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

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Date:	Mon, 11 Jul 2016 09:33:37 +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

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.

Thanks & Regards,
Bharat

> -----Original Message-----
> From: Marc Zyngier [mailto:marc.zyngier@....com]
> Sent: Monday, July 11, 2016 2:18 PM
> To: Bharat Kumar Gogada <bharatku@...inx.com>; linux-
> pci@...r.kernel.org; linux-kernel@...r.kernel.org
> Cc: Arnd Bergmann <arnd@...db.de>; Bjorn Helgaas
> <bhelgaas@...gle.com>
> Subject: Re: PCIe MSI address is not written at pci_enable_msi_range call
>
> On 11/07/16 03:32, Bharat Kumar Gogada wrote:
> > Hi,
> >
> > I have a query.
> > I see that when we use PCI_MSI_IRQ_DOMAIN to handle MSI's, MSI
> address is not being
> > written in to end point's PCI_MSI_ADDRESS_LO/HI at the call
> pci_enable_msi_range.
> >
> > Instead it is being written at the time end point requests irq.
> >
> > Can any one tell the reason why is it handled in this manner ?
>
> Because there is no real need to do it earlier, and in some case you
> cannot allocate MSIs at that stage. pci_enable_msi_range only works out
> how many vectors are required. At least one MSI controller (GICv3 ITS)
> needs to know how many vectors are required before they can be provided
> to the end-point.
>
> Do you see any issue with this?
>
> Thanks,
>
>       M.
> --
> Jazz is not dead. It just smells funny...


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