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

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Date:	Tue, 12 Jul 2016 09:11:51 +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 11/07/16 11:51, 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.
> >>
> >> Interesting. It looks like we're doing something wrong in the MSI flow.
> >> Can you confirm that this is limited to MSI and doesn't affect MSI-X?
> >>
> > I think it's the same issue irrespective of MSI or MSI-X as we are
> > enabling these interrupts before providing the  vectors.
> >
> > So we always have a hole when MSI/MSI-X is 1, and software driver has
> > not registered the irq, and End Point may raise an interrupt (may be
> > due to error) in this point of time.
>
> Looking at the MSI-X part of the code, there is this:
>
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/pci
> /msi.c#n764
>
> which hints that it may not be possible to do otherwise. Damned if you do,
> damned if you don't.
>
MSI-X might not have problem then, how to resolve the issue with MSI ?

Thanks & Regards,
Bharat


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