > To be honest: I don't know.
> > 
> > My gut feeling tells me that it's at the device level, but I really
> > leave that decision to the experts in that field.
> 
> Given the above requirement (single device associated to DMAR), I can
> see two possibilities:
> - we represent DMAR as a single PCI bus: feels a bit artificial
> - we move the MSI domain to the device, as you suggested.
> 
> The second one seems a lot more attractive to me.

And that's very useful if you want to support MSI on non PCI
devices.

> Also, it is not clear to me what is the advantage of getting rid of the
> MSI controller. By doing so, we loose an important part of the topology
> information (the irq domain is another level of abstraction).

That was probably my misunderstanding of the msi controller. I had the
impression it's just there to expose the MSI properties of a device,
i.e. a magic wrapper which can be replaced by the MSI irqdomain work.

If that handles other information as well, then it's probably a
misnomer to begin with.

Thanks,

	tglx


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