Re: [patch 04/16] genirq: Introduce irq_chip.irq_compose_msi_msg() to support stacked irqchip

From: Jiang Liu
Date: Tue Nov 18 2014 - 08:56:01 EST

On 2014/11/18 21:48, Yun Wu (Abel) wrote:
> On 2014/11/18 21:25, Jiang Liu wrote:
>> On 2014/11/18 21:16, Yun Wu (Abel) wrote:
>>> On 2014/11/18 20:43, Jiang Liu wrote:
>>>> On 2014/11/18 19:47, Yun Wu (Abel) wrote:
>>>>> On 2014/11/18 18:02, Thomas Gleixner wrote:
>>>>>> On Tue, 18 Nov 2014, Yun Wu (Abel) wrote:
>>>>>>> On 2014/11/12 21:42, Thomas Gleixner wrote:
>>>>>>>> +int irq_chip_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
>>>>>>>> +{
>>>>>>>> + struct irq_data *pos = NULL;
>>>>>>>> +
>>>>>>>> + for (; data; data = data->parent_data)
>>>>>>>> +#endif
>>>>>>>> + if (data->chip && data->chip->irq_compose_msi_msg)
>>>>>>>> + pos = data;
>>>>>>>> + if (!pos)
>>>>>>>> + return -ENOSYS;
>>>>>>>> +
>>>>>>>> + pos->chip->irq_compose_msi_msg(pos, msg);
>>>>>>>> +
>>>>>>>> + return 0;
>>>>>>>> +}
>>>>>>> Adding message composing routine to struct irq_chip is OK to me, and it should
>>>>>>> be because it is interrupt controllers' duty to compose messages (so that they
>>>>>>> can parse the messages correctly without any pre-defined rules that endpoint
>>>>>>> devices absolutely need not to know).
>>>>>>> However a problem comes out when deciding which parameters should be passed to
>>>>>>> this routine. A message can associate with multiple interrupts, which makes me
>>>>>>> think composing messages for each interrupt is not that appropriate. And we
>>>>>>> can take a look at the new routine irq_chip_compose_msi_msg(). It is called by
>>>>>>> msi_domain_activate() which will be called by irq_domain_activate_irq() in
>>>>>>> irq_startup() for each interrupt descriptor, result in composing a message for
>>>>>>> each interrupt, right? (Unless requiring a judge on the parameter @data when
>>>>>>> implementing the irq_compose_msi_msg() callback that only compose message for
>>>>>>> the first entry of that message. But I really don't like that...)
>>>>>> No, that's not correct. You are looking at some random stale version
>>>>>> of this. The current state of affairs is in
>>>>>> git:// irq/irqdomain
>>>>>> See also
>>>>>> In activate we write the message, which is the right point to do so.
>>>>> I checked the current state, it seems to be the same.
>>>>> Yes, the decision of postponing the actual hardware programming to the point
>>>>> where the interrupt actually gets used is right, but here above I was talking
>>>>> another thing.
>>>>> As I mentioned, a message can associate with multiple interrupts. Enabling
>>>>> any of them will call irq_startup(). So if we don't want to compose or write
>>>>> messages repeatedly, we'd better require performing some checks before
>>>>> activating the interrupts.
>>>> Hi Yun,
>>>> Seems you are talking about the case of multiple MSI support.
>>>> Yes, we have special treatment for multiple MSI, which only writes PCI
>>>> MSI registers when starting up the first MSI interrupt.
>>>> void pci_msi_domain_write_msg(struct irq_data *irq_data, struct msi_msg
>>>> *msg)
>>>> {
>>>> struct msi_desc *desc = irq_data->msi_desc;
>>>> /*
>>>> * For MSI-X desc->irq is always equal to irq_data->irq. For
>>>> * MSI only the first interrupt of MULTI MSI passes the test.
>>>> */
>>>> if (desc->irq == irq_data->irq)
>>>> __pci_write_msi_msg(desc, msg);
>>>> }
>>> Yes, I picked the case of multiple MSI support.
>>> The check should also be performed when composing messages. That's why
>>> I don't like its parameters. The @data only indicates one interrupt,
>>> while I prefer doing compose/write in the unit of message descriptor.
>> Hi Yun,
>> The common abstraction is that every message interrupt could be
>> controlled independently, so have compose_msi_msg()/write_msi_msg() per
>> interrupt. MSI is abstracted as an special message signaled interrupt
>> with hardware limitation where multiple interrupts sharing the same
>> hardware registers. So we filter in pci_msi_domain_write_msg(). On the
>> other handle, the generic MSI framework caches msi_msg in msi_desc,
>> so we don't filter compose_msi_msg().
> It's true that every message interrupt could be controlled independently,
> I mean, by enable/disable/mask/unmask. But the message data & address are
> shared among the interrupts of that message.
> Despite the detailed hardware implementation, MSI and MSI-X are the same
> thing in software view, that is a message related with several consecutive
> interrupts. And the core MSI infrastructure you want to build should not
> be based on any hardware assumptions.
That's the key point. We abstract MSI as using a message to control an
interrupt source instead of controlling several consecutive interrupts.
PCI MSI is just a special case which controls a group of consecutive
interrupts all together due to hardware limitation.

> Thanks,
> Abel
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