[PATCH 3/6] gpu: nova-core: add NVKV decoder

[Eliot Courtney <ecourtney@nvidia.com>]

Add a decoder for NVKV. This is for receiving messages from GSP for
GMCAPI calls. The NVKV format essentially encodes a sequence of function
calls f(key, index, value). This decoder reads an encoded stream and
invokes a type implementing the new `Schema` visitor trait. The
`Schema` trait can either consume the value or not, which is useful for
composing Schemas. If a (key, index, value) is not consumed, error out
depending on `UnknownKeyPolicy`. Whether ignoring unknown keys is ok or
not is per each GMCAPI call.

Add kunit tests for the decoder.

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 drivers/gpu/nova-core/gsp/nvkv.rs        |   3 +
 drivers/gpu/nova-core/gsp/nvkv/decode.rs | 258 +++++++++++++++++++++++++++++++
 2 files changed, 261 insertions(+)

diff --git a/drivers/gpu/nova-core/gsp/nvkv.rs b/drivers/gpu/nova-core/gsp/nvkv.rs
index b908f66e760d..64d8d0118452 100644
--- a/drivers/gpu/nova-core/gsp/nvkv.rs
+++ b/drivers/gpu/nova-core/gsp/nvkv.rs
@@ -20,6 +20,9 @@
 mod encode;
 pub(crate) use encode::*;
 
+mod decode;
+pub(crate) use decode::*;
+
 /// The identifier of an NVKV key.
 pub(crate) type KeyId = u16;
 
