linux-kernel - Re: Masked MSIs expectations

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Message-ID: <20130620133314.GR2575@8bytes.org>
Date:	Thu, 20 Jun 2013 15:33:14 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: Masked MSIs expectations

(In case this topic is still relevant)

On Thu, May 09, 2013 at 06:09:42PM +1000, Benjamin Herrenschmidt wrote:
> Do we provide drivers any guarantee to what happen if an MSI is shot
> while masked with disable_irq() or while not yet request_irq()'ed ?
> 
> Do we guarantee delivery (latched while masked), non-delivery, or
> undefined ?

I am not aware of any guarantees the kernel gives in this situation. I
think it would just drop the IRQ and print a "nobody cared" message.

> I'm bringing up a piece of HW where if it happened, it won't be
> automatically sent to the CPU and can block further MSIs unless I
> explicitly either ditch it or force a resend when unmasking (at the PCI
> Express controller PIC level).
> 
> I'm tempted to just ditch anything that happened while masked, it would
> make everything easier on my side, but maybe drivers have different
> expectations (and of course an LSI would still shoot, that's not an
> issue, only MSIs are in question here).
> 
> I have cases of devices continuing to shoot one or two MSIs after kexec
> and before the new kernel takes over, causing a "loss" of any subsequent
> one unless I deal with that case one way or another.

I would also just ditch such IRQs that happen in that kexec case and
make sure that they will work again when the kexec-kernel device driver
wants to initialize them.


	Joerg


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