linux-kernel - Re: [PATCH v2 3/5] irqchip, gicv3: Workaround for Cavium ThunderX erratum 23154

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Message-ID: <20150814114754.GR1820@rric.localhost>
Date:	Fri, 14 Aug 2015 13:47:54 +0200
From:	Robert Richter <robert.richter@...iumnetworks.com>
To:	Marc Zyngier <marc.zyngier@....com>
CC:	Robert Richter <rric@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH v2 3/5] irqchip, gicv3: Workaround for Cavium ThunderX
 erratum 23154

On 14.08.15 09:28:12, Marc Zyngier wrote:
> On 13/08/15 18:11, Robert Richter wrote:
> > On 13.08.15 17:54:41, Marc Zyngier wrote:
> >> On 13/08/15 17:17, Robert Richter wrote:
> >>> Marc,
> >>>
> >>> thanks for your quick review.
> >>>
> >>> On 13.08.15 16:11:15, Marc Zyngier wrote:
> >>>> On 13/08/15 15:47, Robert Richter wrote:
> >>>>> From: Robert Richter <rrichter@...ium.com>
> >>>
> >>>>>  static const struct gic_capabilities gicv3_errata[] = {
> >>>>>  	{
> >>>>> +		.desc		= "GIC: Cavium erratum 23154",
> >>>>> +		.iidr		= 0xa100034c,	/* ThunderX pass 1.x */
> >>>>> +		.iidr_mask	= 0xffff0fff,
> >>>>> +		.init		= gicv3_enable_cavium_thunderx,
> >>>>> +	},
> >>>>
> >>>> I'm even more puzzled. You're working around a CPU bug based on the ITS
> >>>> ID registers? Or have you swapped the detection methods for the two errata?
> >>>
> >>> :/ Right, I mixed this up... Must have starred on this for too long.
> >>> Will fix that.
> >>>
> >>> Wrt midr: Originally this was written to support iidr. I wanted to
> >>> keep the version check in the driver of the hw, an implementation
> >>> outside of drivers/irqchip looked not appropriate here as it would
> >>> rely then on arch arm64 only. This is the main reason. Apart from
> >>> that, I think an implmentation based on struct arm64_cpu_capabilities,
> >>> etc. would require much rework compared to my current easy
> >>> implementation, e.g:
> >>>
> >>>  * binding flags to callbacks and actually run them,
> >>>
> >>>  * handing over private driver data (base addr for iidr detection) to
> >>>    a capabilty's match function.
> >>>
> >>> Overall this looked bloated. Now, that the MIDR also needs to be
> >>> checked, it looked better to me to keep the gic hw detection at a
> >>> single location in the driver. This also allows us to check a
> >>> combination of midr and iidr values.
> >>>
> >>> I hope this sounds reasonable?
> >>
> >> +Will.
> >>
> >> The point I was trying to make is that a CPU interface bug is a CPU bug,
> >> and that it feels quite weird weird to have the detection in the GIC.
> >> Will, what do you think?
> >>
> >> Also, I don't really buy the combined MIDR/GITS_IIDR detection. These
> >> are two *very* distinct pieces of HW that are not even directly
> >> connected (the redistributors are in between).
> >>
> >> I wouldn't mind having something like:
> >>
> >> struct gic_capabilities {
> >> 	const char *desc;
> >> 	void (*init)(void *data);
> >> 	u32 iidr;
> >> 	u32 iidr_mask;
> >> 	int feature;
> >> };
> > 
> > Yes, once we leave this in the driver it is much easier. But why do
> > the read_cpuid_id() in cpu_errata.c and not in his file? The
> > value/mask pairs will be then on complete different locations for the
> > same kind of hw depending on midr/iidr. And the only reason for using
> > midr is not, that it's a cpu, but just that it needs to be applied to
> > guests too and this is the only way to find out the real hw, otherwise
> > we would use iidr. 
> 
> But that's the thing! Using GITS_IIDR would be the complete wrong thing
> to check for a CPU interface, which is what your ICC_IAR1_EL1 workaround
> is about.
> 
> > Apart from the fact that this looks inconsistent
> > having one errata^Mfeature flag for one errata, but not for the
> > other. And only because one is useing midr for hw detection and the
> > other iidr.
> 
> Because they are architecturally two different things. I strongly
> suspect that you could take the ThunderX core, remove Cavium's own
> implementation of the GIC and slap another GIC implementation in front
> of it, and you would have the exact same CPU interface bug, because
> that's where the issue is.

Ok, I am going to send an update based on your suggestions.

Thanks,

-Robert
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