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

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Message-ID: <8520D5D51A55D047800579B094147198258BC6C4@XAP-PVEXMBX01.xlnx.xilinx.com>
Date:	Wed, 27 Jul 2016 11:14:17 +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>,
	Thomas Gleixner <tglx@...utronix.de>
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
>
> +tglx
>
> 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.
>
> A couple of additional questions:
>
> Does your HW support MSI masking? And if it does, does it resample the
> address/data on unmask?
>
No we do not support masking.

Regards,
Bharat



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