diff --git a/drivers/gpu/nova-core/gsp/nvkv/decode.rs b/drivers/gpu/nova-core/gsp/nvkv/decode.rs
new file mode 100644
index 000000000000..ee8b6ab5a3a4
--- /dev/null
+++ b/drivers/gpu/nova-core/gsp/nvkv/decode.rs
@@ -0,0 +1,258 @@
+// SPDX-License-Identifier: GPL-2.0
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+
+#![cfg_attr(not(CONFIG_KUNIT), expect(dead_code))]
+
+use kernel::prelude::*;
+
+use crate::gsp::nvkv::{
+    Index,
+    KeyId,
+    Op,
+    Opcode, //
+};
+use crate::num;
+
+/// A decoded NVKV value.
+#[derive(Copy, Clone)]
+pub(crate) enum DecoderValue<'a> {
+    Scalar32(u32),
+    Scalar64(u64),
+    Array8(&'a [u8]),
+    Array32(&'a [u32]),
+    Array64(&'a [u64]),
+}
+
+/// Implements `TryFrom` from the given `DecoderValue` variant to the given type.
+///
+/// `TryFrom` is used by the `Schema` implementations in this file to convert from the
+/// `DecoderValue`s into the types to store. Provide the implementations for basic types here.
+macro_rules! impl_try_from_decoder_value {
+    ($ty:ty, $variant:ident) => {
+        impl<'a> TryFrom<DecoderValue<'a>> for $ty {
+            type Error = Error;
+
+            fn try_from(value: DecoderValue<'a>) -> Result<Self> {
+                if let DecoderValue::$variant(v) = value {
+                    Ok(v)
+                } else {
+                    Err(EINVAL)
+                }
+            }
+        }
+    };
+}
+
+impl_try_from_decoder_value!(u32, Scalar32);
+impl_try_from_decoder_value!(u64, Scalar64);
+impl_try_from_decoder_value!(&'a [u8], Array8);
+impl_try_from_decoder_value!(&'a [u32], Array32);
+impl_try_from_decoder_value!(&'a [u64], Array64);
+
+/// A visitor that consumes decoded NVKV and produces a `Target`.
+pub(crate) trait Schema {
+    type Target;
+
+    /// Visits one decoded pair. Returns `Ok(true)` if the schema consumed it.
+    fn visit<'a>(&mut self, key: KeyId, index: Index, value: DecoderValue<'a>) -> Result<bool>;
+
+    /// Returns an initializer that makes the decoded `Target`.
+    fn finish(self) -> impl Init<Self::Target, Error>;
+}
+
+/// A read position in an NVKV stream.
+struct Cursor<'a> {
+    data: &'a [u64],
+}
+
+impl<'a> Cursor<'a> {
+    fn new(data: &'a [u64]) -> Self {
+        Self { data }
+    }
+
+    fn is_empty(&self) -> bool {
+        self.data.is_empty()
+    }
+
+    fn take_u64(&mut self) -> Result<u64> {
+        // PANIC: `take_u64s(1)` returns exactly one element on success.
+        Ok(self.take_u64s(1)?[0])
+    }
+
+    fn take_u8s(&mut self, count: usize) -> Result<&[u8]> {
+        let values = self.take_u64s(count.div_ceil(8))?;
+        values.as_bytes().get(..count).ok_or(EINVAL)
+    }
+
+    fn take_u32s(&mut self, count: usize) -> Result<&[u32]> {
+        let values = self.take_u64s(count.div_ceil(2))?;
+        // SAFETY: `values` is 8 byte aligned and only 4 byte alignment is required. All bit
+        // patterns are valid for `u32`.
+        Ok(unsafe { core::slice::from_raw_parts(values.as_ptr().cast::<u32>(), count) })
+    }
+
+    fn take_u64s(&mut self, count: usize) -> Result<&[u64]> {
+        let (prefix, suffix) = self.data.split_at_checked(count).ok_or(EINVAL)?;
+        self.data = suffix;
+        Ok(prefix)
+    }
+}
+
+/// A decoder for an NVKV stream.
+pub(crate) struct Decoder<'a> {
+    data: &'a [u64],
+    policy: UnknownKeyPolicy,
+}
+
+impl<'a> Decoder<'a> {
+    /// Creates a decoder for `data` that handles unknown keys per `policy`.
+    pub(crate) fn new(data: &'a [u64], policy: UnknownKeyPolicy) -> Self {
+        Self { data, policy }
+    }
+
+    fn visit<S: Schema>(
+        &self,
+        schema: &mut S,
+        key: KeyId,
+        index: Index,
+        value: DecoderValue<'_>,
+    ) -> Result {
+        let consumed = schema.visit(key, index, value)?;
+        if !consumed && self.policy == UnknownKeyPolicy::Error {
+            Err(EINVAL)
+        } else {
+            Ok(())
+        }
+    }
+
+    fn seq_key(base: KeyId, offset: usize) -> Result<KeyId> {
+        base.checked_add(KeyId::try_from(offset)?).ok_or(EINVAL)
+    }
+
+    /// Decodes every pair into `schema` and returns the result of [`Schema::finish`].
+    pub(crate) fn decode<S: Schema>(&self, mut schema: S) -> Result<impl Init<S::Target, Error>> {
+        let mut cursor = Cursor::new(self.data);
+        while !cursor.is_empty() {
+            let op: Op = cursor.take_u64()?.into();
+
+            let key = op.key().into();
+            let index = op.index();
+            let op_value: u32 = op.value().into();
+            match op.opcode()? {
+                Opcode::Imm32 => {
+                    self.visit(&mut schema, key, index, DecoderValue::Scalar32(op_value))?;
