Date:	Sat, 19 May 2012 15:46:07 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Arnd Bergmann <arnd@...db.de>, Magnus Damm <magnus.damm@...il.com>,
	linux-kernel@...r.kernel.org, rjw@...k.pl,
	linus.walleij@...ricsson.com, linux-sh@...r.kernel.org,
	horms@...ge.net.au, olof@...om.net
Subject: Re: [PATCH] gpio: Emma Mobile GPIO driver V2

On Fri, May 18, 2012 at 05:25:39PM -0600, Grant Likely wrote:
> On Thu, 17 May 2012 09:41:25 +0900, Paul Mundt <lethal@...ux-sh.org> wrote:
> > In adding irq domain support for the sh intc subsystem I hacked up some
> > prototype code for doing an in-place update of IORESOURCE_IRQ resources
> > hanging off platform devices that does the hwirq->virq translation and it
> > seems to work fine, albeit hacky, and something I would rather avoid. On
> > the other hand, there's no need for that either if we support a 1:1 hwirq
> > to virq mapping where possible, which is fairly easy to do by just trying
> > to fetch a virq with irq_alloc_desc_at() before falling back on the
> > existing virq hinting logic in irq_create_mapping().
> 
> That's exactly what the current irq_domain code does.  Search for
> 'hint' in the irqdomain code.  It just cannot be relied upon in any
> way when there is more than once irq_domain depending on which one
> maps it first.  Plus I'm planning to remove the 'hint' logic because I
> think it just adds complexity that doesn't need to be there.
> 
There's a subtle functionality difference that the irqdomain code fails
to account for at present, which is the difference between the old
dynamic IRQ API (create_irq()/create_irq_nr() -- and no, not the stupid
signed vs unsigned thing). irq_alloc_desc_at() allocates at a fixed
position within the bitmap, and only at that position. If we're unable to
get the mapping we want, it's an error. irq_alloc_desc_from() simply uses
the hwirq as a hint for where to start looking, and will quite happily
hand you back whatever it finds. At present we have no way to tell the
irqdomain code to allocate a specific virq, which is what we're going to
need for converting static mappings over.

I do agree that getting rid of the hinting logic is a good idea though.
On SH we actually do our hinting the opposite, where we first populate
the IRQ map with all of our static hardware mappings, and then scan from
the beginning to find holes for when we need dynamic IRQs. This gives us
a pretty tightly packed IRQ map, which helps with lookup times, radix
tree utilization, and nr_irqs expansion for sparseirq (which we use
unconditionally on all sh + sh/r-mobile arm platforms).

> - get rid of the legacy map (I certainly won't cry to see it go)
> - switch all legacy users to linear

While this is a good direction to go, it's also important to keep in mind
that not all legacy users will have linear maps (though perhaps out of
the existing users it's one or the other). I've added a couple of API
calls to help with inserting extant mappings and establishing identity
mappings that should permit legacy users to adopt whatever mapping model
works the best for them.

Once the semantics have been agreed upon we can begin converting the
existing users before the _legacy proliferation gets too far along.

> - Add a new api to force-associate hwirqs and linux irqs... something like:
> 	irq_domain_associate(struct irq_domain *d, int irq_base,
> 			     int hwirq_base, int size)
>   - This function will reserve irq numbers (without allocating irq_descs)
>     for the given range.

There's already an irq_reserve_irq{s,}() that can be used to inhibit
accidental mapping. We use this in the SH INTC case to ensure that all of
the possible hardware vectors are reserved before we permit dynamic IRQ
allocation throughout the remaining space (we have a flat vector space
where both static and dynamic mappings co-exist -- I plan to maintain
this behaviour in converting to IRQ domains, which should also make it
easy for DT/non-DT stuff to co-exist in the same vector space).

I've generalized irq_setup_virq() as irq_domain_associate() in order to
allow direct insertion of existing IRQs while still allowing the domain
to do whatever it likes with the mapping in its ->map() (ie, revmap
insertion).

> - When mapping, if an irq has been reserved, then allocate only that
>   irq_desc; otherwise do the normal irq_alloc_desc().

This seems likely to duplicate state. We have no way to determine what
caused an IRQ to be reserved in the bitmap, or to test for reserved vs
normally allocated ones. I think we can avoid the need for this by simply
splitting in to irq_alloc_desc_at() for static and maintaining
irq_alloc_desc_from() for dynamic.

> - In irq_request, if the irq_desc has not yet been allocated and
>   mapped, then do so.
> 
> This does require a number of changes in the irq_desc layer though
> which aren't particularly hard but I haven't wanted to do.
> 
> Alternately (or perhaps as a stepping stone) irq_domain_associate()
> could actually allocate and map irq_descs for the given range.  It
> isn't as memory efficient because unused irq_descs still get mapped,
> but it is a whole lot simpler and easier to understand.
> 
That's possible as well. I've taken a slightly different approach with my
patches that leave the irqdesc behaviour alone, but I'm not sure how well
it will mesh with the DT model.
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