Message-ID: <4EC1AE15.3090101@gmail.com>
Date:	Mon, 14 Nov 2011 16:11:01 -0800
From:	David Daney <ddaney.cavm@...il.com>
To:	Rob Herring <robherring2@...il.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>
CC:	David Daney <ddaney.cavm@...il.com>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
	"ralf@...ux-mips.org" <ralf@...ux-mips.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH 0/2] irq/of: Enchance irq_domain support.

On 11/14/2011 02:41 PM, Rob Herring wrote:
> On 11/14/2011 11:58 AM, David Daney wrote:
>> On 11/11/2011 07:55 PM, Rob Herring wrote:
>>> On 11/11/2011 07:50 PM, ddaney.cavm@...il.com wrote:
>>>> From: David Daney<david.daney@...ium.com>
>>>>
>>>> This is the first cut at hooking up my Octeon port to the irq_domain
>>>> things.
>>>>
>>>> The Octeon specific patches are part of a larger set, and will need to
>>>> be applied with that set, the first patch is stand-alone.
>>>>
>>>> The basic problem being solved taken from one of my other e-mails:
>>>>
>>>>      Unfortunately, although a good idea, kernel/irq/irqdomain.c makes a
>>>>      bunch of assumptions that don't hold for Octeon.  We may be able to
>>>>      improve it so that it flexible enough to suit us.
>>>>
>>>>
>>>>      Here are the problems I see:
>>>>
>>>>      1) It is assumed that there is some sort of linear correspondence
>>>>      between 'hwirq' and 'irq', and that the range of valid values is
>>>>      contiguous.
>>>>
>>>>      2) It is assumed that the concepts of nr_irq, irq_base and
>>>>      hwirq_base have easy to determine values and you can do iteration
>>>>      over their ranges by adding indexes to the bases.
>>>>
>>>
>>> I still think this is the wrong approach.
>>>
>>> Are the gpio interrupts the source of your problem here?
>>
>> No.
>>
>>> That's how I read it.
>>
>> Take a look at Patch 2/2, since the GPIO irqs are contiguous over both
>> irq and hwirq numbers, I use the existing infrastructure with no
>> modifications.
>>
>
> I did. You are adding GPIO support to the existing support.

No, I am adding DT support.  For that I need a working 
irq_create_of_mapping() for non-GPIO interrupts.  I am also adding GPIO, 
but that is not the part giving us problems.

You may recall that in an earlier e-mail you asked me to use irq_domain 
support for my irq_create_of_mapping() rather than my previous ad hoc 
approach.  I agree that this is a good idea, and now we are here.

> It would be
> nice to separate that from add DT support.

Granted, the non-GPIO support for irq_create_of_mapping() and GPIO 
support could be separated into two patches, but that is somewhat of an 
orthogonal issue.  Since that part is isolated in arch/mips, Ralf and I 
can discuss doing that.  For now I would like to concentrate on figuring 
out if it is advantageous to adjust the irq_domain core implementation.

>
>>> You have 16 GPIO irqs directly connected into lines on your
>>> primary interrupt controller which has 128 lines. So for a Linux irq
>>> number, you want to translate to a GPIO hwirq number and/or a CIU hwirq
>>> number. Trying to have 2 hwirq mappings for 1 Linux irq number just
>>> won't work. It seems to me you should use a chained handler here because
>>> you need to process the interrupt at both the primary ctrlr and gpio
>>> ctrlr levels.
>>>
>>
>> All moot as it is based on the false predicate of GPIO irqs being the
>> problem.
>>
> Let me rephrase, if you completely ignore GPIO for a minute, what is the
> issue.

Yes, let's do that.  It is clouding the issue.

> Just that irq_descs are sparsely allocated for the primary
> controller. Then the GPIO interrupts are inserted into the middle of
> that irq space. Handling sparse irqs is a potentially common problem, so
> we should address that in the core irqdomain code.

This is precisely the issue, and what patch 1/2 is doing.

My Patch 2/2 was just included to show how I would be using the changes.

>
>>
>> The root of the problem are all of the irqs that are not GPIO.  I have:
>>
>> o irq numbers currently in the range [9..196], with holes for any given
>> SOC/Board implementation.  SOCs currently in development will have
>> additional irq numbers with even more holes.
>>
>> o Two different interrupt controllers.  One with 128 lines, the other
>> with  512 or more lines, both sparsely populated.  The mapping of hwirq
>> to irq is done at boot time based on the hardware the kernel image is
>> running on.  Note that the second type of irq controller support is not
>> in the kernel.org kernel, but it exists, and I intend on getting support
>> for it merged ASAP.
>
> To be clear, those are not holes in hwirq's, but many lines don't have
> connections so you are not allocating Linux irqs for them. hwirq should
> reflect interrupt numbers from the controller's perspective and be
> directly usable to index to the correct register and bit mask. There's
> no storage associated with a hwirq, so the only cost is iterating over
> them which is not done frequently.

Yes, this is essentially correct.

There are two iterators defined:

1) irq_domain_each_hwirq()

2) irq_domain_for_each_irq()

Yet struct irq_domain has only nr_irq, this clearly cannot be used as 
the range of both iterators.  Furthermore I think the concept of a 
linear iteration through irq values is fundamentally broken.

>
> There is not a clean separation of the primary interrupt controller's
> hwirq numbers and Linux irq numbers in your patch.

?? I don't get what you mean.  They are clearly separate concepts, my 
interrupt controller code even goes as far as having a map to translate 
between the two.

> Then you are
> overlaying the GPIO interrupts into the Linux irq space.
>

Indeed I am, but its only effect is to introduce an additional hole 
(among many others) in the irq number space of the primary controller. 
It is only tangentially related to the real issues of this discussion.


>> At a minimum the loop in irq_domain_add() where we iterate over a linear
>> range of irq numbers is not flexible enough.  You may not like my
>> iterator functions in irq_domain_ops, but we need to provide something
>> better than the irq_domain_for_each_irq() macro.
>
> I'm just trying to back-up some and understand the problem.
>
> How about if .to_irq returns 0, the loop can just continue and skip over
> that hwirq without error.
>

Ok, now we are getting somewhere.

However I am not sufficiently clever to design an iteration macro like 
irq_domain_for_each_irq(), that would use the return value of zero from 
.to_irq to determine if we should continue to the next hwirq value 
without entering the body of the loop.

I am left with my callback iterator thing that is in the patch 1/2.

> I don't think .each_hwirq is being used, so you can delete that.

Yes, I noticed that.  There are two options:

1) Remove it as you suggest, along with irq_domain_for_each_hwirq() and 
irq_domain_each_hwirq().

2) Leave it on the grounds that irq_domain_for_each_hwirq() is there for 
a reason.

David Daney
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