From mboxrd@z Thu Jan 1 00:00:00 1970 From: thierry.reding@gmail.com (Thierry Reding) Date: Wed, 4 Jun 2014 15:44:03 +0200 Subject: [PATCH v2] devicetree: Add generic IOMMU device tree bindings In-Reply-To: <4462924.8iRbdkOPp7@wuerfel> References: <1400877218-4113-1-git-send-email-thierry.reding@gmail.com> <4545972.cM7IP1qTXQ@wuerfel> <87tx87rrp2.fsf@nvidia.com> <4462924.8iRbdkOPp7@wuerfel> Message-ID: <20140604134400.GE28484@ulmo> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 at a { > > ... > > iommus = <&smmu 1 2 3>; # 3 IDs > > }; > > > > master at 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 at a { > > ... > > iommus = <&smmu (BIT(1) | BIT(2) | BIT(3))>; # IDs 1 2 3 > > }; > > > > master at 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 -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 836 bytes Desc: not available URL: