gpu: nova-core: gsp: add mechanism to wait for space on command queue

Add a timeout to `allocate_command` which waits for space on the GSP
command queue. It uses a similar timeout to nouveau.

This lets `send_command` wait for space to free up in the command queue.
This is required to support continuation records which can fill up the
queue.

Tested-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260306-cmdq-continuation-v6-2-cc7b629200ee@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

drivers/gpu/nova-core/gsp/cmdq.rs

@@ -250,6 +250,19 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
         }
     }
 
+    /// Returns the size of the region of the CPU message queue that the driver is currently allowed
+    /// to write to, in bytes.
+    fn driver_write_area_size(&self) -> usize {
+        let tx = self.cpu_write_ptr();
+        let rx = self.gsp_read_ptr();
+
+        // `rx` and `tx` are both in `0..MSGQ_NUM_PAGES` per the invariants of `gsp_read_ptr` and
+        // `cpu_write_ptr`. The minimum value case is where `rx == 0` and `tx == MSGQ_NUM_PAGES -
+        // 1`, which gives `0 + MSGQ_NUM_PAGES - (MSGQ_NUM_PAGES - 1) - 1 == 0`.
+        let slots = (rx + MSGQ_NUM_PAGES - tx - 1) % MSGQ_NUM_PAGES;
+        num::u32_as_usize(slots) * GSP_PAGE_SIZE
+    }
+
     /// Returns the region of the GSP message queue that the driver is currently allowed to read
     /// from.
     ///
@@ -281,15 +294,22 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
     }
 
     /// Allocates a region on the command queue that is large enough to send a command of `size`
-    /// bytes.
+    /// bytes, waiting for space to become available based on the provided timeout.
     ///
     /// This returns a [`GspCommand`] ready to be written to by the caller.
     ///
     /// # Errors
     ///
-    /// - `EAGAIN` if the driver area is too small to hold the requested command.
+    /// - `ETIMEDOUT` if space does not become available within the timeout.
     /// - `EIO` if the command header is not properly aligned.
-    fn allocate_command(&mut self, size: usize) -> Result<GspCommand<'_>> {
+    fn allocate_command(&mut self, size: usize, timeout: Delta) -> Result<GspCommand<'_>> {
+        read_poll_timeout(
+            || Ok(self.driver_write_area_size()),
+            |available_bytes| *available_bytes >= size_of::<GspMsgElement>() + size,
+            Delta::from_micros(1),
+            timeout,
+        )?;
+
         // Get the current writable area as an array of bytes.
         let (slice_1, slice_2) = {
             let (slice_1, slice_2) = self.driver_write_area();
@@ -298,13 +318,6 @@ fn allocate_command(&mut self, size: usize) -> Result<GspCommand<'_>> {
             (slice_1.as_flattened_mut(), slice_2.as_flattened_mut())
         };
 
-        // If the GSP is still processing previous messages the shared region
-        // may be full in which case we will have to retry once the GSP has
-        // processed the existing commands.
-        if size_of::<GspMsgElement>() + size > slice_1.len() + slice_2.len() {
-            return Err(EAGAIN);
-        }
-
         // Extract area for the `GspMsgElement`.
         let (header, slice_1) = GspMsgElement::from_bytes_mut_prefix(slice_1).ok_or(EIO)?;
 
@@ -462,6 +475,9 @@ impl Cmdq {
     /// Number of page table entries for the GSP shared region.
     pub(crate) const NUM_PTES: usize = size_of::<GspMem>() >> GSP_PAGE_SHIFT;
 
+    /// Timeout for waiting for space on the command queue.
+    const ALLOCATE_TIMEOUT: Delta = Delta::from_secs(1);
+
     /// Creates a new command queue for `dev`.
     pub(crate) fn new(dev: &device::Device<device::Bound>) -> Result<Cmdq> {
         let gsp_mem = DmaGspMem::new(dev)?;
@@ -497,7 +513,7 @@ fn notify_gsp(bar: &Bar0) {
     ///
     /// # Errors
     ///
-    /// - `EAGAIN` if there was not enough space in the command queue to send the command.
+    /// - `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.
     ///
@@ -509,7 +525,9 @@ pub(crate) fn send_command<M>(&mut self, bar: &Bar0, command: M) -> Result
         Error: From<M::InitError>,
     {
         let command_size = size_of::<M::Command>() + command.variable_payload_len();
-        let dst = self.gsp_mem.allocate_command(command_size)?;
+        let dst = self
+            .gsp_mem
+            .allocate_command(command_size, Self::ALLOCATE_TIMEOUT)?;
 
         // Extract area for the command itself.
         let (cmd, payload_1) = M::Command::from_bytes_mut_prefix(dst.contents.0).ok_or(EIO)?;