Re: [PATCH 14/27] gpu: nova-core: add GMC API message types

[Zhi Wang <zhiw@nvidia.com>]

On Tue, 18 Aug 2026 20:52:07 -0700
John Hubbard <jhubbard@nvidia.com> wrote:

snip

> +/// GMC API message header.
> +///
> +/// Matches the `GMCAPI_HEADER` struct from Open RM. The `command`
> field carries +/// flags in the high byte and a 24-bit command ID in
> the low bits. `size` is the +/// payload size for both requests and
> responses, while the union at offset 16 +/// carries the maximum
> response size for requests and the status for responses. +#[repr(C)]
> +#[derive(Zeroable)]
> +pub(crate) struct GmcApiHeader {
> +    /// GMC command identifier with flags in the high byte.
> +    pub(crate) command: u32,
> +    /// Payload size in bytes.
> +    pub(crate) size: u32,
> +    /// Sequence number for matching requests to responses.
> +    pub(crate) sequence: u64,
> +    /// Request: maximum response size. Response: `NV_STATUS` code.
> +    pub(crate) max_resp_or_status: u32,
> +    reserved: [u32; 5],
> +}
> +
> +static_assert!(size_of::<GmcApiHeader>() == 40);
> +

Should we use the GMCAPI_HEADER in the r000 bindings?

Z.

> +// SAFETY: All fields are integer types with no uninitialized
> padding bytes. +unsafe impl AsBytes for GmcApiHeader {}
> +
> +// SAFETY: All fields are integer types for which all bit patterns
> are valid. +unsafe impl FromBytes for GmcApiHeader {}
> +
> +/// Header for an unencrypted GMC API message in the GSP command
> queue. +///
> +/// The fields before `gmc` match `GSP_MSG_QUEUE_ELEMENT` through its
> +/// `noEncryption` union member. `gmc` occupies the start of that
> member's +/// flexible payload array, and the GMC command payload
> follows this structure. +#[repr(C)]
> +pub(crate) struct GspGmcMsgElement {
> +    mctp_magic: u32,
> +    mctp_payload_size: u32,
> +    mctp_header: MctpHeader,
> +    nvdm_header: NvdmHeader,
> +    nvdm_payload_size: u32,
> +    reserved: u32,
> +    pub(crate) gmc: GmcApiHeader,
> +}
> +
> +static_assert!(
> +    core::mem::offset_of!(GspGmcMsgElement, mctp_magic)
> +        == core::mem::offset_of!(r000_00::GSP_MSG_QUEUE_ELEMENT,
> mctpMagic) +);
> +static_assert!(
> +    core::mem::offset_of!(GspGmcMsgElement, mctp_payload_size)
> +        == core::mem::offset_of!(r000_00::GSP_MSG_QUEUE_ELEMENT,
> mctpPayloadSize) +);
> +static_assert!(
> +    core::mem::offset_of!(GspGmcMsgElement, mctp_header)
> +        == core::mem::offset_of!(r000_00::GSP_MSG_QUEUE_ELEMENT,
> mctpHeader) +);
> +static_assert!(
> +    core::mem::offset_of!(GspGmcMsgElement, nvdm_header)
> +        == core::mem::offset_of!(r000_00::GSP_MSG_QUEUE_ELEMENT,
> nvdmHeader) +);
> +
> +#[expect(dead_code)]
> +impl GspGmcMsgElement {
> +    /// Creates a new GMC message element.
> +    ///
> +    /// `payload_size` is the size of the command payload following
> this header.
> +    /// `command_id` is the GMC command identifier.
> +    /// `sequence` is the sequence number for request/response
> matching.
> +    /// `max_response_size` is the maximum expected response payload
> size.
> +    pub(crate) fn init(
> +        command_id: u32,
> +        sequence: u64,
> +        payload_size: usize,
> +        max_response_size: u32,
> +    ) -> impl Init<Self, Error> {
> +        try_init!(GspGmcMsgElement {
> +            mctp_magic: MCTP_MAGIC,
> +            // Despite the name, this counts the element header as
> well as the payload.
> +            mctp_payload_size: size_of::<Self>()
> +                .checked_add(payload_size)
> +                .ok_or(EOVERFLOW)?
> +                .try_into()
> +                .map_err(|_| EOVERFLOW)?,
> +            mctp_header: MctpHeader::single_packet(),
> +            nvdm_header: NvdmHeader::new(NvdmType::GmcApi),
> +            nvdm_payload_size: size_of::<GmcApiHeader>()
> +                .checked_add(payload_size)
> +                .ok_or(EOVERFLOW)?
> +                .try_into()
> +                .map_err(|_| EOVERFLOW)?,
> +            reserved: 0u32,
> +            gmc: GmcApiHeader {
> +                command: command_id,
> +                size: payload_size.try_into().map_err(|_|
> EOVERFLOW)?,
> +                sequence,
> +                max_resp_or_status: max_response_size,
> +                reserved: [0; 5],
> +            },
> +        })
> +    }
> +
> +    /// Returns the total length of the message, transport and GMC
> headers included.
> +    pub(crate) fn length(&self) -> usize {
> +        num::u32_as_usize(self.mctp_payload_size)
> +    }
> +
> +    /// Returns the number of elements (i.e. memory pages) used by
> this message.
> +    pub(crate) fn element_count(&self) -> u32 {
> +        self.mctp_payload_size
> +            .div_ceil(num::usize_into_u32::<GSP_PAGE_SIZE>())
> +    }
> +}
> +
> +// SAFETY: All fields are integer types with no uninitialized
> padding bytes. +unsafe impl AsBytes for GspGmcMsgElement {}
> +
> +// SAFETY: All fields are integer types for which all bit patterns
> are valid. +unsafe impl FromBytes for GspGmcMsgElement {}
> +
>  /// Arguments for GSP startup.
>  #[repr(transparent)]
>  #[derive(Zeroable)]
> diff --git a/drivers/gpu/nova-core/mctp.rs
> b/drivers/gpu/nova-core/mctp.rs index 03546213da28..0ae88bef2a05
> 100644 --- a/drivers/gpu/nova-core/mctp.rs
> +++ b/drivers/gpu/nova-core/mctp.rs
> @@ -28,6 +28,8 @@ pub(crate) enum NvdmType with TryFrom<Bounded<u32,
> 8>> { Cot = 0x14,
>          /// FSP command response.
>          FspResponse = 0x15,
> +        /// GMC API message (GSP command queue).
> +        GmcApi = 0x26,
>      }
>  }
>