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

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Message-ID: <87ioibczrj.fsf@approximate.cambridge.arm.com>
Date:	Wed, 19 Nov 2014 09:20:48 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	"Yun Wu \(Abel\)" <wuyun.wu@...wei.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\@linaro.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 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.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.
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