Add external memory (EMEM) read/write operations to the GPU's FSP falcon
engine. These operations use Falcon PIO (Programmed I/O) to communicate
with the FSP through indirect memory access.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 drivers/gpu/nova-core/falcon/fsp.rs | 120 +++++++++++++++++++++++++++-
 drivers/gpu/nova-core/regs.rs       |  12 +++
 2 files changed, 131 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/nova-core/falcon/fsp.rs b/drivers/gpu/nova-core/falcon/fsp.rs
index c5ba1c2412cd..29a68d6934a9 100644
--- a/drivers/gpu/nova-core/falcon/fsp.rs
+++ b/drivers/gpu/nova-core/falcon/fsp.rs
@@ -5,13 +5,26 @@
 //! The FSP falcon handles secure boot and Chain of Trust operations
 //! on Hopper and Blackwell architectures, replacing SEC2's role.
 
+use kernel::{
+    io::{
+        Io,
+        IoCapable, //
+    },
+    prelude::*, //
+};
+
 use crate::{
+    driver::Bar0,
     falcon::{
+        Falcon,
         FalconEngine,
         PFalcon2Base,
         PFalconBase, //
     },
-    regs::macros::RegisterBase,
+    regs::{
+        self,
+        macros::RegisterBase, //
+    },
 };
 
 /// Type specifying the `Fsp` falcon engine. Cannot be instantiated.
@@ -28,3 +41,108 @@ impl RegisterBase<PFalcon2Base> for Fsp {
 impl FalconEngine for Fsp {
     const ID: Self = Fsp(());
 }
+
+/// Maximum addressable EMEM size, derived from the 24-bit offset field
+/// in NV_PFALCON_FALCON_EMEM_CTL.
+const EMEM_MAX_SIZE: usize = 1 << 24;
+
+/// I/O backend for the FSP falcon's external memory (EMEM).
+///
+/// Each 32-bit access programs a byte offset via the EMEM_CTL register,
+/// then reads or writes through the EMEM_DATA register.
+pub(crate) struct Emem<'a> {
+    bar: &'a Bar0,
+}
+
+impl<'a> Emem<'a> {
+    fn new(bar: &'a Bar0) -> Self {
+        Self { bar }
+    }
+}
+
+impl IoCapable<u32> for Emem<'_> {
+    unsafe fn io_read(&self, address: usize) -> u32 {
+        // The Io trait validates that EMEM accesses fit within the 24-bit offset field.
+        let offset = address as u32;
+
+        regs::NV_PFALCON_FALCON_EMEM_CTL::default()
+            .set_rd_mode(true)
+            .set_offset(offset)
+            .write(self.bar, &Fsp::ID);
+
+        regs::NV_PFALCON_FALCON_EMEM_DATA::read(self.bar, &Fsp::ID).data()
+    }
+
+    unsafe fn io_write(&self, value: u32, address: usize) {
+        // The Io trait validates that EMEM accesses fit within the 24-bit offset field.
+        let offset = address as u32;
+
+        regs::NV_PFALCON_FALCON_EMEM_CTL::default()
+            .set_wr_mode(true)
+            .set_offset(offset)
+            .write(self.bar, &Fsp::ID);
+
+        regs::NV_PFALCON_FALCON_EMEM_DATA::default()
+            .set_data(value)
+            .write(self.bar, &Fsp::ID);
+    }
+}
+
+impl Io for Emem<'_> {
+    fn addr(&self) -> usize {
+        0
+    }
+
+    fn maxsize(&self) -> usize {
+        EMEM_MAX_SIZE
+    }
+}
+
+impl Falcon<Fsp> {
+    /// Returns an EMEM I/O accessor for this FSP falcon.
+    pub(crate) fn emem<'a>(&self, bar: &'a Bar0) -> Emem<'a> {
+        Emem::new(bar)
+    }
+
+    /// Writes `data` to FSP external memory at byte `offset`.
+    ///
+    /// Data is interpreted as little-endian 32-bit words.
+    /// Returns `EINVAL` if offset or data length is not 4-byte aligned.
+    #[expect(unused)]
+    pub(crate) fn write_emem(&self, bar: &Bar0, offset: u32, data: &[u8]) -> Result {
+        if offset % 4 != 0 || data.len() % 4 != 0 {
+            return Err(EINVAL);
+        }
+
+        let emem = self.emem(bar);
+        let mut off = offset as usize;
+        for chunk in data.chunks_exact(4) {
+            let word = u32::from_le_bytes([chunk[0], chunk[1], chunk[2], chunk[3]]);
+            emem.try_write32(word, off)?;
+            off += 4;
+        }
+
+        Ok(())
+    }
+
+    /// Reads FSP external memory at byte `offset` into `data`.
+    ///
+    /// Data is stored as little-endian 32-bit words.
+    /// Returns `EINVAL` if offset or data length is not 4-byte aligned.
+    #[expect(unused)]
+    pub(crate) fn read_emem(&self, bar: &Bar0, offset: u32, data: &mut [u8]) -> Result {
+        if offset % 4 != 0 || data.len() % 4 != 0 {
+            return Err(EINVAL);
+        }
+
+        let emem = self.emem(bar);
+        let mut off = offset as usize;
+        for chunk in data.chunks_exact_mut(4) {
+            let word = emem.try_read32(off)?;
+            chunk.copy_from_slice(&word.to_le_bytes());
+            off += 4;
+        }
+
+        Ok(())
+    }
+}
diff --git a/drivers/gpu/nova-core/regs.rs b/drivers/gpu/nova-core/regs.rs
index 53f412f0ca32..f577800db3e3 100644
--- a/drivers/gpu/nova-core/regs.rs
+++ b/drivers/gpu/nova-core/regs.rs
@@ -461,6 +461,18 @@ pub(crate) fn reset_engine<E: FalconEngine>(bar: &Bar0) {
     8:8     br_fetch as bool;
 });
 
+// Falcon EMEM PIO registers (used by FSP on Hopper/Blackwell).
+// These provide the falcon external memory communication interface.
+register!(NV_PFALCON_FALCON_EMEM_CTL @ PFalconBase[0x00000ac0] {
+    23:0    offset as u32;      // EMEM byte offset (must be 4-byte aligned)
+    24:24   wr_mode as bool;    // Write mode
+    25:25   rd_mode as bool;    // Read mode
+});
+
+register!(NV_PFALCON_FALCON_EMEM_DATA @ PFalconBase[0x00000ac4] {
+    31:0    data as u32;        // EMEM data register
+});
+
 // The modules below provide registers that are not identical on all supported chips. They should
 // only be used in HAL modules.
 
-- 
2.53.0