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

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Date:	Mon, 11 Jul 2016 16:50:07 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Bharat Kumar Gogada <bharat.kumar.gogada@...inx.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

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.

	M.
-- 
Jazz is not dead. It just smells funny...