[PATCH v3 4/9] gpu: nova-core: gsp: cmdq: split the transport part of the send path

[Alexandre Courbot <acourbot@nvidia.com>]

Introduce the `CommandElement` trait and `RpcCommandElement` wrapper
type to define how messages (independently of their type) are sent
through the transport layer. `send_single_command` becomes
`send_command_element`, which allocates the queue slots, writes the
element header, and delegates the writing of the message itself to the
implementation of `CommandElement` before computing the checksum,
advancing the write pointer and ringing the doorbell.

The RPC part of `send_single_command` (writing the RPC header and the
command payload) is now part of the `CommandElement` implementation.
This sets things up for moving the RPC code into its own sub-module,
leaving the transport agnostic of the message type.

No functional change intended.

Suggested-by: Eliot Courtney <ecourtney@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 drivers/gpu/nova-core/gsp/cmdq.rs | 133 +++++++++++++++++++++++---------------
 1 file changed, 80 insertions(+), 53 deletions(-)

diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
index bda79f23626d..d8a7716fc500 100644
--- a/drivers/gpu/nova-core/gsp/cmdq.rs
+++ b/drivers/gpu/nova-core/gsp/cmdq.rs
@@ -35,10 +35,7 @@
     },
 };
 
-use continuation::{
-    ContinuationRecord,
-    SplitState, //
-};
+use continuation::SplitState;
 
 use pin_init::pin_init_scope;
 
@@ -67,6 +64,19 @@
 /// reply type are sent using [`Cmdq::send_command_no_wait`].
 pub(crate) struct NoReply;
 
+/// Trait implemented by types that can be sent as a single command queue element.
+///
+/// The command queue allocates `size()` bytes after the `GspMsgElement` header and calls `write()`
+/// to fill them.
+trait CommandElement {
+    /// Size in bytes of the element, not including the `GspMsgElement` header.
+    fn size(&self) -> usize;
+
+    /// Writes the contents of the command into `dst`. `dev` is the queue's device (to be used for
+    /// logging), `seq` is the sequence number of the element.
+    fn write(&self, dev: &device::Device, seq: u32, dst: &mut GspCommand<'_>) -> Result;
+}
+
 /// Trait implemented by types representing a command to send to the GSP.
 ///
 /// The main purpose of this trait is to provide [`Cmdq`] with the information it needs to send
@@ -129,6 +139,62 @@ fn size(&self) -> usize {
     }
 }
 
+/// Wrapper type for sending a RPC command as a command queue element.
+///
+/// [`CommandElement`] cannot be directly implemented for all [`CommandToGsp`] with a blanket
+/// implementation as it would conflict with other future command types.
+struct RpcCommandElement<M>(M);
+
+impl<M> CommandElement for RpcCommandElement<M>
+where
+    M: CommandToGsp,
+    Error: From<M::InitError>,
+{
+    fn size(&self) -> usize {
+        self.0.size()
+    }
+
+    fn write(&self, dev: &device::Device, seq: u32, dst: &mut GspCommand<'_>) -> Result {
+        let command = &self.0;
+        let size_in_bytes = command.size();
+        // Extract area for the command itself. The GSP message header and the command header
+        // together are guaranteed to fit entirely into a single page, so it's ok to only look
+        // at `dst.contents.0` here.
+        let (cmd, payload_1) = M::Command::from_bytes_mut_prefix(dst.contents.0).ok_or(EIO)?;
+        let rpc_header_init = RpcMessageHeader::init(size_in_bytes, M::FUNCTION);
+        // SAFETY: `dst.header.rpc_header_mut()` is a valid reference, and is not touched if the
+        // initializer fails.
+        unsafe {
+            pin_init::raw_try_init(
+                core::ptr::from_mut(dst.header.rpc_header_mut()),
+                rpc_header_init,
+            )?;
+        }
+        // SAFETY: `cmd` is a valid reference, and is not touched if the initializer fails.
+        unsafe {
+            pin_init::raw_try_init(core::ptr::from_mut(cmd), command.init())?;
+        }
+
+        // Fill the variable-length payload, which may be empty.
+        let mut sbuffer = SBufferIter::new_writer([&mut payload_1[..], &mut dst.contents.1[..]]);
+        command.init_variable_payload(&mut sbuffer)?;
+
+        if !sbuffer.is_empty() {
+            return Err(EIO);
+        }
+
+        dev_dbg!(
+            dev,
+            "GSP RPC: send: seq# {}, function={:?}, length=0x{:x}\n",
+            seq,
+            M::FUNCTION,
+            size_in_bytes,
+        );
+
+        Ok(())
+    }
+}
+
 /// Trait representing messages received from the GSP.
 ///
 /// This trait tells [`Cmdq::receive_msg`] how it can receive a given type of message.
@@ -638,24 +704,18 @@ impl CmdqInner<'_> {
     /// Timeout for waiting for space on the command queue.
     const ALLOCATE_TIMEOUT: Delta = Delta::from_secs(1);
 
