Message-ID: <20140905090528.GG2172@leverpostej>
Date:	Fri, 5 Sep 2014 10:05:28 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	"computersforpeace@...il.com" <computersforpeace@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 2/2] Documentation: bcm7120-l2: Add Broadcom
 BCM7120-style L2 binding

On Wed, Sep 03, 2014 at 05:59:58PM +0100, Florian Fainelli wrote:
> On 09/03/2014 05:43 AM, Mark Rutland wrote:
> > On Wed, Sep 03, 2014 at 01:13:02PM +0100, Thomas Gleixner wrote:
> >> You forgot to CC the device tree dudes. We want an ack on the bindings
> >> before they materialize in Linus tree.
> > 
> > Thanks for the Cc.
> > 
> > Florian, in future could you please Cc for both the binding and driver?
> > So long as it's obvious which patch introduces the binding other can
> > choose to ignore the driver, but for me it's useful context.
> > 
> >>
> >> Thanks,
> >>
> >> 	tglx
> >>
> >> On Thu, 28 Aug 2014, Florian Fainelli wrote:
> >>
> >>> This patch adds the Device Tree binding document for the Broadcom
> >>> BCM7120-style Set-top-box Level 2 interrupt controller hardware.
> >>>
> >>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> >>> ---
> >>>  .../interrupt-controller/brcm,bcm7120-l2-intc.txt  | 44 ++++++++++++++++++++++
> >>>  1 file changed, 44 insertions(+)
> >>>  create mode 100644 Documentation/devicetree/bindings/interrupt-controller/brcm,bcm7120-l2-intc.txt
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/interrupt-controller/brcm,bcm7120-l2-intc.txt b/Documentation/devicetree/bindings/interrupt-controller/brcm,bcm7120-l2-intc.txt
> >>> new file mode 100644
> >>> index 000000000000..3818ffed7347
> >>> --- /dev/null
> >>> +++ b/Documentation/devicetree/bindings/interrupt-controller/brcm,bcm7120-l2-intc.txt
> >>> @@ -0,0 +1,44 @@
> >>> +Broadcom BCM7120-style Level 2 interrupt controller
> >>> +
> >>> +Required properties:
> >>> +
> >>> +- compatible: should be "brcm,bcm7120-l2-intc"
> > 
> > Is this a custom block for the bcm7120?
> 
> This is a custom block that started being used with bcm7120 and that we
> inherited, unmodified, in newer BCM7xxx designs since then, hence the
> name we picked up.

Ok.

> > 
> > Does the IP block not have a more specific name?
> 
> I wish there was one, I had to dig through the verilog sources to find
> which chip this interrupt controller first started to appear, and the
> filename was not too helpful either.

Fair enough.

[...]

> >>> +- brcm,int-map-mask: 32-bits bit mask describing how the interrupts
> >>> at this level
> >>> +  map to their respective parents. Should match exactly the number of interrupts
> >>> +  specified in the 'interrupts' property.
> > 
> > I don't follow.
> > 
> > Surely this should be static, and we know the 1-1 mapping, or this is
> > dynamic and should be SW-configured?
> 
> This is static for a given instance of this interrupt controller on a
> particular brcmstb chip. BCM7445 will have something different here than
> e.g: BCM7366 or BCM7439 which are also from the same family.
> 
> Not all 32 bits within this interrupt controller will map to wired
> interrupts, so the point of this bitmask is to:
> 
> - tell how many valid interrupt sources there are
> - tell how a given bitmask maps to its corresponding L1 interrupt line
> number
> 
> So this is not much to know the 1:1, but to know the the 1:many mapping,
> the example might make it a little clearer how this works

I'll have to give the example another scan, it's not immediately clear
how the latter portion works.

> >>> +Optional properties:
> >>> +
> >>> +- interrupt-names: if present, the litteral names for the parent interrupts
> >>> +  specified in the 'interrupts' property.
> > 
> > If you use the interrupt-names property, it should contain the names of
> > the interrupts from the POV of this device. Those names must be
> > specified in the binding doc.
> 
> Since this also varies on a per-chip basis, I can certainly add each
> valid names for each chips out there, but that does not sound useful,
> maybe let's just drop that property, we don't use it anyway since we
> perform lookups by indexes, and that is safe to do.

Ok. It sounds like the index would be the thing to use here.

> >>> +- brcm,irq-can-wake: if present, this means the L2 controller can be used as a
> >>> +  wakeup source for system suspend/resume.
> > 
> > How variable is this?
> 
> There's two instances of this interrupt controller on most SoCs, one
> that can wake up the system, one that cannot, see below.
> 
> > 
> > I realise have properties like this elsewhere, but it seems to be
> > hacking around the lack of a decent power domain interface for figuring
> > this out.
> 
> Humm, I kind of see your point here with the power domains, I don't see
> a big problem with specifying that property though, at most this becomes
> redundant when we have a power domain representation (which will be very
> simple: always-on and everything else).

Sure, we seem to have done that elsewhere.

> >>> +- brcm,int-fwd-mask: if present, a 32-bits bit mask describing the interrupts
> >>> +  which need to be enabled in this controller to flow to the higher level
> >>> +  interrupt controller. This is typically needed for the UARTs interrupts to
> >>> +  flow through the top-level interrupt controller (e.g: ARM GIC on ARM-based
> >>> +  platforms).
> >>> +
> > 
> > I don't follow why this property is needed at all. Is this a mechanism
> > to bypass this controller entirely? Why should this be described as a
> > fixed HW property?
> 
> This interrupt controller has traditionally (not necessarily for good
> reasons) been the placeholder for special bits that control whether our
> UARTs level 1 interrupts (wired to the ARM GIC) will flow to the L1
> interrupt.

So basically setting these bits unmasks some irq lines inpout to the
GIC?

> We discussed initially with Arnd Bergman about how to best approach
> this, and he was happy with a bitmask since:
> 
> a) that is a one-time initialization thing that can happen anywhere in
> the kernel before UART interrupts get used (so before user-space gets
> scheduled)

That feels a little dodgy to me, but perhaps that's ok.

> 
> b) we need to save/restore that bitmask during suspend/resume
> 
> c) this is not a real interrupt bit, we need to set this bit, but we
> will not get any interrupt at this particular interrupt controller level
> for UARTs, so this is totally transparent for the UART driver
> 
> Once again, the bitmask values varies on a per-chip basis, though the
> fundamentals are the same.

Thanks for the explanation.

Mark.
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