[PATCH v9 06/23] gpu: nova-core: mm: Add support to use PRAMIN windows to write to VRAM

[Joel Fernandes <joelagnelf@nvidia.com>]

PRAMIN apertures are a crucial mechanism to direct read/write to VRAM.
Add support for the same.

Cc: Nikola Djukic <ndjukic@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 drivers/gpu/nova-core/mm.rs        |   5 +
 drivers/gpu/nova-core/mm/pramin.rs | 293 +++++++++++++++++++++++++++++
 drivers/gpu/nova-core/nova_core.rs |   1 +
 drivers/gpu/nova-core/regs.rs      |   6 +
 4 files changed, 305 insertions(+)
 create mode 100644 drivers/gpu/nova-core/mm.rs
 create mode 100644 drivers/gpu/nova-core/mm/pramin.rs

diff --git a/drivers/gpu/nova-core/mm.rs b/drivers/gpu/nova-core/mm.rs
new file mode 100644
index 000000000000..7a5dd4220c67
--- /dev/null
+++ b/drivers/gpu/nova-core/mm.rs
@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Memory management subsystems for nova-core.
+
+pub(crate) mod pramin;
diff --git a/drivers/gpu/nova-core/mm/pramin.rs b/drivers/gpu/nova-core/mm/pramin.rs
new file mode 100644
index 000000000000..707794f49add
--- /dev/null
+++ b/drivers/gpu/nova-core/mm/pramin.rs
@@ -0,0 +1,293 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Direct VRAM access through the PRAMIN aperture.
+//!
+//! PRAMIN provides a 1MB sliding window into VRAM through BAR0, allowing the CPU to access
+//! video memory directly. Access is managed through a two-level API:
+//!
+//! - [`Pramin`]: The parent object that owns the BAR0 reference and synchronization lock.
+//! - [`PraminWindow`]: A guard object that holds exclusive PRAMIN access for its lifetime.
+//!
+//! The PRAMIN aperture is a 1MB region at BAR0 + 0x700000 for all GPUs. The window base is
+//! controlled by the `NV_PBUS_BAR0_WINDOW` register and is 64KB aligned.
+//!
+//! # Examples
+//!
+//! ## Basic read/write
+//!
+//! ```no_run
+//! use crate::driver::Bar0;
+//! use crate::mm::pramin;
+//! use kernel::devres::Devres;
+//! use kernel::prelude::*;
+//! use kernel::sync::Arc;
+//!
+//! fn example(devres_bar: Arc<Devres<Bar0>>, vram_region: core::ops::Range<u64>) -> Result<()> {
+//!     let pramin = Arc::pin_init(pramin::Pramin::new(devres_bar, vram_region)?, GFP_KERNEL)?;
+//!     let mut window = pramin.window()?;
+//!
+//!     // Write and read back.
+//!     window.try_write32(0x100, 0xDEADBEEF)?;
+//!     let val = window.try_read32(0x100)?;
+//!     assert_eq!(val, 0xDEADBEEF);
+//!
+//!     Ok(())
+//! }
+//! ```
+//!
+//! ## Auto-repositioning across VRAM regions
+//!
+//! ```no_run
+//! use crate::driver::Bar0;
+//! use crate::mm::pramin;
+//! use kernel::devres::Devres;
+//! use kernel::prelude::*;
+//! use kernel::sync::Arc;
+//!
+//! fn example(devres_bar: Arc<Devres<Bar0>>, vram_region: core::ops::Range<u64>) -> Result<()> {
+//!     let pramin = Arc::pin_init(pramin::Pramin::new(devres_bar, vram_region)?, GFP_KERNEL)?;
+//!     let mut window = pramin.window()?;
+//!
+//!     // Access first 1MB region.
+//!     window.try_write32(0x100, 0x11111111)?;
+//!
+//!     // Access at 2MB - window auto-repositions.
+//!     window.try_write32(0x200000, 0x22222222)?;
+//!
+//!     // Back to first region - window repositions again.
+//!     let val = window.try_read32(0x100)?;
+//!     assert_eq!(val, 0x11111111);
+//!
+//!     Ok(())
+//! }
+//! ```
+
+#![expect(unused)]
+
+use core::ops::Range;
+
+use crate::{
+    driver::Bar0,
+    num::IntoSafeCast,
+    regs, //
+};
+
+use kernel::{
+    devres::Devres,
+    io::Io,
+    new_mutex,
+    prelude::*,
+    revocable::RevocableGuard,
+    sizes::{
+        SZ_1M,
+        SZ_64K, //
+    },
+    sync::{
+        lock::mutex::MutexGuard,
+        Arc,
+        Mutex, //
+    },
+};
+
+/// Target memory type for the BAR0 window register.
+///
+/// Only VRAM is supported; Hopper+ GPUs do not support other targets.
+#[repr(u8)]
+#[derive(Debug, Default)]
+pub(crate) enum Bar0WindowTarget {
+    /// Video RAM (GPU framebuffer memory).
