Re: [RFC PATCH 5/7] iommu/riscv: Expose global QoS IDs in sysfs

[Drew Fustini <fustini@kernel.org>]

On Tue, Jul 14, 2026 at 09:06:55PM +0800, Zhanpeng Zhang wrote:
> The RISC-V IOMMU QoS extension provides iommu_qosid as a per-IOMMU
> global default tag. It is used for IOMMU-originated DDT, CQ, FQ, PQ, and
> MSI accesses, and for device-originated requests when DDTP is in BARE
> mode.
> 
> Initialize iommu_qosid to RCID 0 and MCID 0 when the hardware advertises
> QOSID support. Preserve reserved and WPRI bits with read-modify-write,
> and use register readback to reject values which the WARL fields do not
> retain.
> 
> Add a qosid attribute to the RISC-V IOMMU class device. Reading returns
> the current RCID and MCID values. Writing the documented
> 'rcid=<rcid> mcid=<mcid>' form updates both fields while preserving the
> other register bits.
> 
> Keep this interface separate from resctrl group QoS. The sysfs attribute
> controls the IOMMU-wide default, while resctrl device assignment programs
> per-device DC.ta in translated modes.
> 
> Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper@bytedance.com>
> ---
>  .../ABI/testing/sysfs-class-iommu-riscv-iommu |  27 +++
>  MAINTAINERS                                   |  10 ++
>  drivers/iommu/riscv/iommu.c                   | 159 +++++++++++++++++-
>  drivers/iommu/riscv/iommu.h                   |   9 +-
>  4 files changed, 202 insertions(+), 3 deletions(-)
>  create mode 100644 Documentation/ABI/testing/sysfs-class-iommu-riscv-iommu
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-iommu-riscv-iommu b/Documentation/ABI/testing/sysfs-class-iommu-riscv-iommu
> new file mode 100644
> index 000000000000..b0cd68997f17
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-class-iommu-riscv-iommu
> @@ -0,0 +1,27 @@
> +What:		/sys/class/iommu/<iommu>/qosid
> +Date:		June 2026
> +KernelVersion:	6.18
> +Contact:	Zhanpeng Zhang <zhangzhanpeng.jasper@bytedance.com>
> +Description:
> +		The RISC-V IOMMU global default QoS IDs for this IOMMU.
> +		The file is present only when the IOMMU reports the QOSID
> +		capability.
> +
> +		Reading the file returns the RCID and MCID fields from the
> +		iommu_qosid register:
> +
> +		  rcid=<rcid> mcid=<mcid>
> +
> +		Writing the file updates the RCID and MCID fields while
> +		preserving reserved/WPRI bits:
> +
> +		  rcid=<rcid> mcid=<mcid>

I think it might fit sysfs better if the rcid and mcid are separate
files in that directory. Is there a reason you combine them into one
file?

Thanks,
Drew