linux-kernel - Re: [patch 08/16] genirq: Introduce callback irq_chip.irq_write_msi

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Date:	Wed, 10 Dec 2014 17:26:26 +0800
From:	"Yun Wu (Abel)" <wuyun.wu@...wei.com>
To:	Marc Zyngier <marc.zyngier@....com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Bjorn Helgaas" <bhelgaas@...gle.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>,
	"Yijing Wang" <wangyijing@...wei.com>
Subject: Re: [patch 08/16] genirq: Introduce callback irq_chip.irq_write_msi_msg

On 2014/11/19 17:20, Marc Zyngier wrote:

> On Wed, Nov 19 2014 at  6:57:25 am GMT, "Yun Wu (Abel)" <wuyun.wu@...wei.com> wrote:
>> On 2014/11/18 22:32, Thomas Gleixner wrote:
>>
>>> On Tue, 18 Nov 2014, Yun Wu (Abel) wrote:
>>>
>>> Can you please trim the messages when you're replying?
>>>
>>>> The above you described is absolutely right, but not the things I want
>>>> to know. :)
>>>> Take GICv3 ITS for example, it deals with both PCI and non PCI message
>>>> interrupts. IIUC, several irq_chips need to be implemented in the ITS
>>>> driver (i.e. pci_msi_chip, A_msi_chip and B_msi_chip). What should we
>>>> do to the ITS driver if new MSI-capable devices come out?
>>>
>>> You seem to miss the stacking here
>>>
>>> PCI-MSI	->
>>> A-MSI	->   ITS  -> GIC
>>> B-MSI	->
>>>
>>> So each of the device types has its own MSI controller. Each of them
>>> will have their own callbacks and are backed by the underlying ITS/GIC
>>> implementation.
>>
>> Yes, this hits the key point. Once a new device type becomes available,
>> we need to add pieces of code outside the new device's driver to make
>> it work, which in my opinion is due to lack of core infrastructure.
>> More specifically, the core infrastructure needs to support mechanism
>> of MSI, not the various types of devices.
>>
>>>
>>> And that's the only sensible solution.
>>>
>>
>> It's sensible, but not perfect.
>> What I suggested is to add a generic layer:
>>
>> PCI-MSI	->
>> A-MSI	-> (generic layer) -> ITS -> GICR
>> B-MSI	->
>>
>> The PCI/A/B/... passes its hardware properties to the generic layer which
>> gets configurations ready by calling ITS's domain/chip callbacks. When
>> a new device type arrives, the only thing need to do is to implement the
>> driver of that device, with nothing to do with the generic layer or ITS.
> 
> I really don't get your "generic layer" story. To me, it looks like a
> glorified set of function pointers. And that's exactly what stacked
> domains are giving you:
> 
> A-MSI -> ITS -> GIC
> 
> This "A-MSI" is responsible for:
> - implementing the "prepare" callback, which allocates the ITT
> - implementing the "irq_compose_msi_msg"
> 
> Hardly anything to change in the ITS driver, and I can probably make it
> so that you don't even have to write a single line of code in the ITS
> driver.
> 
> If the generic MSI layer we now have is not enough for you, then please
> submit detailed use cases.
> 

Hi Marc,

As I said, I never thought Gerry's patch don't work, I am just trying to
make it better. :)

As to the "generic layer" story, please check the following URL:
https://lkml.org/lkml/2014/12/10/93

Thanks,
	Abel

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