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

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Message-Id: <200807251212.41980.jbarnes@virtuousgeek.org>
Date:	Fri, 25 Jul 2008 12:12:41 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	David Vrabel <david.vrabel@....com>,
	Michal Schmidt <mschmidt@...hat.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: PCI: MSI interrupts masked using prohibited method

On Friday, July 25, 2008 9:56 am Matthew Wilcox wrote:
> On Fri, Jul 25, 2008 at 05:37:49PM +0100, David Vrabel wrote:
> > The spec says that system software should enable MSI before setting
> > message address and data (PCI 3.0 section 6.8.3.1 MSI configuration).
> > The kernel doesn't do this.
>
> I think you meant "disable"?  I can't find anything in 6.8.3.1 of 3.0
> that refers to this.
>
> > I really don't think we should be enabling/disabling MSI while
> > interrupts might be being generated.  There are cases where interrupts
> > will be lost.  Consider PCIe where we might end up with a situation
> > where MSI is disabled and then enabled sufficiently quickly that no
> > periodic line interrupt message is sent by the device.
>
> I don't think there's a difference here between PCIe and conventional
> PCI.  A device raising a line based interrupt is perfectly equivalent to
> a device sending an INTx message.
>
> > The message address and data should only be modified while the vector is
> > masked (to avoid the aforementioned 'tearing').  This means that setting
> > IRQ affinity cannot be done on devices without per-vector mask bits.  I
> > don't think this is a problem.
>
> I agree.  I think it's fine to have this limitation.
>
> > In vague psuedo-code, set_affinity() should be something like this:
> >
> > int did_mask = msi_mask_vector();
> > if (!did_mask) {
> >     return -ENOTSUPP;
> > }
> > /* fiddle with address and mask now */
> > msi_unmask_vector();
>
> Yes, something like that.

Yeah, that reflects what we actually support...

David, can you resend your "don't mask MSIs using the MSI enable bit" patch 
against the latest bits so I can apply it?  Did you also want to hack up the 
above for the affinity code?

Thanks,
Jesse
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