linux-kernel - Re: [PATCH 1/4] arm64: gicv3: its: Encode domain number in PCI stream id

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Date:	Fri, 22 May 2015 22:57:40 +0000
From:	"Chalamarla, Tirumalesh" <Tirumalesh.Chalamarla@...iumnetworks.com>
To:	Marc Zyngier <marc.zyngier@....com>
CC:	"Richter, Robert" <Robert.Richter@...iumnetworks.com>,
	Robert Richter <rric@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	Radha Mohan Chintakuntla <rchintakuntla@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/4] arm64: gicv3: its: Encode domain number in PCI stream
 id


> On May 22, 2015, at 1:26 AM, Marc Zyngier <marc.zyngier@....com> wrote:
> 
> On 20/05/15 13:48, Robert Richter wrote:
>> Mark,
>> 
>> thanks for review, also of the other patches of this series.
>> 
>> See below
>> 
>> On 20.05.15 13:11:38, Marc Zyngier wrote:
>>>> -	dev_alias->dev_id = alias;
>>>> +	dev_alias->dev_id = (pci_domain_nr(pdev->bus) << 16) | alias;
>> 
>>> This feels very scary. We're now assuming that the domain number will
>>> always be presented to the doorbell. What guarantee do we have that
>>> this is always the case, irrespective of the platform?
>>> 
>>> Also, domains have no PCI reality, they are a Linux thing. And they can
>>> be "randomly" assigned, unless you force the domain in DT with a
>>> linux,pci-domain property. This looks even more wrong, specially
>>> considering ACPI.
>> 
>> The main problem here is that device ids (32 bits) are system
>> specific. Since we have more than one PCI root complex we need the
>> upper 16 bits in the devid for mapping. Using pci_domain_nr for this
>> fits our needs for now and shouldn't affect systems with a single RC
>> only as the domain nr is zero then.
>> 
>> The domain number is incremented during initialization beginnig with
>> zero and the order of it is fixed since it is taken from DT or ACPI
>> tables. So we have full controll of it. I don't see issues here.
> 
> This may match what you have on ThunderX (as long as the kernel doesn't
> adopt another behaviour when allocating the domain number). But other
> platforms may have a completely different numbering, which will mess
> them up entirely.
> 
>>> It really feels like we need a way to describe how the BDF numbering is
>>> augmented. We also need to guarantee that we get the actual bridge
>>> number, as opposed to the domain number.
>> 
>> But true, the obove is just intermediate. In the end we need some sort
>> of handler that is setup during cpu initialization that registers a
>> callback for the gic to determine the device id of that paricular
>> system.
> 
> I don't really like the idea of a callback from the GIC - I'd prefer it
> to be standalone, and rely on the topology information to build the
> DeviceID. Mark Rutland had some ideas for DT (he posted an RFC a while
> ago), maybe it would be good to get back to that and find out what we
> can do. ACPI should also have similar information (IORT?).
> 

How can some one pass this from DT, especially in GIC entry. i still think it is bus owner responsibility and call back is better idea. 
but if some one has a better idea for DT and ACPI, we are fine as long as it works on ThunderX.   

Thanks,
Tirumalesh. 


> Thanks,
> 
> 	M.
> -- 
> Jazz is not dead. It just smells funny...

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