+    #[default]
+    Vram = 0,
+}
+
+impl From<Bar0WindowTarget> for u8 {
+    fn from(value: Bar0WindowTarget) -> Self {
+        value as u8
+    }
+}
+
+impl TryFrom<u8> for Bar0WindowTarget {
+    type Error = Error;
+
+    fn try_from(value: u8) -> Result<Self> {
+        match value {
+            0 => Ok(Self::Vram),
+            _ => Err(EINVAL),
+        }
+    }
+}
+
+/// PRAMIN aperture base offset in BAR0.
+const PRAMIN_BASE: usize = 0x700000;
+
+/// PRAMIN aperture size (1MB).
+const PRAMIN_SIZE: usize = SZ_1M;
+
+/// Generate a PRAMIN read accessor.
+macro_rules! define_pramin_read {
+    ($name:ident, $ty:ty) => {
+        #[doc = concat!("Read a `", stringify!($ty), "` from VRAM at the given offset.")]
+        pub(crate) fn $name(&mut self, vram_offset: usize) -> Result<$ty> {
+            let (bar_offset, new_base) =
+                self.compute_window(vram_offset, ::core::mem::size_of::<$ty>())?;
+
+            if let Some(base) = new_base {
+                Self::write_window_base(&self.bar, base);
+                *self.state = base;
+            }
+            self.bar.$name(bar_offset)
+        }
+    };
+}
+
+/// Generate a PRAMIN write accessor.
+macro_rules! define_pramin_write {
+    ($name:ident, $ty:ty) => {
+        #[doc = concat!("Write a `", stringify!($ty), "` to VRAM at the given offset.")]
+        pub(crate) fn $name(&mut self, vram_offset: usize, value: $ty) -> Result {
+            let (bar_offset, new_base) =
+                self.compute_window(vram_offset, ::core::mem::size_of::<$ty>())?;
+
+            if let Some(base) = new_base {
+                Self::write_window_base(&self.bar, base);
+                *self.state = base;
+            }
+            self.bar.$name(value, bar_offset)
+        }
+    };
+}
+
+/// PRAMIN aperture manager.
+///
+/// Call [`Pramin::window()`] to acquire exclusive PRAMIN access.
+#[pin_data]
+pub(crate) struct Pramin {
+    bar: Arc<Devres<Bar0>>,
+    /// Valid VRAM region. Accesses outside this range are rejected.
+    vram_region: Range<u64>,
+    /// PRAMIN aperture state, protected by a mutex.
+    ///
+    /// # Safety
+    ///
+    /// This lock is acquired during the DMA fence signaling critical path.
+    /// It must NEVER be held across any reclaimable CPU memory / allocations
+    /// (`GFP_KERNEL`), because the memory reclaim path can call
+    /// `dma_fence_wait()`, which would deadlock with this lock held.
+    #[pin]
+    state: Mutex<u64>,
+}
+
+impl Pramin {
+    /// Create a pin-initializer for PRAMIN.
+    ///
+    /// `vram_region` specifies the valid VRAM address range.
+    pub(crate) fn new(
+        bar: Arc<Devres<Bar0>>,
+        vram_region: Range<u64>,
+    ) -> Result<impl PinInit<Self>> {
+        let bar_access = bar.try_access().ok_or(ENODEV)?;
+        let current_base = Self::read_window_base(&bar_access);
+
+        Ok(pin_init!(Self {
+            bar,
+            vram_region,
+            state <- new_mutex!(current_base, "pramin_state"),
+        }))
+    }
+
+    /// Acquire exclusive PRAMIN access.
+    ///
+    /// Returns a [`PraminWindow`] guard that provides VRAM read/write accessors.
+    /// The [`PraminWindow`] is exclusive and only one can exist at a time.
+    pub(crate) fn window(&self) -> Result<PraminWindow<'_>> {
+        let bar = self.bar.try_access().ok_or(ENODEV)?;
+        let state = self.state.lock();
+        Ok(PraminWindow {
+            bar,
+            vram_region: self.vram_region.clone(),
+            state,
+        })
+    }
+
+    /// Read the current window base from the BAR0_WINDOW register.
+    fn read_window_base(bar: &Bar0) -> u64 {
+        let reg = regs::NV_PBUS_BAR0_WINDOW::read(bar);
+        // TODO: Convert to Bounded<u64, 40> when available.
+        u64::from(reg.window_base()) << 16
+    }
+}
+
+/// PRAMIN window guard for direct VRAM access.
+///
+/// This guard holds exclusive access to the PRAMIN aperture. The window auto-repositions
