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Date:   Mon, 7 Nov 2022 10:48:31 +0100
From:   Thierry Reding <thierry.reding@gmail.com>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Ulf Hansson <ulf.hansson@linaro.org>,
        Prathamesh Shete <pshete@nvidia.com>, joro@8bytes.org,
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        linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org,
        will@kernel.org, iommu@lists.linux.dev, anrao@nvidia.com,
        smangipudi@nvidia.com, kyarlagadda@nvidia.com,
        Thierry Reding <treding@nvidia.com>
Subject: Re: [PATCH v10 1/4] iommu: Always define struct iommu_fwspec
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On Thu, Nov 03, 2022 at 05:35:19PM +0000, Robin Murphy wrote:
[...]
> Now, there does happen to be a tacit contract between the ACPI IORT code =
and
> the Arm SMMU drivers for how SMMU StreamIDs are encoded in their respecti=
ve
> fwspecs, but it was never intended for wider consumption. If Tegra drivers
> want to have a special relationship with arm-smmu then fair enough, but t=
hey
> can do the same as MSM and formalise it somewhere that the SMMU driver
> maintainers are at least aware of, rather than holding the whole generic
> IOMMU API hostage.

Are you talking about qcom_adrena_smmu_is_gpu_device()? That's the only
place I can find where MSM uses iommu_fwspec directly and in a "special"
way.

> Since apparently it wasn't clear, what I was proposing is a driver helper=
 at
> least something like this:
>=20
> int tegra_arm_smmu_streamid(struct device *dev)
> {
> #ifdef CONFIG_IOMMU_API
> 	struct iommu_fwspec *fwspec =3D dev_iommu_fwspec_get(dev)
>=20
> 	if (fwspec && fwspec->num_ids =3D=3D 1)
> 		return fwspec->ids[0] & 0xffff;
> #endif
> 	return -EINVAL;
> }

We actually also use this mechanism on devices that predate the ARM
SMMU, so it'd need to be even more generic. Also, since we need to
access this from a wide range of subsystems, it'd need to be in a
centralized place. Do you think iommu.h would be acceptable for this?

How about if I also add a comment to struct iommu_fwspec about the
intended use?

Thierry

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