Re: [PATCH v10 1/4] iommu: Always define struct iommu_fwspec

[Thierry Reding <thierry.reding@gmail.com>]

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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 respective
> 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 they
> 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:
> 
> int tegra_arm_smmu_streamid(struct device *dev)
> {
> #ifdef CONFIG_IOMMU_API
> 	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev)
> 
> 	if (fwspec && fwspec->num_ids == 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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