linux-kernel - Re: [PATCH v2 09/10] irqchip: ti-sci-inta: Add support for Interrupt Aggregator driver

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Date:   Sat, 27 Oct 2018 01:49:41 +0530
From:   Lokesh Vutla <lokeshvutla@...com>
To:     Marc Zyngier <marc.zyngier@....com>
CC:     Nishanth Menon <nm@...com>,
        Device Tree Mailing List <devicetree@...r.kernel.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        <jason@...edaemon.net>, Peter Ujfalusi <peter.ujfalusi@...com>,
        Sekhar Nori <nsekhar@...com>, <linux-kernel@...r.kernel.org>,
        Tero Kristo <t-kristo@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Santosh Shilimkar <ssantosh@...nel.org>, <tglx@...utronix.de>,
        Linux ARM Mailing List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 09/10] irqchip: ti-sci-inta: Add support for Interrupt
 Aggregator driver

Hi Marc,

[..snip..]
>> [...]
>>
>>>>> +/**
>>>>> + * ti_sci_inta_register_event() - Register a event to an interrupt aggregator
>>>>> + * @dev:	Device pointer to source generating the event
>>>>> + * @src_id:	TISCI device ID of the event source
>>>>> + * @src_index:	Event source index within the device.
>>>>> + * @virq:	Linux Virtual IRQ number
>>>>> + * @flags:	Corresponding IRQ flags
>>>>> + * @ack_needed:	If explicit clearing of event is required.
>>>>> + *
>>>>> + * Creates a new irq and attaches to IA domain if virq is not specified
>>>>> + * else attaches the event to vint corresponding to virq.
>>>>> + * When using TISCI within the client drivers, source indexes are always
>>>>> + * generated dynamically and cannot be represented in DT. So client
>>>>> + * drivers should call this API instead of platform_get_irq().
>>>>
>>>> NAK. Either this fits in the standard model, or we adapt the standard
>>>> model to catter for your particular use case. But we don't define a new,
>>>> TI specific API.
>>>>
>>>> I have a hunch that if the IDs are generated dynamically, then the model
>>>> we use for MSIs would fit this thing. I also want to understand what
>>>
>>> hmm..I haven't thought about using MSI. Will try to explore it. But
>>> the "struct msi_msg" is not applicable in this case as device does not
>>> write to a specific location.
>>
>> It doesn't need to. You can perfectly ignore the address field and
>> only be concerned with the data. We already have MSI users that do not
>> need programming of the doorbell address, just the data.
> 

Just one more clarification.

First let me explain the IRQ routes a bit deeply. As I said earlier there are 
three ways in which IRQ can flow in AM65x SoC
1) Device directly connected to GIC
	- Device IRQ --> GIC
2) Device connected to INTR.
	- Device IRQ --> INTR --> GIC
3) Devices connected to INTA.
	- Device IRQ --> INTA --> INTR --> GIC

1 and 2 are straight forward and we use DT for IRQ representation. Coming to 3 
the trickier part is that Input to INTA and output from INTA and dynamically 
managed. To be more specific:
- By hardware design there are certain set of physical global events(interrupts) 
attached to an INTA. Out of which a certain range are assigned to the current 
linux host that can be queried from system-controller.
- Similarly out of all the INTA outputs(referenced as vints) a certain range can 
be used by the current linux host.


So for configuring an IRQ route in case 3, the following steps are needed:
- Device id and device resource index for which the interrupt is needed
- A free event id from the range assigned to the INTA in this host context
- A free vint from the range assigned to the INTA in this host context
- A free gic IRQ from the range assigned to the INTR in this host context.

With the above information, linux should send a message to system-controller 
using TISCI protocol. After policing the given information, system-controller 
does the following:
- Attaches the interrupt(INTA input) to the device resource index
- Muxes the interrupt(INTA input) to corresponding vint(INTA output)
- Muxes the vint(INTR input) to GIC irq(INTR output).

For grouping of interrupts, the same vint number is to be passed to 
system-controller for all the requests.

Keeping all the above in mind, I see the following as software IRQ Domain Hierarchy:

1) INTA multi MSI --> 2)INTA  -->3) MSI --> 4) INTR  -->5) GIC

INTA driver has to set a chained IRQ using virq allocated from its parent MSI. 
This is to differentiate the grouped interrupts within INTA.

Inorder to cover the above two MSI domains, a new bus driver has to be created 
as I couldn't find a fit with the existing bus drivers.

Does the above approach make sense? Please correct me if i am wrong.

Thanks and regards,
Lokesh