Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

From: Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
To: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
Cc: Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>,
	"devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"linux-samsung-soc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-samsung-soc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Pawel Moll <pawel.moll-5wv7dgnIgG8@public.gmane.org>,
	Ian Campbell
	<ijc+devicetree-KcIKpvwj1kUDXYZnReoRVg@public.gmane.org>,
	Grant Grundler <grundler-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>,
	Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>,
	Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>,
	"linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Marc Zyngier <marc.zyngier-5wv7dgnIgG8@public.gmane.org>,
	Linux IOMMU
	<iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>,
	Rob Herring <robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Kumar Gala <galak-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>,
	"linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Cho KyongHo <pullip.cho-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>,
	Dave Martin <Dave.Martin-5wv7dgnIgG8@public.gmane.org>,
	"linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org"
	<linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org>
Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings
Date: Wed, 4 Jun 2014 15:44:03 +0200	[thread overview]
Message-ID: <20140604134400.GE28484@ulmo> (raw)
In-Reply-To: <4462924.8iRbdkOPp7@wuerfel>

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On Fri, May 30, 2014 at 09:54:37PM +0200, Arnd Bergmann wrote:
> On Friday 30 May 2014 22:29:13 Hiroshi Doyu wrote:
> > 
> > IIUC the original problem, "a master with 8 streamIDs" means something
> > like below, where some devices have multiple IDs but some have a
> > single. A sinle #address-cells cannot afford those 2 masters at once.
> > 
> >        iommu {
> >                /* the specifier represents the ID of the master */
> >                #address-cells = <1>;
> >                #size-cells = <0>;
> >        };
> > 
> >         master@a {
> >                 ...
> >                 iommus = <&smmu 1 2 3>; # 3 IDs
> >         };
> > 
> >         master@b {
> >                 ...
> >                 iommus = <&smmu 4>;     # 1 ID
> >         };
> 
> This would not be the usual format really. It should instead be
> 
> 		iommus = <&smmu 1>, <&smmu 2>, <&smmu 3>;
> 
> which can be tedious to type.

"Tedious to type" doesn't sound like a good argument to me. I don't see
why the above would necessarily be a bad notation. It's very much up to
the point and very explicit. This very obviously translates to:

	This device has three master interfaces, one for smmu ID 1, one
	for smmu ID 2 and one for smmu ID 3.

> > Tegra,SMMU has a similar problem and we have used a fixed size bitmap(64
> > bit) to afford 64 stream IDs so that a single device can hold multiple
> > IDs. If we apply the same bitmap to the above exmaple:
> > 
> >        iommu {
> >                /* the specifier represents the ID of the master */
> >                #address-cells = <1>;
> >                #size-cells = <0>;
> >        };
> > 
> >         master@a {
> >                 ...
> >                 iommus = <&smmu (BIT(1) | BIT(2) | BIT(3))>; # IDs 1 2 3
> >         };
> > 
> >         master@b {
> >                 ...
> >                 iommus = <&smmu BIT(4)>;     # ID 4
> >         };
> > 
> > The disadvantage of this is that this limits the max number of streamIDs
> > to support. If # of streamID is increased later more than 64, this
> > format cannot cover any more. You have to predict the max # of streamIDs
> > in advance if steamID is statically assigned.
> > 
> 
> Well, the iommu specific binding could allow a variable #address-cells.
> That way, you just need to know the number of stream IDs for that instance
> of the iommu.

That sounds fairly complicated to me. I don't see what that buys us over
the clarity and simplicity that the above explicit notation gives us. Is
it not more common for a device to have a single master rather than a
whole bunch of them?

Thierry

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