linux-kernel - Re: [PATCH 4/6] pci: altera: Add Altera PCIe MSI driver

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Date:	Wed, 29 Jul 2015 10:15:39 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Ley Foon Tan <lftan@...era.com>
CC:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Russell King <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 4/6] pci: altera: Add Altera PCIe MSI driver

On 29/07/15 09:52, Ley Foon Tan wrote:
> On Wed, Jul 29, 2015 at 1:58 AM, Marc Zyngier <marc.zyngier@....com> wrote:
>> Hi Ley,
>>
>> On 28/07/15 11:45, Ley Foon Tan wrote:
>>> This patch adds Altera PCIe MSI driver. This soft IP supports configurable
>>> number of vectors, which is a dts parameter.
>>
>> Can't you read this configuration from the HW?
> No, we can't read from HW.

Ah, that's a shame. Specially on HW that is configurable by design.

[...]

>>> +
>>> +                     irq = irq_find_mapping(msi->msi_domain->parent, offset);
>>
>> This would tend to indicate that you don't really need to store the
>> msi_domain pointer, but the inner_domain pointer instead.
> Will store the inner_domain pointer. But, I think we still need
> msi_domain for irq_domain_remove.
> Or do we have any way to retrieve msi_domain from inner_domain?

Do you have any case where you remove the domains, aside from the
obvious error cases?

[...]

>>> +
>>> +static struct msi_domain_info altera_msi_domain_info = {
>>> +     .flags  = (MSI_FLAG_USE_DEF_DOM_OPS | MSI_FLAG_USE_DEF_CHIP_OPS),
>>
>> So you don't support MSIX? That's a bit weird.
> Yes, this MSI IP doesn't support it.

This is not really a function of the MSI IP, but of the PCI device. In
your case, this is just a set of doorbells, so I hardly see what would
prevent MSI-X to be supported. Multi-MSI, I can see why.

[...]

>>> +static int altera_msi_set_affinity(struct irq_data *irq_data,
>>> +                              const struct cpumask *mask, bool force)
>>> +{
>>> +      return irq_set_affinity(irq_data->hwirq, mask);
>>
>> There is no way this can be right. irq_data->hwirq can *never* be passed
>> as a Linux IRQ. This really should be the IRQ to the GIC.
>>
>> Which raises another issue: as you only have a single interrupt to the
>> GIC, changing the affinity of a single MSI is going to affect all the
>> other MSIs as well. This doesn't seem like a desirable behaviour.
> Do we must provide '.irq_set_affinity" callback to msi domain? I have
> tried set it to NULL,
> but kernel got the NULL pointer deference error from msi_domain_set_affinity().
> Recently, I saw this new patch for pci-tegra.c from [1], it doesn't
> set the ".irq_set_affinity".
> Just wonder how it can work.
> 
> Do you have any recommendation way for this?
> 
> [1] https://git.kernel.org/cgit/linux/kernel/git/maz/arm-platforms.git/commit/drivers/pci/host?h=irq/gsi-irq-domain-v2&id=17c22fc4e60e6ad54c7e3b73868cbc057360fa63

Please realize that I *never* tested this patch (I don't think I ever
posted it officially, I just keep here for convenience), and I wouldn't
take it as a reference.

When it comes to msi_domain_set_affinity issue, this look more like an
oversight. I'll cook a patch for that.

Anyway, whichever way you want to do it, you need to fix this. You could
always return -EINVAL in the meantime,

Thanks,

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