linux-kernel - Re: [PATCH V2 13/14] dt-bindings: arm-gic: Add documentation for Tegra210 AGIC

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Date:	Fri, 22 Apr 2016 12:12:57 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Mark Rutland <mark.rutland@....com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <marc.zyngier@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Kevin Hilman <khilman@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	<linux-tegra@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 13/14] dt-bindings: arm-gic: Add documentation for
 Tegra210 AGIC


On 22/04/16 11:00, Mark Rutland wrote:
> On Wed, Apr 20, 2016 at 12:03:56PM +0100, Jon Hunter wrote:
>> The Tegra AGIC interrupt controller is compatible with the ARM GIC-400
>> interrupt controller.
> 
> The cover letter says it _is_ a GIC-400, just used in a slightly unusual
> manner (i.e. not directly connected to CPUs).

Correct.

>> The Tegra AGIC requires two clocks, namely the
>> "ape" (functional) and "apb2ape" (interface) clocks, to operate. Add
>> the compatible string and clock information for the AGIC to the GIC
>> device-tree binding documentation.
> 
> The GIC-400 spec only describes "CLK" (which is what I imagine "ape" is.
> There isn't an APB clock described, and the manual seems to show GIC-400
> directly connected to AXI rather than APB, so that doesn't seem to even
> be the usual "apb_pclk".
> 
> Is there some wrapper logic around a GIC-400 to giove it an APB
> interface? Or am I misudnerstanding the spec?

Looking at the Tegra documentation what we have is ...

APB --> AXI switch --> AGIC (GIC400)

I am not sure how such a switch would typically be modeled in DT but we
need the apb clock to interface to the GIC registers. I am not sure if
something like simple-pm-bus is appropriate here.

>> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
>> ---
>>
>> I am not sure if it will be popular to add Tegra specific clock names
>> to the GIC DT docs. However, in that case, then possibly the only
>> alternative is to move the Tegra AGIC driver into its own file and
>> expose the GIC APIs for it to use. Then we could add our own DT doc
>> for the Tegra AGIC as well (based upon the ARM GIC).
> 
> The clock-names don't seem right to me, as they sound like provide names
> or global clock line names rather than consumer-side names ("clk" and
> "apb_pclk").

Yes that would be fine with me.

> I'm also not certain about the compatible string; if this really is a
> GIC-400 then I would at least expect "arm,gic-400" as a fallback.

OK.

Cheers
Jon