-    /// Sends `command` to the GSP, without splitting it.
+    /// Allocate enough send slots to store `command`, initialize them using
+    /// [`CommandElement::write`], and send the command to the GSP.
     ///
     /// # Errors
     ///
     /// - `EMSGSIZE` if the command exceeds the maximum queue element size.
     /// - `ETIMEDOUT` if space does not become available within the timeout.
-    /// - `EIO` if the variable payload requested by the command has not been entirely
-    ///   written to by its [`CommandToGsp::init_variable_payload`] method.
     ///
-    /// Error codes returned by the command initializers are propagated as-is.
-    fn send_single_command<M>(&mut self, command: M) -> Result
-    where
-        M: CommandToGsp,
-        // This allows all error types, including `Infallible`, to be used for `M::InitError`.
-        Error: From<M::InitError>,
-    {
+    /// Error codes returned by [`CommandElement::write`] are returned as-is.
+    fn send_command_element<E: CommandElement>(&mut self, command: E) -> Result {
         let size_in_bytes = command.size();
-        let dst = self
+        let mut dst = self
             .gsp_mem
             .allocate_command(size_in_bytes, Self::ALLOCATE_TIMEOUT)?;
 
@@ -666,40 +726,8 @@ fn send_single_command<M>(&mut self, command: M) -> Result
             pin_init::raw_try_init(core::ptr::from_mut(dst.header), msg_element_init)?;
         }
 
-        // Extract area for the command itself. The GSP message header and the command header
-        // together are guaranteed to fit entirely into a single page, so it's ok to only look
-        // at `dst.contents.0` here.
-        let (cmd, payload_1) = M::Command::from_bytes_mut_prefix(dst.contents.0).ok_or(EIO)?;
-        let rpc_header_init = RpcMessageHeader::init(size_in_bytes, M::FUNCTION);
-        // SAFETY: `dst.header.rpc_header_mut()` is a valid reference, and is not touched if the
-        // initializer fails.
-        unsafe {
-            pin_init::raw_try_init(
-                core::ptr::from_mut(dst.header.rpc_header_mut()),
-                rpc_header_init,
-            )?;
-        }
-        // SAFETY: `cmd` is a valid reference, and is not touched if the initializer fails.
-        unsafe {
-            pin_init::raw_try_init(core::ptr::from_mut(cmd), command.init())?;
-        }
-
-        // Fill the variable-length payload, which may be empty.
-        let mut sbuffer = SBufferIter::new_writer([&mut payload_1[..], &mut dst.contents.1[..]]);
-        command.init_variable_payload(&mut sbuffer)?;
-
-        if !sbuffer.is_empty() {
-            return Err(EIO);
-        }
-        drop(sbuffer);
-
-        dev_dbg!(
-            &self.dev,
-            "GSP RPC: send: seq# {}, function={:?}, length=0x{:x}\n",
-            self.seq,
-            M::FUNCTION,
-            size_in_bytes,
-        );
+        // Write the message payload.
+        command.write(self.dev, self.seq, &mut dst)?;
 
         // Compute checksum now that the whole message is ready.
         dst.header
@@ -735,13 +763,12 @@ fn send_command<M>(&mut self, command: M) -> Result
         Error: From<M::InitError>,
     {
         match SplitState::new(command)? {
-            SplitState::Single(command) => self.send_single_command(command),
+            SplitState::Single(command) => self.send_command_element(RpcCommandElement(command)),
             SplitState::Split(command, mut continuations) => {
-                self.send_single_command(command)?;
+                self.send_command_element(RpcCommandElement(command))?;
 
                 while let Some(continuation) = continuations.next() {
-                    // Turbofish needed because the compiler cannot infer M here.
-                    self.send_single_command::<ContinuationRecord<'_>>(continuation)?;
+                    self.send_command_element(RpcCommandElement(continuation))?;
                 }
 
                 Ok(())

-- 
2.55.0