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

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Date:	Wed, 13 Jul 2016 16:39:55 +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 16:34, Bharat Kumar Gogada wrote:
>> 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.
>>
> Hi Marc,
> 
> I tested with the patch you provided, now it is working for us.

Thanks, I'll repost this as a proper patch with your Tested-by.

> Can you please point to any doc related to affinity in MSI, until now we
> came across affinity for MSI-X. I will explore more on it.

I don't have anything at hand, but simply look at how MSI (and MSI-X) is
implemented on x86, for example: each CPU has its own doorbell, and
changing the affinity of a MSI is done by changing the target address of
that interrupt. And it doesn't seem that the kernel switches the Enable
bit off and on for those.

Thanks,

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