+                }
+                Opcode::Seq32 => {
+                    let values = cursor.take_u32s(num::u32_as_usize(op_value))?;
+                    for (i, &value) in values.iter().enumerate() {
+                        let key = Self::seq_key(key, i)?;
+                        self.visit(&mut schema, key, index, DecoderValue::Scalar32(value))?;
+                    }
+                }
+                Opcode::Seq64 => {
+                    let values = cursor.take_u64s(num::u32_as_usize(op_value))?;
+                    for (i, &value) in values.iter().enumerate() {
+                        let key = Self::seq_key(key, i)?;
+                        self.visit(&mut schema, key, index, DecoderValue::Scalar64(value))?;
+                    }
+                }
+                Opcode::Array8 => {
+                    let value = cursor.take_u8s(num::u32_as_usize(op_value))?;
+                    self.visit(&mut schema, key, index, DecoderValue::Array8(value))?;
+                }
+                Opcode::Array32 => {
+                    let value = cursor.take_u32s(num::u32_as_usize(op_value))?;
+                    self.visit(&mut schema, key, index, DecoderValue::Array32(value))?;
+                }
+                Opcode::Array64 => {
+                    let value = cursor.take_u64s(num::u32_as_usize(op_value))?;
+                    self.visit(&mut schema, key, index, DecoderValue::Array64(value))?;
+                }
+            };
+        }
+        Ok(schema.finish())
+    }
+}
+
+/// This is defined per call.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum UnknownKeyPolicy {
+    Ignore,
+    Error,
+}
+
+#[kunit_tests(nova_core_nvkv_decode)]
+mod tests {
+    use super::*;
+
+    use crate::gsp::nvkv::Encoder;
+
+    // Tests that basic decoding into a manually implemented `Schema` works correctly.
+    #[test]
+    fn decode_raw_schema() -> Result {
+        // Decodes an IMM32 pair and a SEQ64 pair (the encoder emits a u64 as a single-element
+        // SEQ64) with a hand written `Schema`. Keys and value constants chosen to distinguish e.g.
+        // saving the wrong value to the wrong location.
+        const SCALAR32_KEY: KeyId = 0x1001;
+        const SCALAR64_KEY: KeyId = 0x1002;
+        const UNKNOWN_KEY: KeyId = 0x2001;
+
+        const SCALAR32_VALUE: u32 = 0x1111_2222;
+        const SCALAR64_VALUE: u64 = 0x3333_4444_5555_6666;
+
+        // The output type of the hand written Schema. In this case, we can have it also implement
+        // `Schema` on itself rather than having a separate carrier type, since the `Schema`
+        // implementation is completely stateless.
+        #[derive(Default)]
+        struct RawSchema {
+            scalar32: u32,
+            scalar64: u64,
+        }
+
+        impl Schema for RawSchema {
+            type Target = Self;
+
+            fn visit(&mut self, key: KeyId, index: Index, value: DecoderValue<'_>) -> Result<bool> {
+                if index != Index::new::<0>() {
+                    return Err(EINVAL);
+                }
+                match key {
+                    SCALAR32_KEY => self.scalar32 = value.try_into()?,
+                    SCALAR64_KEY => self.scalar64 = value.try_into()?,
+                    _ => return Ok(false),
+                }
+                Ok(true)
+            }
+
+            fn finish(self) -> impl Init<Self::Target, Error> {
+                Ok(self)
+            }
+        }
+
+        let mut encoder = Encoder::new();
+        encoder.encode_u32(SCALAR32_KEY, Index::new::<0>(), SCALAR32_VALUE)?;
+        encoder.encode_u64(SCALAR64_KEY, Index::new::<0>(), SCALAR64_VALUE)?;
+        let serialized = encoder.finish();
+
+        let decoder = Decoder::new(&serialized, UnknownKeyPolicy::Error);
+        let decoded = KBox::try_init(decoder.decode(RawSchema::default())?, GFP_KERNEL)?;
+
+        assert_eq!(decoded.scalar32, SCALAR32_VALUE);
+        assert_eq!(decoded.scalar64, SCALAR64_VALUE);
+
+        // An unknown key should fail with under `UnknownKeyPolicy::Error` and be skipped under
+        // `UnknownKeyPolicy::Ignore`.
+        let mut encoder = Encoder::new();
+        encoder.encode_u32(UNKNOWN_KEY, Index::new::<0>(), 1)?;
+
+        let serialized = encoder.finish();
+        let decoder = Decoder::new(&serialized, UnknownKeyPolicy::Error);
+        assert!(decoder.decode(RawSchema::default()).is_err());
+
+        let decoder = Decoder::new(&serialized, UnknownKeyPolicy::Ignore);
+        let decoded = KBox::try_init(decoder.decode(RawSchema::default())?, GFP_KERNEL)?;
+        assert_eq!(decoded.scalar32, 0);
+
+        Ok(())
+    }
+}

-- 
2.55.0