linux-kernel - Re: [PATCH] irqchip, gicv3-its, numa: Workaround for Cavium ThunderX erratum 23144

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Message-ID: <55DB205E.9070205@arm.com>
Date:	Mon, 24 Aug 2015 14:47:10 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Ganapatrao Kulkarni <gpkulkarni@...il.com>
CC:	Robert Richter <robert.richter@...iumnetworks.com>,
	Will Deacon <Will.Deacon@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Tirumalesh Chalamarla <tirumalesh.chalamarla@...iumnetworks.com>,
	Robert Richter <rric@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ganapatrao Kulkarni <ganapatrao.kulkarni@...iumnetworks.com>
Subject: Re: [PATCH] irqchip, gicv3-its, numa: Workaround for Cavium ThunderX
 erratum 23144

On 24/08/15 14:27, Ganapatrao Kulkarni wrote:
> On Mon, Aug 24, 2015 at 6:15 PM, Marc Zyngier <marc.zyngier@....com> wrote:

>>>>>  static void its_enable_cavium_thunderx(void *data)
>>>>>  {
>>>>> -     struct its_node *its = data;
>>>>> +     struct its_node __maybe_unused *its = data;
>>>>>
>>>>> -     its->flags |= ITS_FLAGS_CAVIUM_THUNDERX;
>>>>> +#ifdef CONFIG_CAVIUM_ERRATUM_22375
>>>>> +     its->flags |= ITS_WORKAROUND_CAVIUM_22375;
>>>>> +     pr_info("ITS: Enabling workaround for 22375, 24313\n");
>>>>> +#endif
>>>>> +
>>>>> +#ifdef CONFIG_CAVIUM_ERRATUM_23144
>>>>> +     if (num_possible_nodes() > 1) {
>>>>> +             its->numa_node = its_get_node_thunderx(its);
>>>>
>>>> I'd rather see numa_node being always initialized to something useful.
>>>> If you're adding numa support, why can't this be initialized via
>>>> standard topology bindings?
>>> IIUC, topology defines only cpu topology.
>>
>> Well, welcome to a much more complex system where both your CPUs and
>> your IOs have some degree of affinity. This needs to be described
>> properly, and not hacked on the side.
> ok, will add description for the function.

I sense that you misunderstood what I meant. What I'd like to see is
some topology information coming from DT, showing the relationship
between a device (your ITS) and a given node (your socket). This can
then be used from two purposes:

- find the optimal affinity for a MSI so that it doesn't default to a
foreign node (a reasonable performance expectation),
- work around implementation bugs where an LPI cannot be routed to a
redistributor that is on a foreign node.

I really don't feel like adding a hack just for the second point, and
I'd rather get the big picture right so that your workaround is just a
special case of the generic one.

Thanks,

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