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

From: Grygorii Strashko
Date: Wed Oct 31 2018 - 12:39:41 EST

Hi Marc,

On 10/23/18 8:50 AM, Marc Zyngier wrote:
On Mon, 22 Oct 2018 15:35:41 +0100,
Lokesh Vutla <lokeshvutla@xxxxxx> wrote:
On Friday 19 October 2018 08:52 PM, Marc Zyngier wrote:
On 18/10/18 16:40, Lokesh Vutla wrote:
Texas Instruments' K3 generation SoCs has an IP Interrupt Aggregator
which is an interrupt controller that does the following:
- Converts events to interrupts that can be understood by
an interrupt router.
- Allows for multiplexing of events to interrupts.
- Allows for grouping of multiple events to a single interrupt.

Aren't the last two points the same thing? Also, can you please define
what an "event" is? What is its semantic? If they look like interrupts,
can we just name them as such?

Event is actually a message sent by a master via an Event transport
lane. Based on the id within the message, each message is directed to
corresponding Interrupt Aggregator(IA). In turn IA raises a
corresponding interrupt as configured for this event.

Ergo, this is an interrupt, and there is nothing more to it. HW folks
may want to give it a sexy name, but as far as SW is concerned, it has
the properties of an interrupt and should be modelled as such.


+ for_each_set_bit(bit, vint_desc->event_map, MAX_EVENTS_PER_VINT) {
+ val = 1 << bit;
+ __raw_writeq(val, inta->base + data->hwirq * 0x1000 +
+ }

If you need an ack callback, why is this part of the eoi? We have
interrupt flows that allow you to combine both, so why don't you use that?

Actually I started with ack_irq. But I did not see this callback being
triggered when interrupt is raised. Then I was suggested to use
irq_roi. Will see why ack_irq is not being triggered and update it in
next version.

It is probably because you're not using the right interrupt flow.

Also, the __raw_writeq call is probably wrong, as it assumes that both
the CPU and the INTA have the same endianness.

hmm.. May I know what is the right call to use here?

writeq_relaxed is most probably what you want. I assume this code will
never run on a 32bit platform, right?


+ * ti_sci_inta_irq_domain_free() - Free an IRQ from the IRQ domain
+ * @domain: Domain to which the irqs belong
+ * @virq: base linux virtual IRQ to be freed.
+ * @nr_irqs: Number of continuous irqs to be freed
+ */
+static void ti_sci_inta_irq_domain_free(struct irq_domain *domain,
+ unsigned int virq, unsigned int nr_irqs)
+ struct ti_sci_inta_irq_domain *inta = domain->host_data;
+ struct ti_sci_inta_vint_desc *vint_desc;
+ struct irq_data *data, *gic_data;
+ int event_index;
+ data = irq_domain_get_irq_data(domain, virq);
+ gic_data = irq_domain_get_irq_data(domain->parent->parent, virq);

That's absolutely horrid...

I agree. But I need to get GIC irq for sending TISCI message. Can you
suggest a better way of doing it?

I'd say "fix the firmware to have a layered approach". But I guess
that's not an option, right?


+ * ti_sci_allocate_event_irq() - Allocate an event to a IA vint.
+ * @inta: Pointer to Interrupt Aggregator IRQ domain
+ * @vint_desc: Virtual interrupt descriptor to which the event gets attached.
+ * @src_id: TISCI device id of the event source
+ * @src_index: Event index with in the device.
+ * @dst_irq: Destination host irq
+ * @vint: Interrupt number within interrupt aggregator.
+ *
+ * Return 0 if all went ok else appropriate error value.
+ */
+static int ti_sci_allocate_event_irq(struct ti_sci_inta_irq_domain *inta,
+ struct ti_sci_inta_vint_desc *vint_desc,
+ u16 src_id, u16 src_index, u16 dst_irq,
+ u16 vint)
+ struct ti_sci_inta_event_desc *event;
+ unsigned long flags;
+ u32 free_bit;
+ int err;
+ raw_spin_lock_irqsave(&vint_desc->lock, flags);
+ free_bit = find_first_zero_bit(vint_desc->event_map,
+ if (free_bit != MAX_EVENTS_PER_VINT)
+ set_bit(free_bit, vint_desc->event_map);
+ raw_spin_unlock_irqrestore(&vint_desc->lock, flags);

Why disabling the interrupts? Do you expect to take this lock
concurrently with an interrupt? Why isn't it enough to just have a mutex

I have thought about this while coding. We are attaching multiple
events to the same interrupt. Technically the events from different
IPs can be attached to the same interrupt or events from the same IP
can be registered at different times. So I thought it is possible that
when an event is being allocated to an interrupt, an event can be
raised that belongs to the same interrupt.

I strongly dispute this. Events are interrupts, and we're not
requesting an interrupt from an interrupt handler. That would be just


+ * 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.

this event is, and how drivers get notified that such an event has fired.

As said above, Event is a message being sent by a device using a
hardware protocol. This message is sent over an Event Transport
Lane(ETL) that understands this protocol. Based on the message ETL re
directs the message to a specificed target(In our case it is interrupt

From a client drivers(that generates this event) prespective, the
following needs to be done:
- Get an index that is free and allocate it to a particular task.
- Request INTA driver to assign an irq for this index.
- do a request_irq baseed on the return value from the above step.

