Message-ID: <4E78AFD6.9030209@ti.com>
Date:	Tue, 20 Sep 2011 17:23:02 +0200
From:	"Cousson, Benoit" <b-cousson@...com>
To:	Rob Herring <robherring2@...il.com>
CC:	Grant Likely <grant.likely@...retlab.ca>,
	Thomas Abraham <thomas.abraham@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	"jamie@...ieiles.com" <jamie@...ieiles.com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>
Subject: Re: [PATCH 5/5] ARM: gic: add OF based initialization

On 9/19/2011 11:53 PM, Rob Herring wrote:
> On 09/19/2011 04:14 PM, Grant Likely wrote:
>> On Mon, Sep 19, 2011 at 7:48 AM, Rob Herring<robherring2@...il.com>  wrote:
>>> On 09/19/2011 07:09 AM, Cousson, Benoit wrote:
>>>> On 9/18/2011 11:23 PM, Rob Herring wrote:
>>>>> I was headed down the path of implementing the 2nd option above,
>>>>> but had a dilemma. What would be the numbering base for PPIs in
>>>>> this case? Should it be 0 in the DT as proposed for SPIs or does it
>>>>> stay at 16?
>>>>
>>>> Both SGI and PPI are internal to the CortexA9 MP core, and referring
>>>> to the CortexA9 MP core TRM [1], you can see that the PPI# ->  ID#
>>>> mapping is already documented: - Private timer, PPI(2) Each Cortex-A9
>>>> processor has its own private timers that can generate interrupts,
>>>> using ID29. - Watchdog timers, PPI(3) Each Cortex-A9 processor has
>>>> its own watchdog timers that can generate interrupts, using ID30.
>>>>
>>>> So in that case, it can makes sense to use the ID. But it is
>>>> interesting to note that the PPI is identified with a 0 based index
>>>> number.
>>>>
>>> It's even worse than I thought: we could use 13 (ID16 == PPI0), 29 or 2
>>> for the timer interrupt. The first would match 0 based SPI convention.
>>> The last 2 would both match the documentation. We could never use 2 as
>>> this will for sure be different and the GIC code will have no way to
>>> know how to do the translation to ID. The only sane choice is using the
>>> ID as you say.
>>>
>>> But you can't have it both ways. It does not make sense to use the ID
>>> for some interrupts and a different scheme for others.
>>
>> Hmmm, it seems to me that some orthogonal issues are getting
>> conflated.  Specifically, the binding vs. what the GIC driver using
>> internally.  For my own understanding, let me see if I can summarize
>> and clarify the problem.
>>
>> Each GIC IRQ is represented in 5 different ways:
>> 1) the hardware documentation (PPI[0-15] or SPI[988] input pin)
>> 2) The DT binding to represent the connection.
>> 3) The Interrupt ID as specified by the GIC architecture reference[1]
>> (SGI:[0-15], PPI:[16-31], SPI:[32-1019], special:[1020-1023])
>> 4) The internal HWIRQ representation used by the GIC driver
>> 5) The Linux VIRQ number that #4 maps to.
>>
>> [1] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ihi0048b/BCGBFHCH.html
>>
>> Some thoughts:
>> - Generally the DT binding (#2) should reflect the HW view of the
>> system (#1) since that is the number most likely to be represented in
>> hardware manuals.  The interrupt ID is an internal detail of the GIC,
>> and isn't really exposed in the block diagram of the hardware.
>> - Presumably it is preferable for the GIC to directly use the
>> Interrupt ID (#3) as the HWIRQ number (#4) because it is the most
>> efficient from an interrupt handling perspective, and indeed this is
>> currently what the GIC driver does.
>> - Translation between the DT binding (#2) and the Interrupt ID / HWIRQ
>> (#3/#4) is trivial, and easily managed by the GIC's irq_domain.
>> - Though not necessarily as trivial, the mapping between Linux VIRQ
>> and HWIRQ is not fixed, and when migrating to DT it should be assumed
>> to be assigned at runtime.  Perhaps not so important for a core IRQ
>> controller like the GIC (as opposed to an i2c irq expander), but
>> assuming an fixed offset still should be avoided.  We may still force
>> a SPI0->VIRQ32 on the root GIC as an optimization, but it is not
>> necessary and the driver still needs to support remapping for a
>> secondary GIC.
>
> The irq base is dynamic in my series, but is typically still GIC ID =
> VIRQ for a primary GIC for now. A platform can adjust this with
> irq_alloc_descs if necessary (but recommended not to of course).
>
>>
>> So, for the GIC DT binding, I'm inclined to agree with Benoit that the
>> binding should reflect the hardware connections, not the values used
>> by software for decoding IRQs.  Also, I see absolutely no need to use
>> separate nodes for each GIC interrupt space.  The DT interrupt
>> specifier number space can more than handle the features of the GIC in
>> a clear and concise manor.  So, here's my counter proposal for a GIC
>> bindings (using Rob's text as the starting point):
>>
>> ----
>>
>> * ARM Generic Interrupt Controller
>>
>> ARM SMP cores are often associated with a GIC, providing per processor
>> interrupts (PPI), shared processor interrupts (SPI) and software
>> generated interrupts (SGI).
>>
>> Primary GIC is attached directly to the CPU and typically has PPIs and SGIs.
>> Secondary GICs are cascaded into the upward interrupt controller and do not
>> have PPIs or SGIs.
>>
>> Main node required properties:
>>
>> - compatible : should be one of:
>>         "arm,cortex-a9-gic"
>>         "arm,arm11mp-gic"
>> - interrupt-controller : Identifies the node as an interrupt controller
>> - #interrupt-cells : Specifies the number of cells needed to encode an
>>    interrupt source.  The type shall be a<u32>  and the value shall be 3.
>>
>>    The 1st cell is the interrupt type; 0 for SPI interrupts, 1 for PPI
>> interrupts.
>>    The 2nd cell contains the interrupt number for the interrupt type.
>> SPI interrupts are in the range [0-987].  PPI interrupts are in the
>> range [0-15].
>>    The 3rd cell is the flags, encoded as follows:
>>          bits[3:0] trigger type and level flags.
>>                      1 = low-to-high edge triggered
>>                      2 = high-to-low edge triggered
>>                      4 = active high level-sensitive
>>                      8 = active low level-sensitive
>>          bits[15:8] PPI interrupt cpu mask.  Each bit corresponds to
>> each of the 8 possible cpus attached to the GIC.  A bit set to '1'
>> indicated the interrupt is wired to that CPU.  Only valid for PPI
>> interrupts.
>>
> How about a cpu mask of 0 means SPI and non-zero means PPI? Then we can
> drop the first cell.
>
>> (Alternately, if there is no need for a CPU mask because PPI
>> interrupts will never be wired to more than one CPU, then it would be
>> better to encode the CPU number into the second cell with the SPI
>> number).
> You meant PPI number, right?                                    ^^^
>
> The common case at least on the A9 is a PPI is routed to all cores. QC
> is different though. This was discussed previously. Basically, anything
> is possible here, so the mask is needed for sure.
>
> Overall I'm fine with this and just happy to have some conclusion. I
> will send out an updated series if there are no further comments.

I'm OK with that proposal too. That solve all the concerns I had.

Thanks for the good discussion,
Benoit
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