+/// when accessing VRAM offsets outside the current 1MB range.
+///
+/// Only one [`PraminWindow`] can exist at a time per [`Pramin`] instance (enforced by the
+/// internal `MutexGuard`).
+pub(crate) struct PraminWindow<'a> {
+    bar: RevocableGuard<'a, Bar0>,
+    vram_region: Range<u64>,
+    state: MutexGuard<'a, u64>,
+}
+
+impl PraminWindow<'_> {
+    /// Write a new window base to the BAR0_WINDOW register.
+    fn write_window_base(bar: &Bar0, base: u64) {
+        // CAST: The caller (compute_window) validates that base is within the
+        // VRAM region which is always <= 40 bits. After >> 16, a 40-bit base
+        // becomes 24 bits, which fits in u32.
+        regs::NV_PBUS_BAR0_WINDOW::default()
+            .set_target(Bar0WindowTarget::Vram)
+            .set_window_base((base >> 16) as u32)
+            .write(bar);
+    }
+
+    /// Compute window parameters for a VRAM access.
+    ///
+    /// Returns (`bar_offset`, `new_base`) where:
+    /// - `bar_offset`: The BAR0 offset to use for the access.
+    /// - `new_base`: `Some(base)` if window needs repositioning, `None` otherwise.
+    fn compute_window(
+        &self,
+        vram_offset: usize,
+        access_size: usize,
+    ) -> Result<(usize, Option<u64>)> {
+        // Validate VRAM offset is within the valid VRAM region.
+        let vram_addr = vram_offset as u64;
+        let end_addr = vram_addr.checked_add(access_size as u64).ok_or(EINVAL)?;
+        if vram_addr < self.vram_region.start || end_addr > self.vram_region.end {
+            return Err(EINVAL);
+        }
+
+        // Check if access fits within the current 1MB window.
+        let current_base = *self.state;
+        if vram_addr >= current_base {
+            let offset_in_window: usize = (vram_addr - current_base).into_safe_cast();
+            if offset_in_window + access_size <= PRAMIN_SIZE {
+                return Ok((PRAMIN_BASE + offset_in_window, None));
+            }
+        }
+
+        // Access doesn't fit in current window - reposition.
+        // Hardware requires 64KB alignment for the window base register.
+        let needed_base = vram_addr & !(SZ_64K as u64 - 1);
+        let offset_in_window: usize = (vram_addr - needed_base).into_safe_cast();
+
+        // Verify access fits in the 1MB window from the new base.
+        if offset_in_window + access_size > PRAMIN_SIZE {
+            return Err(EINVAL);
+        }
+
+        Ok((PRAMIN_BASE + offset_in_window, Some(needed_base)))
+    }
+
+    define_pramin_read!(try_read8, u8);
+    define_pramin_read!(try_read16, u16);
+    define_pramin_read!(try_read32, u32);
+    define_pramin_read!(try_read64, u64);
+
+    define_pramin_write!(try_write8, u8);
+    define_pramin_write!(try_write16, u16);
+    define_pramin_write!(try_write32, u32);
+    define_pramin_write!(try_write64, u64);
+}
diff --git a/drivers/gpu/nova-core/nova_core.rs b/drivers/gpu/nova-core/nova_core.rs
index b5caf1044697..c5a78d6388e5 100644
--- a/drivers/gpu/nova-core/nova_core.rs
+++ b/drivers/gpu/nova-core/nova_core.rs
@@ -13,6 +13,7 @@
 mod gfw;
 mod gpu;
 mod gsp;
+mod mm;
 mod num;
 mod regs;
 mod sbuffer;
diff --git a/drivers/gpu/nova-core/regs.rs b/drivers/gpu/nova-core/regs.rs
index ea0d32f5396c..8ec35b8c4b28 100644
--- a/drivers/gpu/nova-core/regs.rs
+++ b/drivers/gpu/nova-core/regs.rs
@@ -32,6 +32,7 @@
         Architecture,
         Chipset, //
     },
+    mm::pramin::Bar0WindowTarget,
     num::FromSafeCast,
 };
 
@@ -102,6 +103,11 @@ fn fmt(&self, f: &mut kernel::fmt::Formatter<'_>) -> kernel::fmt::Result {
     31:16   frts_err_code as u16;
 });
 
+register!(NV_PBUS_BAR0_WINDOW @ 0x00001700, "BAR0 window control for PRAMIN access" {
+    25:24   target as u8 ?=> Bar0WindowTarget;
+    23:0    window_base as u32, "Window base address (bits 39:16 of FB addr)";
+});
+
 // PFB
 
 // The following two registers together hold the physical system memory address that is used by the
-- 
2.34.1