linux-kernel - Re: [Patch V1 0/6] Refine generic/PCI MSI irqodmian interfaces

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Date:	Fri, 14 Nov 2014 16:13:27 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Yijing Wang <wangyijing@...wei.com>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Tony Luck <tony.luck@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Patch V1 0/6] Refine generic/PCI MSI irqodmian interfaces

On 14/11/14 15:54, Jiang Liu wrote:
> On 2014/11/14 5:28, Marc Zyngier wrote:
>> On 13/11/14 21:11, Thomas Gleixner wrote:
>>> On Thu, 13 Nov 2014, Marc Zyngier wrote:
>>>> With the new stacked irq domains, it becomes pretty tempting
>>>> to allocate an MSI domain per PCI bus, which would remove
>>>> the requirement of either relying on arch-specific code, or
>>>> a default PCI MSI domain.
>>>
>>> Right. That's what I roughly had in mind. And that would solve the
>>> multi-iommu issue on x86 nicely as well. We establish the association
>>> at the time where the bus gets populated. So the whole lookup magic
>>> simply goes away.
>>
>> Great. I've pushed the whole thing out with this patch, the couple
>> of fixes I mentioned earlier, as well as the whole ITS code:
>>
>> git://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git irq/stacked-its-v2
> Hi Marc,
> 	I have looked at the code, and found some issues.
> 1) With my next version, no need to implemeent its_pci_msi_free()
> anymore. msi_domain_free_irqs() will reset desc->irq to zero
> for all architectures.

Great. The less code, the better.

> 2) This piece of code in its_msi_prepare() may run into trouble
> for PCI device with both MSI and MSIX capability. I will change
> msi_prepare() prototype to pass in the "nvec" parameter. And you
> may access first_pci_msi_entry()->msi_attrib.is_msix to get
> allocation type if needed.
> 	nvec = pci_msix_vec_count(pdev);
> 	if (nvec < 0)
> 		nvec = pci_msi_vec_count(pdev);
> 	if (nvec < 0)
> 		return nvec;

Ah, good point. Having nvec will make things simpler indeed. I have the
feeling that the ITS driver may not be the only one to require such a thing.

> 3) Do we need to increase the default of NUM_MSI_ALLOC_SCRATCHPAD_REGS
> to 4? 2 is a little too limited.

I'd say that 2 is enough for now. The ITS only uses one (the second one
is for debug only). Should the need arise, we can bump it up later.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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