linux-kernel - Re: [patch 04/16] genirq: Introduce irq_chip.irq_compose_msi

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Date:	Tue, 18 Nov 2014 21:25:35 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	"Yun Wu (Abel)" <wuyun.wu@...wei.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Grant Likely <grant.likely@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>,
	Yijing Wang <wangyijing@...wei.com>
Subject: Re: [patch 04/16] genirq: Introduce irq_chip.irq_compose_msi_msg()
 to support stacked irqchip

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().
Regards!
Gerry
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