Re: [PATCH v11 2/6] iommu/tegra: Add tegra_dev_iommu_get_stream_id() helper

Ulf Hansson
2022-11-18T08:08:58Z

On Thu, 17 Nov 2022 at 19:27, Thierry Reding <thierry.reding@gmail.com> wrote:
>
> From: Thierry Reding <treding@nvidia.com>
>
> Access to the internals of struct iommu_fwspec by non-IOMMU drivers is
> discouraged. Many drivers for Tegra SoCs, however, need access to their
> IOMMU stream IDs so that they can be programmed into various hardware
> registers.
>
> Formalize this access into a common helper to make it easier to audit
> and maintain.
>
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
> include/linux/iommu.h | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index dc5e6f5c14fc..f0d72acad7db 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -1186,4 +1186,25 @@ static inline u32 iommu_sva_get_pasid(struct iommu_sva *handle)
> }
> #endif /* CONFIG_IOMMU_SVA */
>
> +/*
> + * Newer generations of Tegra SoCs require devices' stream IDs to be directly programmed into
> + * some registers. These are always paired with a Tegra SMMU or ARM SMMU, for which the contents
> + * of the struct iommu_fwspec are known. Use this helper to formalize access to these internals.
> + */
> +#define TEGRA_STREAM_ID_BYPASS 0x7f
> +
> +static inline bool tegra_dev_iommu_get_stream_id(struct device *dev, u32 *stream_id)
> +{
> +#ifdef CONFIG_IOMMU_API
> + struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
> +
> + if (fwspec && fwspec->num_ids == 1) {
> + *stream_id = fwspec->ids[0] & 0xffff;
> + return true;
> + }
> +#endif

It's totally up to iommu maintainers to decide, but I personally would
prefer to replace the below part with a stub function, defined when
CONFIG_IOMMU_API is unset.

> +
> + return false;
> +}
> +
> #endif /* __LINUX_IOMMU_H */
> --
> 2.38.1
>

Other than that, both patch1 and patch2 look good to me. I can pick up
the series, after an ack from Robin or Joerg.

Kind regards
Uffe