All of that can be done in the using the current MSI framework. You
can either implement your own bus framework or use the platform MSI
stuff. You can then rewrite the INTA driver to be what it really is,
an interrupt multiplexer.

In case of grouping events, the client drivers has its own mechanism
to identify the index that caused an interrupt(at least that is the
case for the existing user).

This simply isn't acceptable. Each event must be the result of a
single interrupt allocation from the point of view of the driver. If
events are shared, they should be modelled as a shared interrupt.

Overall, I'm extremely concerned that you're reinventing the wheel and
coming up with a new "concept" that seems incredibly similar to what
we already have everywhere else, just offering an incompatible
API. This means that your drivers become specialised for your new API,
and this isn't going to fly.

I can only urge you to reconsider the way you provide these events,
and make sure that you use the existing API to its full potential. If
something is not up to the task, we can then fix it in core code.

I'd try to provide some additional information here.
(Sry, I'll still use term "events")

As Lokesh explained in other mail on K3 SoC everything is generic and most
of resources allocated dynamicaly:
- generic DMA channels
- generic HW rings (used by DMA channel)
- generic events (assigned to the rings) and muxed to different cores/hosts

So, when some driver would like to perform DMA transaction It's
required to build (configure) DMA channel by allocating different type of
resources and link them together to get finally working Data Movement path
(situation complicated by ti-sci firmware which policies resources between cores/hosts):
- get UDMA channel from available range
- get HW rings and attach them to the UDMA channel
- get event, assign it to the ring and mux it to the core/host through IA->IR-> chain
(and this step is done by ti_sci_inta_register_event() - no DT as everything is dynamic).

Next, how this is working now - ti_sci_inta_register_event():
- first call does similar things as regular DT irq mapping (end up calling irq_create_fwspec_mapping()
and builds IRQ chain as below:
linux_virq = ti_sci_inta_register_event(dev, <ringacc tisci_dev_id>,
<ringacc id>, 0, IRQF_TRIGGER_HIGH, false);

| IA |
+--------+ | +------+ | +--------+ +------+
| ring 1 +----->evtA+----->VintX +----------> IR +---------> GIC +-->
+--------+ | +------+ | +--------+ +------+ Linux IRQ Y
evtA | |
| |

- second call updates only IA input part while keeping other parts of IRQ chain the same
if valid <linux_virq> passed as input parameter:
linux_virq = ti_sci_inta_register_event(dev, <ringacc tisci_dev_id>,
<ringacc id>, linux_virq, IRQF_TRIGGER_HIGH, false);
| IA |
+--------+ | +------+ | +--------+ +------+
| ring 1 +----->evtA+--^-->VintX +----------> IR +---------> GIC +-->
+--------+ | | +------+ | +--------+ +------+ Linux IRQ Y
| | |
+--------+ | | |
| ring 2 +----->evtB+--+ |
+--------+ | |

As per above, irq-ti-sci-inta and tisci fw creates shared IRQ on HW layer by attaching
events to already established IA->IR->GIC IRQ chain. Any Rings events will trigger
Linux IRQ Y line and keep it active until Rings are not empty.

Now why this was done this way?
Note. I'm not saying this is right, but it is the way we've done it as of now. And I hope MSI
will help to move forward, but I'm not very familiar with it.

The consumer of this approach is K3 Networking driver, first of all, and
this approach allows to eliminate runtime overhead in Networking hot path and
provides possibility to implement driver's specific queues/rings handling policies
- like round-robin vs priority.

CPSW networking driver doesn't need to know exact ring generated IRQ - it
need to know if there is packet for processing, so current IRQ handling sequence we have (simplified):
- any ring evt -> IA -> IR -> GIC -> Linux IRQ Y
handle_fasteoi_irq() -> cpsw_irq_handler -> disable_irq() -> napi_schedule()
soft_irq() -> cpsw_poll():
- [1] for each ring from Hi prio to Low prio
[2] get packet
[3] if (packet) process packet & goto [2]
else goto [1]
if (no more packets) goto [4]
[4] enable_irq()

As can be seen there is no intermediate IRQ dispatchers on IA/IR levels and no IRQs-per-rings,
and NAPI poll cycle allows to implement driver's specific rings handling policy.

Next: depending on the use case following optimizations are possible:
1) throughput: split all TX (or RX) rings on X groups, where X = num_cpus
and allocate assign IRQ to each group for Networking XPS/RPS/RSS.
For example, CPSW2G has 8 TX channels and so 8 completion rings, 4 CPUs:
rings[0,1] -(IA/IR) - Linux IRQ 1
rings[2,3] -(IA/IR) - Linux IRQ 2
rings[4,5] -(IA/IR) - Linux IRQ 3
rings[6,7] -(IA/IR) - Linux IRQ 4
each Linux IRQ assigned to separate CPU.

2) min latency:
Ring X is used by RT application for TX/RX some traffic (using AF_XDP sockets for example)
Ring X can be assigned with separate IRQ while other rings still grouped to
produce 1 IRQ
rings[0,6] - (IA/IR) - Linux IRQ 1
rings[7] - (IA/IR) - Linux IRQ 2
Linux IRQ 2 assigned to separate CPU where RT application is running.

Hope above will help to clarify some K3 AM6 IRQ generation questions and
find the way to move forward.

Thanks a lot for you review and comments.