Re: [PATCH v2 5/5] firmware: tegra: bpmp: Add MBWT sysfs interface

[Mikko Perttunen <mperttunen@nvidia.com>]

On Wednesday, July 22, 2026 8:05 PM Aniruddha Rao wrote:
> Different workloads can place different memory-bandwidth demands on the
> system. Selecting an appropriate bandwidth limit depends on the runtime
> workload mix and on the devices carrying that traffic, such as PCIe
> traffic or GPU traffic connected over the chip-to-chip link (NVCLINK).
> That information is not available to the kernel.
> 
> BPMP firmware of some Tegra platforms provides Memory Bandwidth
> Throttler (MBWT) controls for PCIe and GPU traffic connected over
> NVCLINK on the path to DRAM. Each PCIe bandwidth group has a single
> shared cap for all traffic in that group. Bandwidth for a group can be
> set per traffic type.
> 
> Add sysfs attributes on the tegra-bpmp platform device to expose a
> narrow userspace interface for MBWT control. The attributes are arranged
> as mbwt/pcie0..pcie5/pcie_read/bandwidth,
> pcie_write/bandwidth and nvclink/bandwidth files. Each pcieN directory
> identifies a PCIe bandwidth group and each traffic directory identifies
> the traffic type.
> 
> Reading a bandwidth attribute queries firmware for the selected bandwidth
> group and traffic type. Writing an integer programs the target bandwidth
> cap in GB/s for that bandwidth group and traffic type.
> 
> Register the attributes only when BPMP firmware reports support for the
> MBWT GET_BW and SET_BW requests through its query ABI.
> 
> Signed-off-by: Aniruddha Rao <anrao@nvidia.com>
> ---
> Changes since v1:
> - Register the interface based on MBWT firmware support, not ACPI.
> - Replace the stateful tuple attribute with per-traffic bandwidth files.
> - Remove software bandwidth range checks.
> - Build the sysfs interface under CONFIG_SYSFS.
> - Rename the registration helper to tegra_bpmp_init_sysfs().
> - Avoid platform-specific or transport-specific MBWT wording.
> 
>  .../ABI/testing/sysfs-platform-tegra-bpmp     |  48 ++++
>  drivers/firmware/tegra/Makefile               |   1 +
>  drivers/firmware/tegra/bpmp-private.h         |   9 +
>  drivers/firmware/tegra/bpmp-tegra-sysfs.c     | 205 ++++++++++++++++++
>  drivers/firmware/tegra/bpmp.c                 |   4 +
>  5 files changed, 267 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-platform-tegra-bpmp
>  create mode 100644 drivers/firmware/tegra/bpmp-tegra-sysfs.c
> 
> diff --git a/Documentation/ABI/testing/sysfs-platform-tegra-bpmp b/Documentation/ABI/testing/sysfs-platform-tegra-bpmp
> new file mode 100644
> index 000000000000..36440e696e7d
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-platform-tegra-bpmp
> @@ -0,0 +1,48 @@
> +What:		/sys/bus/platform/devices/<bpmp-device>/mbwt/pcie[0-5]/pcie_read/bandwidth
> +What:		/sys/bus/platform/devices/<bpmp-device>/mbwt/pcie[0-5]/pcie_write/bandwidth
> +What:		/sys/bus/platform/devices/<bpmp-device>/mbwt/pcie[0-5]/nvclink/bandwidth
> +Date:		July 2026
> +KernelVersion:	7.2
> +Contact:	Aniruddha TVS Rao <anrao@nvidia.com>
> +Description:
> +		Provides access to Memory Bandwidth Throttler (MBWT)
> +		controls exposed by BPMP firmware for PCIe traffic and GPU
> +		traffic connected over the chip-to-chip link (NVCLINK) on
> +		the path to DRAM.
> +
> +		The attributes are present only when BPMP firmware reports
> +		support for the MBWT GET_BW and SET_BW requests through the
> +		MBWT query ABI.
> +
> +		Each pcieN directory identifies one PCIe bandwidth group.
> +		Each bandwidth group has a single shared cap for all traffic
> +		in that group. A group may contain only PCIe devices, only a
> +		GPU connected over NVCLINK, or both PCIe and GPU traffic in a
> +		bifurcated topology.
> +
> +		The pcie_read, pcie_write and nvclink directories select the
> +		traffic type for the selected group:
> +
> +		pcie_read
> +			PCIe read traffic
> +
> +		pcie_write
> +			PCIe write traffic
> +
> +		nvclink
> +			GPU traffic connected over NVCLINK
> +
> +		Reading a bandwidth attribute returns the bandwidth cap in GB/s
> +		reported by firmware for that bandwidth group and traffic
> +		type.
> +
> +		Writing an integer to a bandwidth attribute programs the target
> +		bandwidth cap in GB/s for that bandwidth group and traffic
> +		type.
> +
> +		Examples:
> +			cat .../mbwt/pcie0/pcie_write/bandwidth
> +			echo 100 > .../mbwt/pcie0/pcie_write/bandwidth
> +
> +Users:		Platform integration and bandwidth tuning on systems with BPMP
> +		firmware MBWT support.
> diff --git a/drivers/firmware/tegra/Makefile b/drivers/firmware/tegra/Makefile
> index 41e2e4dc31d6..59085e183fbd 100644
> --- a/drivers/firmware/tegra/Makefile
> +++ b/drivers/firmware/tegra/Makefile
> @@ -6,5 +6,6 @@ tegra-bpmp-$(CONFIG_ARCH_TEGRA_194_SOC)	+= bpmp-tegra186.o
>  tegra-bpmp-$(CONFIG_ARCH_TEGRA_234_SOC)	+= bpmp-tegra186.o
>  tegra-bpmp-$(CONFIG_ARCH_TEGRA_264_SOC)	+= bpmp-tegra186.o
>  tegra-bpmp-$(CONFIG_DEBUG_FS)	+= bpmp-debugfs.o
> +tegra-bpmp-$(CONFIG_SYSFS)	+= bpmp-tegra-sysfs.o
>  obj-$(CONFIG_TEGRA_BPMP)	+= tegra-bpmp.o
>  obj-$(CONFIG_TEGRA_IVC)		+= ivc.o
> diff --git a/drivers/firmware/tegra/bpmp-private.h b/drivers/firmware/tegra/bpmp-private.h
> index b3770e71020e..3a15a7aa0ffc 100644
> --- a/drivers/firmware/tegra/bpmp-private.h
> +++ b/drivers/firmware/tegra/bpmp-private.h
> @@ -33,4 +33,13 @@ int tegra_bpmp_mbwt_get(struct tegra_bpmp *bpmp, unsigned int instance,
>  int tegra_bpmp_mbwt_set(struct tegra_bpmp *bpmp, unsigned int instance,
>  			unsigned int vc_type, unsigned int bandwidth);
>  
> +#ifdef CONFIG_SYSFS
> +int tegra_bpmp_init_sysfs(struct tegra_bpmp *bpmp);
> +#else
> +static inline int tegra_bpmp_init_sysfs(struct tegra_bpmp *bpmp)
> +{
> +	return 0;
> +}
> +#endif
> +
>  #endif
> diff --git a/drivers/firmware/tegra/bpmp-tegra-sysfs.c b/drivers/firmware/tegra/bpmp-tegra-sysfs.c

The directory already says 'tegra', so the file can be just 
'bpmp-sysfs.c'. That matches the existing 'bpmp-debugfs.c' as well.