linux-kernel - Re: [PATCH v4 1/2] PCI: Add tango MSI controller support

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Message-ID: <f1a57255-be75-e8c4-6b8d-0254f4365d61@arm.com>
Date:   Wed, 24 May 2017 11:00:43 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Mason <slash.tmp@...e.fr>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiang Liu <jiang.liu@...ux.intel.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        David Laight <david.laight@...lab.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Phuong Nguyen <phuong_nguyen@...madesigns.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] PCI: Add tango MSI controller support

On 23/05/17 20:15, Mason wrote:
> On 23/05/2017 20:03, Robin Murphy wrote:
>> On 23/05/17 18:54, Mason wrote:
>>> On 23/05/2017 19:03, Bjorn Helgaas wrote:
>>>> On Wed, May 17, 2017 at 04:56:08PM +0200, Marc Gonzalez wrote:
>>>>> On 20/04/2017 16:28, Marc Gonzalez wrote:
>>>>>
>>>>>> +static int tango_set_affinity(struct irq_data *data,
>>>>>> +		const struct cpumask *mask, bool force)
>>>>>> +{
>>>>>> +	return -EINVAL;
>>>>>> +}
>>>>>> +
>>>>>> +static struct irq_chip tango_chip = {
>>>>>> +	.irq_ack		= tango_ack,
>>>>>> +	.irq_mask		= tango_mask,
>>>>>> +	.irq_unmask		= tango_unmask,
>>>>>> +	.irq_set_affinity	= tango_set_affinity,
>>>>>> +	.irq_compose_msi_msg	= tango_compose_msi_msg,
>>>>>> +};
>>>>>
>>>>> Hmmm... I'm wondering why .irq_set_affinity is required.
>>>>>
>>>>> static int setup_affinity(struct irq_desc *desc, struct cpumask *mask)
>>>>> first calls __irq_can_set_affinity() to check whether
>>>>> desc->irq_data.chip->irq_set_affinity) exists.
>>>>>
>>>>> then calls irq_do_set_affinity(&desc->irq_data, mask, false);
>>>>> which calls chip->irq_set_affinity(data, mask, force);
>>>>> = msi_domain_set_affinity()
>>>>> which calls parent->chip->irq_set_affinity() unconditionally.
>>>>>
>>>>> Would it make sense to test that the callback is implemented
>>>>> before calling it?
>>>>>
>>>>>
>>>>> [    0.723895] Unable to handle kernel NULL pointer dereference at virtual address 00000000
>>>>
>>>> I'm not sure what you're asking.
>>>>
>>>> Is this a bug report for the v4 tango driver?
>>>
>>> No.
>>>
>>>> Or are you asking whether msi_domain_set_affinity() should be changed
>>>> to check whether parent->chip->irq_set_affinity is implemented?
>>>
>>> Yes. The way things are implemented now, drivers are forced
>>> to define an irq_set_affinity callback, even if it just returns
>>> an error, otherwise, the kernel crashes, because of the
>>> unconditional function pointer deref. 
>>>
>>>> msi_domain_set_affinity() has called parent->chip->irq_set_affinity()
>>>> without checking since it was added in 2014 by f3cf8bb0d6c3 ("genirq: Add
>>>> generic msi irq domain support"), so if there's a problem here, it's most
>>>> likely in the tango code.
>>>
>>> The issue is having to define an "empty" function.
>>> (Unnecessary code bloat and maintenance.)
>>
>> AFAICS, only one driver (other than this one) implements a "do nothing"
>> set_affinity callback - everyone else who doesn't do anything hardware
>> specific just passes it along via irq_chip_set_affinity_parent().
> 
> I counted 4. Where did I mess up?
> 
> advk_msi_set_affinity
> altera_msi_set_affinity
> nwl_msi_set_affinity
> vmd_irq_set_affinity
> tango_set_affinity

Fair point - I went through drivers/irqchip and the various
arch-specific instances and found ls_scfg_msi_set_affinity(), but
somehow skipped over drivers/pci. Anyway, I think the question stands of
why are these handful of drivers *not* using irq_chip_set_affinity_parent()?

As an outsider, it naively seems logical that the affinity of an MSI
which just gets translated to a wired interrupt should propagate through
to the affinity of that wired interrupt, but maybe there are reasons not
to; I really don't know.

Robin.