linux-kernel - Re: PCI: MSI interrupts masked using prohibited method

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Message-ID: <4864DA54.6000205@csr.com>
Date:	Fri, 27 Jun 2008 13:17:24 +0100
From:	David Vrabel <david.vrabel@....com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
CC:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: PCI: MSI interrupts masked using prohibited method

Jesse Barnes wrote:
> On Tuesday, June 24, 2008 3:46 am David Vrabel wrote:
>> PCI MSI interrupts are masked and unmasked using a method (by writing
>> the MSI Enable capability bit) that is prohibited by the PCI specification.
> 
> Yeah, it's probably quite a bit slower too (I assume you're talking about 
> io_apic_64's msi_mask_irq).  Seems like masking this at the ioapic level 
> would make more sense anyway...
> 
>> This behaviour can cause missed interrupts with some devices if the
>> interrupt is asserted by the hardware while MSI is disabled.
>>
>> I believe the interrupt should be masked/unmasked on the interrupt
>> controller (the APIC on x86, for example).   I'm going to test this now
>> and see if it works.

After further research it seems that MSI interrupts aren't routed via
the IO-APIC, so this cannot be done.

I think the only solution is to not perform any sort of masking and rely
on the device driver being able to handle this.

David
-- 
David Vrabel, Senior Software Engineer, Drivers
CSR, Churchill House, Cambridge Business Park,  Tel: +44 (0)1223 692562
Cowley Road, Cambridge, CB4 0WZ                 http://www.csr.com/

View attachment "pci-dont-mask-msi-with-msi-enable-bit.patch" of type "text/x-diff" (1923 bytes)