[RFC PATCH 00/11] Refactor MSI to support Non-PCI device

[Bharat.Bhushan at freescale.com]

Hi Yijing

> -----Original Message-----
> From: linux-pci-owner at vger.kernel.org [mailto:linux-pci-owner at vger.kernel.org]
> On Behalf Of Yijing Wang
> Sent: Monday, August 04, 2014 8:34 AM
> To: Basu Arnab-B45036
> Cc: Xinwei Hu; Wuyun; Bjorn Helgaas; linux-pci at vger.kernel.org;
> Paul.Mundt at huawei.com; James E.J. Bottomley; Marc Zyngier; linux-arm-
> kernel at lists.infradead.org; Russell King; linux-arch at vger.kernel.org;
> virtualization at lists.linux-foundation.org; Hanjun Guo; linux-
> kernel at vger.kernel.org
> Subject: Re: [RFC PATCH 00/11] Refactor MSI to support Non-PCI device
> 
> >> MSI was introduced in PCI Spec 2.2. Currently, kernel MSI driver
> >> codes are bonding with PCI device. Because MSI has a lot advantages in
> design.
> >> More and more non-PCI devices want to use MSI as their default interrupt.
> >> The existing MSI device include HPET. HPET driver provide its own MSI
> >> code to initialize and process MSI interrupts. In the latest GIC v3
> >> spec, legacy device can deliver MSI by the help of a relay device
> >> named consolidator.
> >> Consolidator can translate the legacy interrupts connected to it to
> >> MSI/MSI-X. And new non-PCI device will be designed to support MSI in
> >> future. So make the MSI driver code be generic will help the non-PCI
> >> device use MSI more simply.
> >
> > As per my understanding the GICv3 provides a service that will convert writes
> to a specified address to IRQs delivered to the core and as you mention above
> MSIs are part of the PCI spec. So I can see a strong case for non-PCI devices to
> want MSI like functionality without being fully compliant with the requirements
> of the MSI spec.
> 
> In GICv3, MBI is named for the service, but there is no more detailed
> information about it, only we can know is MBI is analogous to MSI, MBI devices
> must have address/data registers, but other registers like enable/mask/ctrl are
> not mandatory requirement.
> I don't know whether the MBI spec will be release, but anyway I think MSI
> refactoring is make sense, there are some existing Non-PCI MSI device like hpet,
> dmar.
> For simplicity, let name MSI and MBI to MSI temporarily.
> >
> > My question is do we necessarily want to rework so much of the PCI-MSI layer
> to support non PCI devices? Or will it be sufficient to create a framework to
> allow non PCI devices to hook up with a device that can convert their writes to
> an IRQ to the core.
> >
> > As I understand it, the msi_chip is (almost) such a framework. The only
> problem being that it makes some PCI specific assumptions (such as PCI specific
> writes from within msi_chip functions). Won't it be sufficient to make the
> msi_chip framework generic enough to be used by non-PCI devices and let each
> bus/device manage any additional requirements (such as configuration flow, bit
> definitions etc) that it places on message based interrupts?
> 
> msi_chip framework is important to support that, but I think maybe it's not
> enough, msi_chip is only responsible for IRQ allocation, teardown, etc..
> 
> The key difference between PCI device and Non-PCI MSI is the interfaces to
> access hardware MSI registers.
> for instance, currently, msi_chip->setup_irq() to setup MSI irq and configure
> the MSI address/data registers, so we need to provide device specific
> write_msi_msg() interface, then when we call msi_chip->setup_irq(), the device
> MSI registers can be configured appropriately.

What if we can register/override the setup_irq() from bus-driver (not sure, but may be device-driver itself). Example PCI bus-driver will provide setup_irq() (or the part of setup_irq which set address and data in h/w) by PCI bus, which configure address/data in h/w as per PCI standard. 

We in Freescale will be using MSI for the devices behind a new-bus (which is not PCI based), We have a separate bus driver for same. And this new bus driver register/provide its own address/data write function which is based on that specific bus protocol.

Thanks
-Bharat

> 
> My patchset is just a RFC draft, I will update it later, all we want to do is
> make kernel support Non-PCI MSI devices.
> 
> Thanks!
> Yijing.
> 
> 
> >
> > Thanks
> > Arnab
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> >
> 
> 
> --
> Thanks!
> Yijing
> 
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