Re: [PATCH v5 3/5] ACPI / processor: Introduce map_gic_id() to get apic id from MADT or _MAT method

[Marc Zyngier <marc.zyngier@arm.com>]

On 2014-02-22 10:21, Hanjun Guo wrote:
> On 2014-2-21 20:37, Sudeep Holla wrote:
>> Hi Hanjun,
>>
>> (Adding MarcZ for his views on GIC)
>>
>> On 20/02/14 03:59, Hanjun Guo wrote:
>>> Hi Sudeep,
>>>
>>> Thanks for your comments, please refer to the replies below. :)
>>>
>>> On 2014年02月19日 22:33, Sudeep Holla wrote:
>>>> Hi Hanjun,
>>>>
>>>> On 18/02/14 16:23, Hanjun Guo wrote:
>>>>> Get apic id from MADT or _MAT method is not implemented on 
>>>>> arm/arm64,
>>>>> and ACPI 5.0 introduces GIC Structure for it, so this patch 
>>>>> introduces
>>>>> map_gic_id() to get apic id followed the ACPI 5.0 spec.
>>>>>
>>>>> Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
>>>>> ---
>>>>>   drivers/acpi/processor_core.c |   26 ++++++++++++++++++++++++++
>>>>>   1 file changed, 26 insertions(+)
>>>>>
>>>>> diff --git a/drivers/acpi/processor_core.c 
>>>>> b/drivers/acpi/processor_core.c
>>>>> index 4dcf776..d316d9b 100644
>>>>> --- a/drivers/acpi/processor_core.c
>>>>> +++ b/drivers/acpi/processor_core.c
>>>>> @@ -71,6 +71,27 @@ static int map_lsapic_id(struct 
>>>>> acpi_subtable_header *entry,
>>>>>   	return 0;
>>>>>   }
>>>>>
>>>>> +static int map_gic_id(struct acpi_subtable_header *entry,
>>>>> +		int device_declaration, u32 acpi_id, int *apic_id)
>>>>> +{
>>>>> +	struct acpi_madt_generic_interrupt *gic =
>>>>> +		(struct acpi_madt_generic_interrupt *)entry;
>>>>> +
>>>>> +	if (!(gic->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 && (gic->uid == acpi_id)) {
>>>>> +		*apic_id = gic->gic_id;
>>>> I have mentioned this earlier, it's not clear yet to me how does 
>>>> this work ?
>>>> It needs more clarity in the form of comment here at-least as the 
>>>> ACPIv5.0 is
>>>> also not so clear or explicit on how to handle this.
>>>
>>> Yes, I noticed your comments and had a reply for that, after a
>>> long consideration for this, I would withdraw my previous comments
>>> before, please refer to the comments below.
>>>
>>>>
>>>> Here you are expecting gic->uid = acpi_id which is fine, while 
>>>> acpi_map_cpuid
>>>> matches apic_id with cpu_physical_id(which must be MPIDR in 
>>>> ARM{32,64}). The
>>>> latter imposes restriction that gic->gic_id has to be MPIDR. Does 
>>>> that mean we
>>>> are imposing restriction on GIC ID to be MPIDR ? If so please 
>>>> document it here
>>>> and please explain the reason behind that choice.
>>>
>>> On x86 and IA64, APIC/SAPIC ID is the hardware id of the logical
>>> processor, and UID is just a unique ID to identify the processor in 
>>> DSDT, it
>>> can be any value, and even can be strings defined in ASL if I 
>>> remember
>>> that correctly.
>>>
>> OK, but that's not the case on ARM{32,64}. My main concern here is 
>> if we don't
>> make this definitions clear enough, the vendors might produce ACPI 
>> tables with
>> whatever suits them and we may end up supporting them. Since we are 
>> starting
>> with clean slate, we can avoid getting into such situations. I will 
>> be to be
>> more elaborate this time.
>
> I agree.
>
>>
>> The GIC ID is referred as the local GIC’s hardware ID in ACPIv5.0.
>> IIUC, since GICC is per-cpu entry, it has to GIC CPU interface ID.
>>
>> Now how does it differ from MPIDR ? e.g. ARM TC2(multi cluster 
>> system)
>> 	GIC ID		MPIDR			Comment
>> 	0		0x000			CA15_0
>> 	1		0x001			CA15_1
>> 	2		0x100			CA7_0
>> 	3		0x101			CA7_1
>> 	4		0x102			CA7_2
>
> Yes, obvious different. I know GIC ID can matche the bit index of the
> associated processor
> in the distributor's GICD_ITARGETSR register, and it a clear
> statement in GICv1/GICv2, my
> question is that is this consistent in GICv3/v4 too? this will have
> some impact on the
> code implementation.

For GICv3/v4, the only way you can match a CPU with its local 
redistributor is by using the CPU MPIDR. The GIC CPU ID is an 
implementation choice that may not exist (it doesn't in a distributed 
implementation), so anything that relies on a GIC CPU ID is broken for 
GICv3.

         M.
-- 
Fast, cheap, reliable. Pick two.