Re: [PATCH 2/7] dt-bindings: arm-smmu: Add binding for nvidia, smmu-v2

From: Thierry Reding <thierry.reding@gmail.com>
To: Krishna Reddy <vdumpa@nvidia.com>
Cc: Timo Alho <talho@nvidia.com>, Thierry Reding <treding@nvidia.com>,
	Mikko Perttunen <mperttunen@nvidia.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Pritesh Raithatha <praithatha@nvidia.com>,
	"Thomas Zeng \(SW-TEGRA\)" <thomasz@nvidia.com>,
	Sachin Nikam <Snikam@nvidia.com>,
	"linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
	Yu-Huan Hsu <YHsu@nvidia.com>,
	Juha Tukkinen <jtukkinen@nvidia.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Alexander Van Brunt <avanbrunt@nvidia.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 2/7] dt-bindings: arm-smmu: Add binding for nvidia, smmu-v2
Date: Mon, 2 Sep 2019 09:38:06 +0200	[thread overview]
Message-ID: <20190902073806.GA930@ulmo> (raw)
In-Reply-To: <BYAPR12MB271012F225E35C1459E58D07B3BD0@BYAPR12MB2710.namprd12.prod.outlook.com>

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On Fri, Aug 30, 2019 at 06:12:08PM +0000, Krishna Reddy wrote:
> >> +                        "nidia,smmu-v2"
> >>                           "qcom,smmu-v2"
> 
> >I agree with Mikko that the compatible must be at least SoC-specific, but potentially even instance-specific (e.g. "nvidia,tegra194-gpu-smmu")
> > depending on how many of these parallel-SMMU configurations might be hiding in current and future SoCs.
> 
> I am correcting the spelling mistake pointed by Mikko.  The NVIDIA SMMUv2 implementation is getting used beyond  Tegra194 SOC.  
> To be able to use the smmu compatible string across multiple SOC's, "nvidia,smmu-v2" compatible string is chosen.
> Are you suggesting to make it soc specific and add another one in future?

Yeah, I think that's the safest thing to do. Even if we're using the
same implementation in future SoCs, chances are there will be some
changes. Even if the changes are just fixes, having a SoC-specific
compatible string will ensure we can apply workarounds only to the
implementations that are missing the fixes.

So I think "nvidia,tegra194-smmu" is a good candidate. It uniquely
identifies the instantiation of the IP in Tegra194. Also, if it ever
turns out that the instantiation of the SMMU in the next Tegra
generation is *exactly* the same (even if highly unlikely), there's
nothing wrong with reusing the "nvidia,tegra194-smmu".

We've done similar things in the past, where some new IP was mostly
compatible with old IP. Typically we still include a new compatible
string in case any errata are discovered subsequently. It's not uncommon
to see things like:

	compatible = "nvidia,tegra124-xyz", "nvidia,tegra20-xyz";

Basically this means that this is the IP that was also used in Tegra20
and the same Tegra20 driver can be used to drive this hardware on
Tegra124. The Tegra124-specific compatible string may enable newer
features if there's a driver that supports it.

Thierry

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