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

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Message-ID: <5785F8DC.4060100@arm.com>
Date:	Wed, 13 Jul 2016 09:16:28 +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 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...

> Would anybody change MSI address in between wouldn't it cause race condition ?

Changing the affinity of an interrupt is always racy, and the kernel
deals with it.

Thanks,

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