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

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Date:	Wed, 13 Jul 2016 10:40:33 +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 13/07/16 10:36, Bharat Kumar Gogada wrote:
>> 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

OK, so you're not affected by this affinity setting issue. Please let me
know if the patch I sent yesterday improve things for you once you have
a chance to test it.

Thanks,

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