Re: [PATCH v7 14/28] xen/arm: ITS: Initialize physical ITS and export lpi support

[Julien Grall <julien.grall@citrix.com>]

On 22/09/2015 11:05, Ian Campbell wrote:
> On Tue, 2015-09-22 at 10:45 +0100, Julien Grall wrote:
>>
>> On 22/09/2015 10:17, Vijay Kilari wrote:
>>>> Why do you need a command line option to enable/disable the physical
>>>> ITS?
>>>
>>> I have added this to remove ITS support?.
>>> Did I misunderstood it. May be I have to avoid generating its dt node
>>> to disable its for dom0?
>>
>> My question is why do you want to let the user disabling the ITS using
>> the command line? What's the use case?
>
> It's always useful to be able to nobble this sort of thing, either for
> debugging/development purposes or to allow users to workaround issues.

This is can be done via the firmware table (see the property "status" in 
the DT). I see no advantage to use the command line for a such purpose, 
mainly for the debugging/development point.

Also what kind of workaround do you expect to be fixed by disabling ITS? 
I know that it will disable all the PCI MSI in general. But should not 
it be done with a PCI related parameter?

> Assuming the switch is simple an reasonably self contained (and it really
> should be, since it should just be checked at start of day and then arrange
> to behave like no ITS is present) then I can't see why not to have it.

I think that disabling the ITS is more complex than not using in Xen. 
This will be tied with PCI passthrough very soon and we will have to 
spread this decision everywhere.

We would also have to remove everything related to ITS (i.e 
msi-parent...) to the DT to avoid to pass an half-valid DT to DOM0. 
Otherwise we may just not be able to boot or using PCI (even without MSI).

Regards,

-- 
Julien Grall