linux-kernel - RE: PCIe MSI address is not written at pci_enable_msi

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Date:	Wed, 13 Jul 2016 09:36:37 +0000
From:	Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>
To:	Marc Zyngier <marc.zyngier@....com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Arnd Bergmann <arnd@...db.de>, Bjorn Helgaas <bhelgaas@...gle.com>,
	nofooter <nofooter@...inx.com>
Subject: RE: PCIe MSI address is not written at pci_enable_msi_range call

> Subject: Re: PCIe MSI address is not written at pci_enable_msi_range call
>
> On 13/07/16 10:10, Bharat Kumar Gogada wrote:
> >> Subject: Re: PCIe MSI address is not written at pci_enable_msi_range
> >> call
> >>
> >> On 13/07/16 09:33, Bharat Kumar Gogada wrote:
> >>>> Subject: Re: PCIe MSI address is not written at
> >>>> pci_enable_msi_range call
> >>>>
> >>>> On 13/07/16 07:22, Bharat Kumar Gogada wrote:
> >>>>>> Subject: Re: PCIe MSI address is not written at
> >>>>>> pci_enable_msi_range call
> >>>>>>
> >>>>>> On 11/07/16 10:33, Bharat Kumar Gogada wrote:
> >>>>>>> Hi Marc,
> >>>>>>>
> >>>>>>> Thanks for the reply.
> >>>>>>>
> >>>>>>> From PCIe Spec:
> >>>>>>> MSI Enable Bit:
> >>>>>>> If 1 and the MSI-X Enable bit in the MSI-X Message Control
> >>>>>>> register (see Section 6.8.2.3) is 0, the function is permitted
> >>>>>>> to use MSI to request service and is prohibited from using its INTx#
> pin.
> >>>>>>>
> >>>>>>> From Endpoint perspective, MSI Enable = 1 indicates MSI can be
> >>>>>>> used
> >>>>>> which means MSI address and data fields are available/programmed.
> >>>>>>>
> >>>>>>> In our SoC whenever MSI Enable goes from 0 --> 1 the hardware
> >>>>>>> latches
> >>>>>> onto MSI address and MSI data values.
> >>>>>>>
> >>>>>>> With current MSI implementation in kernel, our SoC is latching
> >>>>>>> on to incorrect address and data values, as address/data are
> >>>>>>> updated much later
> >>>>>> than MSI Enable bit.
> >>>>>>
> >>>>>> As a side question, how does setting the affinity work on this
> >>>>>> end-point if this involves changing the address programmed in the
> >>>>>> MSI
> >>>> registers?
> >>>>>> Do you expect the enabled bit to be toggled to around the write?
> >>>>>>
> >>>>>
> >>>>> Yes,
> >>>>
> >>>> Well, that's pretty annoying, as this will not work either. But
> >>>> maybe your
> >> MSI
> >>>> controller has a single doorbell? You haven't mentioned which HW
> >>>> that
> >> is...
> >>>>
> >>> The MSI address/data is located in config space, in our SoC for the
> >>> logic
> >> behind PCIe
> >>> to become aware of new address/data  MSI enable transition is used
> >>> (0 to
> >> 1).
> >>> The logic cannot keep polling these registers in configuration space
> >>> as it
> >> would consume power.
> >>>
> >>> So the logic uses the transition in MSI enable to latch on to address/data.
> >>
> >> I understand the "why". I'm just wondering if your SoC needs to have
> >> the MSI address changed when changing the affinity of the MSI? What
> >> MSI controller are you using? Is it in mainline?
> >>
> > Can you please give more information on MSI affinity ?
> > For cpu affinity for interrupts we would use MSI-X.
> >
> > We are using GIC 400 v2.
>
> None of that is relevant. GIC400 doesn't have the faintest notion of what an
> MSI is, and MSI-X vs MSI is an end-point property.
>
> Please answer these questions: does your MSI controller have a unique
> doorbell, or multiple doorbells? Does it use wired interrupts (SPIs) connected
> to the GIC? Is the support code for this MSI controller in mainline or not?
>

It has single doorbell.
The MSI decoding is part of our PCIe bridge, and it has SPI to GIC.
Our root driver is in mainline drivers/pci/host/pcie-xilinx-nwl.c

Regards,
Bharat


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