Re: [PATCH v2 01/14] dt-bindings: remoteproc: Add TI PRUSS bindings

[Roger Quadros]

On 14/02/19 10:37, Linus Walleij wrote:
> On Thu, Feb 14, 2019 at 4:13 AM Suman Anna <s-anna@xxxxxx> wrote:
>> [Me]
> 
>>> To be able to use hierarchical interrupt domain in the kernel, the top
>>> interrupt controller must use the hierarchical (v2) irqdomain, so
>>> if this is anything else than the ARM GIC it will be an interesting
>>> undertaking to handle this.
>>
>> These are interrupt lines coming towards the host processor running
>> Linux and are directly connected to the ARM GIC. This INTC module is
>> actually an PRUSS internal interrupt controller that can take in 64 (on
>> most SoCs) external events/interrupt sources and multiplexing them
>> through two layers of many-to-one events-to-intr channels &
>> intr-channels-to-host interrupts. Couple of the host interrupts go to
>> the PRU cores themselves while the remaining ones come out of the IP to
>> connect to other GICs in the SoC.
> 
> If the muxing is static (like set up once at probe) so that while the system is
> running, there is one and one only event mapped to the GIC from
> the component below it, then it is hierarchical.

This is how it looks.

[GIC]<---8---[INTC]<---64---[events from peripherals]

The 8 interrupt lines from INTC to the GIC are 1:1 mapped and fixed per SoC.
The muxing between 64 inputs to INTC and its 8 outputs are programmable
and might not necessarily be static per boot/probe as it depends on what firmware
is loaded on the PRU.

A typical PRUSS use case will usually use just one firmware per boot but if required it
can switch at runtime and the muxing might change.

> 
>> We have implemented this as an irqchip using chained interrupt handlers
>> with the consumers using the event numbers on the Linux-side. The PRUs
>> also access some of the associated registers for clearing an event source.
> 
> Chaining with cascading is when two or more interrupts fire the
> same upper level (say GIC) IRQ. If there is a 1:1 mapping,
> it is not chained/cascaded but hierarchical.
> 
> I understand you used old irqdomain/chip frameworks in the past,
> because everyone was working around the fact that they didn't have
> an abstraction for hierarchical IRQs. Using chained interrupts
> and custom 1:1 maps and assigning a long list of IRQs like this
> patch does was the most common workaround. But we should
> step out of that habit now.
> 
> Different levels of the IRQ handling having to do different stuff is
> what hierarchical irqdomains do best, so it sounds like a good fit.
> 
> We handle some stuff at our level of the hierarchy and then fall
> up to the next higher level using calls such as
> irq_chip_ack_parent(), irq_chip_mask_parent() and friends.
> 
> Yours,
> Linus Walleij
> 

--
cheers,
-roger
 
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