netdev - Re: MSI interrupts and disable

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Message-ID: <4714F736.5000302@pobox.com>
Date:	Tue, 16 Oct 2007 13:39:02 -0400
From:	Jeff Garzik <jgarzik@...ox.com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
CC:	Manfred Spraul <manfred@...orfullife.com>,
	Ayaz Abdulla <aabdulla@...dia.com>,
	nedev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: MSI interrupts and disable_irq

Yinghai Lu wrote:
> On 10/15/07, Jeff Garzik <jgarzik@...ox.com> wrote:
>> Manfred Spraul wrote:
>>> Jeff Garzik wrote:
>>>> I think the scenario you outline is an illustration of the approach's
>>>> fragility:  disable_irq() is a heavy hammer that originated with INTx,
>>>> and it relies on a chip-specific disable method (kernel/irq/manage.c)
>>>> that practically guarantees behavior will vary across MSI/INTx/etc.
>>>>
>>> I checked the code: IRQ_DISABLE is implemented in software, i.e.
>>> handle_level_irq() only calls handle_IRQ_event() [and then the nic irq
>>> handler] if IRQ_DISABLE is not set.
>>> OTHO: The last trace looks as if nv_do_nic_poll() is interrupted by an irq.
>>>
>>> Perhaps something corrupts dev->irq? The irq is requested with
>>>    request_irq(np->pci_dev->irq, handler, IRQF_SHARED, dev->name, dev)
>>> and disabled with
>>>    disable_irq_lockdep(dev->irq);
>>>
>>> Someone around with a MSI capable board? The forcedeth driver does
>>>    dev->irq = pci_dev->irq
>>> in nv_probe(), especially before pci_enable_msi().
>>> Does pci_enable_msi() change pci_dev->irq? Then we would disable the
>>> wrong interrupt....
>> Remember, fundamentally MSI-X is a one-to-many relationship, when you
>> consider a single PCI device might have multiple vectors.
> 
> msi-x is using other entry
> 
>                if (np->msi_flags & NV_MSI_X_ENABLED)
> 
> enable_irq_lockdep(np->msi_x_entry[NV_MSI_X_VECTOR_ALL].vector);

Correct, but the overall point was that MSI-X conceptually conflicts 
with the existing "lockless" disable_irq() schedule, which was written 
when there was a one-one relationship between irq, PCI device, and work 
to be done.

	Jeff



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