linux-kernel - Re: [RFC 2/3] arm_pmu: acpi: spe: Add initial MADT/SPE probing

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Message-ID: <20190215150015.GA6803@fuggles.cambridge.arm.com>
Date:   Fri, 15 Feb 2019 15:00:15 +0000
From:   Will Deacon <will.deacon@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, devel@...ica.org,
        catalin.marinas@....com, mark.rutland@....com,
        robert.moore@...el.com, erik.schmauss@...el.com,
        rafael.j.wysocki@...el.com, lenb@...nel.org
Subject: Re: [RFC 2/3] arm_pmu: acpi: spe: Add initial MADT/SPE probing

On Thu, Feb 14, 2019 at 12:03:57PM -0600, Jeremy Linton wrote:
> On 2/14/19 11:11 AM, Will Deacon wrote:
> > On Fri, Feb 08, 2019 at 06:47:17PM -0600, Jeremy Linton wrote:
> > > +/*
> > > + * For lack of a better place, hook the normal PMU MADT walk
> > > + * and create a SPE device if we detect a recent MADT with
> > > + * a homogeneous PPI mapping.
> > > + */
> > > +static int arm_spe_acpi_parse_irqs(void)
> > > +{
> > > +	int cpu, ret, irq;
> > > +	u16 gsi = 0;
> > > +	bool first = true;
> > > +
> > > +	struct acpi_madt_generic_interrupt *gicc;
> > > +
> > > +	/*
> > > +	 * sanity check all the GICC tables for the same interrupt number
> > > +	 * for now we only support homogeneous ACPI/SPE machines.
> > > +	 */
> > > +	for_each_possible_cpu(cpu) {
> > > +		gicc = acpi_cpu_get_madt_gicc(cpu);
> > > +
> > > +		if (gicc->header.length < ACPI_MADT_GICC_SPE)
> > > +			return -ENODEV;
> > > +
> > > +		if (first) {
> > > +			gsi = gicc->spe_overflow_interrupt;
> > > +			if (!gsi)
> > > +				return -ENODEV;
> > > +			first = false;
> > > +		} else if (gsi != gicc->spe_overflow_interrupt) {
> > > +			pr_warn("ACPI: SPE must have homogeneous interrupts\n");
> > > +			return -EINVAL;
> > > +		}
> > 
> > Unfortunately, I don't think this is sufficient to detect a homogeneous
> > system: we'll have to check the MIDRs instead, which is nasty. I would
> > personally be in favour of enforcing homogeneity for ACPI systems when we
> > bring up secondary CPUs, but I suspect others would disagree.
> 
> Given that all the SPE capable machines i'm aware of at the moment are
> homogeneous, are we ok with just doing an online CPU MIDR check for now, and
> cleaning that up if/when someone builds a machine and complains?

Yeah, I think we added a new bit to the PPTT to tell you that the machine is
homogenous, so just check that first and bail if it's not set.

Will