Re: Using irq-crossbar.c

From: Mason <slash.tmp@free.fr>
To: Marc Zyngier <marc.zyngier@arm.com>
Cc: Sebastian Frias <sf84@laposte.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Grygorii Strashko <grygorii.strashko@ti.com>,
	Mans Rullgard <mans@mansr.com>
Subject: Re: Using irq-crossbar.c
Date: Sun, 12 Jun 2016 15:50:22 +0200	[thread overview]
Message-ID: <575D689E.9080205@free.fr> (raw)
In-Reply-To: <20160612110048.3dd5ea17@arm.com>

On 12/06/2016 12:00, Marc Zyngier wrote:

> Mason wrote:
> 
>> The problem with some Linux APIs is that they're logical and obvious
>> to people who've been using them for years. For newcomers, it's not
>> always so obvious.
>>
>> In this specific instance, the problem statement seems rather simple,
>> on the surface. An interrupt controller, X=0..127 lines in, Y=0..23
>> lines out (connected to GIC interrupt lines 0..23) and "all" we need
>> is a way to map Xs to Ys.
>>
>> As a first order approximation, it's enough to map all Xs to 0.
>> And provide a way for the kernel to check the registers containing
>> the bit-vectors indicating which interrupt(s) fired.
> 
> If that's what your hardware is, then you are taking the wrong
> approach. The irq-crossbar driver does not do that at all: it has x
> inputs and y outputs, but connects exactly *one input to one output*.
> No multiplexing.

Connecting one input to one output is possible iff x=y right?
(In other words, a bijection.)

> And the hierarchical domain infrastructure enforces a similar property:
> a Linux interrupt is dealt with at each level of the hierarchy without
> multiplexing: the "irq" is the same, while the "hwirq" varies to
> reflect the "input pin" for a given interrupt controller.
> 
> In your particular case, you have an evolved chained interrupt
> controller, and nothing else.

Is it possible to support such an "evolved chained intc" through DT only,
or does it require a few function calls from driver code?

>> Is this Doxygen format? Is there a make target to generate
>> some documentation?
> 
> Try "make help".

Documentation targets:
 Linux kernel internal documentation in different formats:
  htmldocs        - HTML
  pdfdocs         - PDF
  psdocs          - Postscript
  xmldocs         - XML DocBook
  mandocs         - man pages
  installmandocs  - install man pages generated by mandocs
  cleandocs       - clean all generated DocBook files

>>> - You've changed the default interrupt controller to be your crossbar.
>>>   Which means that all the sub-nodes are inheriting it. Have you
>>>   checked that this was valid for all of these nodes?
>>
>> I'm not sure I follow. All platform interrupts flow into the platform
>> controller. Maybe other platforms have more complex setups, with
>> several cascaded controllers?
> 
> Most embedded platforms do.

My imagination is lacking, I don't see why it needs to be more
complex than N platform input lines, and M output lines feeding
into the GIC (with M <= N)

(Our previous intc had N=64 and M=3; the new one has N=128 and M=24)

Regards.