RE: Question on MSI support in PCI and PCI-E devices

[McKay, Luke]

Legacy INTx is shared amongst multiple devices.  Since it is a level sensitive simulation of the interrupt line, it only takes one device (or driver) to forget to clear the interrupt, and then it stuck and won't work for any of the devices using it.

If you're working with one particular device that seems to be causing these sorts of problems then you can verify misbehaving hardware with a PCIe analyzer.  With the analyzer you can verify that when the driver informs the device that it has processed the interrupt that the device sends the deassertion message for the INTx line.

Or if that isn't available, simply verifying that interrupt being cleared by the driver on the end device is taken correctly and then verifying the chain of propagation that clears the interrupt status.  It can be verified through any switch that is in the path, to the root port where the legacy PCI interrupt controller that the interrupt is cleared, to the top level interrupt controller.

Regards,
Luke

-- 
Luke McKay 
Senior Engineer
Cobham AvComm
T : +1 (316) 529 5585

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-----Original Message-----
From: Roger Heflin [mailto:rogerheflin@xxxxxxxxx] 
Sent: Wednesday, March 04, 2015 10:31 AM
To: McKay, Luke
Cc: Andrey Utkin; Andrey Utkin; Stephen Hemminger; kernel-mentors@xxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; kernelnewbies
Subject: Re: Question on MSI support in PCI and PCI-E devices

I know from some data I have seen that between the Intel Sandy Bridge and Intel Ivy Bridge the same motherboards stopped delivering INTx reliably (int lost under load around 1x every 30 days, driver and
firmware has no method to recover from failure)   We had to transition
to using MSI on some PCI cards that had this issue. Our issue was duplicated on a large number of different physical machines so if it was a hardware error is was a lot of different physical machines that had the defect.

On Wed, Mar 4, 2015 at 10:03 AM, McKay, Luke <Luke.McKay@xxxxxxxxxxxx> wrote:
> I don't personally know of any PCI drivers that use polling instead of interrupts, since that would really mean the hardware is broke.
>
> Basically all you need to do is create a timer, and have it's callback set to your driver routine that can check the device status registers to determine if there is work to be done.  The status register(s) would be the same indicators that should have generated an interrupt.
>
> Regards,
> Luke
>
>
> --
> Luke McKay
> Senior Engineer
> Cobham AvComm
> T : +1 (316) 529 5585
>
> Please consider the environment before printing this email
>
>
>
> -----Original Message-----
> From: Andrey Utkin [mailto:andrey.utkin@xxxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, March 03, 2015 8:29 AM
> To: McKay, Luke
> Cc: Andrey Utkin; Stephen Hemminger; kernel-mentors@xxxxxxxxxxx; 
> linux-kernel@xxxxxxxxxxxxxxx; kernelnewbies
> Subject: Re: Question on MSI support in PCI and PCI-E devices
>
> On Mon, Mar 2, 2015 at 4:02 PM, McKay, Luke <Luke.McKay@xxxxxxxxxxxx> wrote:
>> It doesn't appear that your device supports MSI.  If it did lspci -v should list the MSI capability and whether or not it is enabled.
>>
>> i.e. Something like...
>> Capabilities: [68] MSI: Enable+ Count=1/1 Maskable- 64bit+
>>
>> Without a listing that shows the capability is present, there is nothing to enable.
>>
>> Have you tried polling instead of using interrupts?  Definitely not ideal, but it might help you to determine whether hardware is dropping/missing an interrupt or whether the hardware is being completely hung up.
>>
>> Do you know if this missing interrupt is occurring in other systems as well?  How about whether it happens with different boards in the same system?  Answers to these questions would help to determine whether you might have a defective board, or some sort of incompatibility with the system.
>
> We have just three setups reproducing this. We have no boards for replacement experiments, unfortunately.
> Polling instead of using interrupts sounds interesting. Is there an example of such usage in any other PCI device driver?
>
> --
> Bluecherry developer.
>
>
> Aeroflex is now a Cobham company


Aeroflex is now a Cobham company

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