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 09:26:25 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Robert Richter <robert.richter@...iumnetworks.com>
CC:	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@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/4] arm64: gicv3: its: Encode domain number in PCI stream
 id

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?).

Thanks,

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