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

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Date:	Wed, 20 Jul 2016 13:19:34 +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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: PCIe MSI address is not written at pci_enable_msi_range call

+tglx

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.

A couple of additional questions:

Does your HW support MSI masking? And if it does, does it resample the
address/data on unmask?

Thanks,

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