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

[Jiang Liu <jiang.liu@linux.intel.com>]

On 2014/11/18 22:03, Yun Wu (Abel) wrote:
> On 2014/11/18 21:55, Jiang Liu wrote:
> 
>> 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;
>>>>>>>>>> +
>>>>>>>>>> +#ifdef	CONFIG_IRQ_DOMAIN_HIERARCHY
>>>>>>>>>> +	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://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git irq/irqdomain
>>>>>>>>
>>>>>>>> See also https://lkml.org/lkml/2014/11/17/764
>>>>>>>>
>>>>>>>> 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.
>>
> 
> Oh, I see. We abstract it in different ways...
> And sounds like you treat multiple MSI as a broken implementation?
Basically, yes:)