linux-kernel - Re: [PATCH v5 12/18] ACPI / processor: Make it possible to get CPU hardware ID via GICC

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Date:	Wed, 29 Oct 2014 10:43:22 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Hanjun Guo <hanjun.guo@...aro.org>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Mark Rutland <Mark.Rutland@....com>,
	Olof Johansson <olof@...om.net>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	"graeme.gregory@...aro.org" <graeme.gregory@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Sudeep Holla <Sudeep.Holla@....com>,
	"jcm@...hat.com" <jcm@...hat.com>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Mark Brown <broonie@...nel.org>, Rob Herring <robh@...nel.org>,
	Robert Richter <rric@...nel.org>,
	Lv Zheng <lv.zheng@...el.com>,
	Robert Moore <robert.moore@...el.com>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	"Kangkang.Shen@...wei.com" <Kangkang.Shen@...wei.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>
Subject: Re: [PATCH v5 12/18] ACPI / processor: Make it possible to get CPU
 hardware ID via GICC

On Mon, Oct 27, 2014 at 09:58:10AM +0000, Hanjun Guo wrote:

[...]

> >> +static int map_gicc_mpidr(struct acpi_subtable_header *entry,
> >> +		int device_declaration, u32 acpi_id, int *mpidr)
> >> +{
> >> +	struct acpi_madt_generic_interrupt *gicc =
> >> +	    container_of(entry, struct acpi_madt_generic_interrupt, header);
> >> +
> >> +	if (!(gicc->flags & ACPI_MADT_ENABLED))
> >> +		return -ENODEV;
> >> +
> >> +	/* In the GIC interrupt model, logical processors are
> >> +	 * required to have a Processor Device object in the DSDT,
> >> +	 * so we should check device_declaration here
> >> +	 */
> >> +	if (device_declaration && (gicc->uid == acpi_id)) {
> >> +		/*
> >> +		 * Only bits [0:7] Aff0, bits [8:15] Aff1, bits [16:23] Aff2
> >> +		 * and bits [32:39] Aff3 are meaningful, so pack the Affx
> >> +		 * fields into a single 32 bit identifier to accommodate the
> >> +		 * acpi processor drivers.
> >> +		 */
> >> +		*mpidr = ((gicc->arm_mpidr & 0xff00000000) >> 8)
> >> +			 | gicc->arm_mpidr;
> > 
> > The simple fact that you define a function to pack the mpidr value and
> > you can't use it here because this is *generic* code is telling, and
> > a very bad omen. At the cost of sounding like a broken record, I do not
> > like this mpidr->apic->logical_cpu song and dance at all.
> > ACPI is peppered with code (eg hotplug is another example, CPUidle driver
> > even worse) that is supposed to be generic but contains x86 code to carry
> > out this cpuid conversion, I really think that in order to start an ARM64
> > ACPI port properly we should at least try to factor out this physical to
> > logical cpu id conversion, and it is not the first time that I mention this
> > on the lists.
> 
> I know, thanks for pointing this out. As I replied in previous version
> of this patch set, apic_id is x86/ia64 specific, but the meaning behind
> it is not. It means the CPU hardware id to identify itself in the system,
> it just like MPIDR on ARM.

Yes, except that it is called apic_id.

> I will send out a patch for RFC to convert apic_id to physid which
> is generic for all platforms.

That seems a good idea to sound out if I am the only one having an issue
with the current approach.

> > I will also talk to Rafael about this at the earliest opportunity, I
> > guess that x86 code relies on apic-id because some ACPI versions could
> > not rely on the acpi-id or some other reasons I have to investigate.
> 
> As ACPI spec (section 8.4, Declaring Processors) said, Each processor
> in the system must be declared in the ACPI namespace, so each cpu will
> have acpi_id in all ACPI versions, and in theory we can map acpi_id to
> logical cpu id if we want to.
> 
> But things are complicated, apic_id is connected to many
> tables, MADT for smp init, DSDT for device driver, and SRAT for NUMA (there
> is no acpi_id in it, ONLY has apic_id in the table for x86/ia64), so if we
> want to factor the code to map acpi_id to logical cpu id, we need to modify:
> 
>  - ACPI drivers;
>  - SMP init for x86 and ia64
>  - the mappings for NUMA init for x86 and ia64
> that will be lots of work I think.
> 
> I'm willing to discuss this further and come out a solution, please
> comment on what I said and share your ideas :)

Factoring out apic_id to a common cpu_physical_id is ok to me, because
basically that's what you are doing except for the naming. I do not
have any particular preference for the acpi_id, I mentioned that only
as a means to implement a generic cpu_physical_id, arch agnostic.

Drop the RFC you mentioned above please on the list, we will restart debating
from there.

Thanks,
Lorenzo

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