The buddy allocator manages the actual usable VRAM. On my GA102 Ampere
with 24GB video memory, that is ~23.7GB on a 24GB GPU enabling proper
GPU memory allocation for driver use.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 drivers/gpu/nova-core/gpu.rs          | 62 ++++++++++++++++++++++-----
 drivers/gpu/nova-core/gsp/boot.rs     |  7 ++-
 drivers/gpu/nova-core/gsp/commands.rs |  2 -
 3 files changed, 57 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index a1bcf6679e2a..dd05ad23f763 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
+use core::cell::Cell;
+
 use kernel::{
     device,
     devres::Devres,
@@ -7,7 +9,7 @@
     gpu::buddy::GpuBuddyParams,
     pci,
     prelude::*,
-    sizes::{SZ_1M, SZ_4K},
+    sizes::SZ_4K,
     sync::Arc, //
 };
 
@@ -28,6 +30,13 @@
     regs,
 };
 
+/// Parameters extracted from GSP boot for initializing memory subsystems.
+#[derive(Clone, Copy)]
+struct BootParams {
+    usable_vram_start: u64,
+    usable_vram_size: u64,
+}
+
 macro_rules! define_chipset {
     ({ $($variant:ident = $value:expr),* $(,)* }) =>
     {
@@ -270,6 +279,13 @@ pub(crate) fn new<'a>(
         devres_bar: Arc<Devres<Bar0>>,
         bar: &'a Bar0,
     ) -> impl PinInit<Self, Error> + 'a {
+        // Cell to share boot parameters between GSP boot and subsequent initializations.
+        // Contains usable VRAM region from FbLayout and BAR1 PDE base from GSP info.
+        let boot_params: Cell<BootParams> = Cell::new(BootParams {
+            usable_vram_start: 0,
+            usable_vram_size: 0,
+        });
+
         try_pin_init!(Self {
             spec: Spec::new(pdev.as_ref(), bar).inspect(|spec| {
                 dev_info!(pdev.as_ref(),"NVIDIA ({})\n", spec);
@@ -291,18 +307,42 @@ pub(crate) fn new<'a>(
 
             sec2_falcon: Falcon::new(pdev.as_ref(), spec.chipset)?,
 
-            // Create GPU memory manager owning memory management resources.
-            // This will be initialized with the usable VRAM region from GSP in a later
-            // patch. For now, we use a placeholder of 1MB.
-            mm: GpuMm::new(devres_bar.clone(), GpuBuddyParams {
-                base_offset_bytes: 0,
-                physical_memory_size_bytes: SZ_1M as u64,
-                chunk_size_bytes: SZ_4K as u64,
-            })?,
-
             gsp <- Gsp::new(pdev),
 
-            gsp_static_info: { gsp.boot(pdev, bar, spec.chipset, gsp_falcon, sec2_falcon)?.0 },
+            // Boot GSP and extract usable VRAM region for buddy allocator.
+            gsp_static_info: {
+                let (info, fb_layout) = gsp.boot(pdev, bar, spec.chipset, gsp_falcon, sec2_falcon)?;
+
+                let usable_vram = fb_layout.usable_vram.as_ref().ok_or_else(|| {
+                    dev_err!(pdev.as_ref(), "No usable FB regions found from GSP\n");
+                    ENODEV
+                })?;
+
+                dev_info!(
+                    pdev.as_ref(),
+                    "Using FB region: {:#x}..{:#x}\n",
+                    usable_vram.start,
+                    usable_vram.end
+                );
+
+                boot_params.set(BootParams {
+                    usable_vram_start: usable_vram.start,
+                    usable_vram_size: usable_vram.end - usable_vram.start,
+                });
+
+                info
+            },
+
+            // Create GPU memory manager owning memory management resources.
+            // Uses the usable VRAM region from GSP for buddy allocator.
+            mm: {
+                let params = boot_params.get();
+                GpuMm::new(devres_bar.clone(), GpuBuddyParams {
+                    base_offset_bytes: params.usable_vram_start,
+                    physical_memory_size_bytes: params.usable_vram_size,
+                    chunk_size_bytes: SZ_4K as u64,
+                })?
+            },
 
             bar: devres_bar,
         })
diff --git a/drivers/gpu/nova-core/gsp/boot.rs b/drivers/gpu/nova-core/gsp/boot.rs
index 75f949bc4864..a034e2e80a4b 100644
--- a/drivers/gpu/nova-core/gsp/boot.rs
+++ b/drivers/gpu/nova-core/gsp/boot.rs
@@ -150,7 +150,7 @@ pub(crate) fn boot(
 
         let gsp_fw = KBox::pin_init(GspFirmware::new(dev, chipset, FIRMWARE_VERSION), GFP_KERNEL)?;
 
-        let fb_layout = FbLayout::new(chipset, bar, &gsp_fw)?;
+        let mut fb_layout = FbLayout::new(chipset, bar, &gsp_fw)?;
         dev_dbg!(dev, "{:#x?}\n", fb_layout);
 
         Self::run_fwsec_frts(dev, gsp_falcon, bar, &bios, &fb_layout)?;
@@ -252,6 +252,11 @@ pub(crate) fn boot(
             Err(e) => dev_warn!(pdev.as_ref(), "GPU name unavailable: {:?}\n", e),
         }
 
+        // Populate usable VRAM from GSP response.
+        if let Some((base, size)) = info.usable_fb_region() {
+            fb_layout.set_usable_vram(base, size);
+        }
+
         Ok((info, fb_layout))
     }
 }
diff --git a/drivers/gpu/nova-core/gsp/commands.rs b/drivers/gpu/nova-core/gsp/commands.rs
index d619cf294b9c..4a7eda512789 100644
--- a/drivers/gpu/nova-core/gsp/commands.rs
+++ b/drivers/gpu/nova-core/gsp/commands.rs
@@ -191,7 +191,6 @@ pub(crate) struct GetGspStaticInfoReply {
     gpu_name: [u8; 64],
     bar1_pde_base: u64,
     /// First usable FB region (base, size) for memory allocation.
-    #[expect(dead_code)]
     usable_fb_region: Option<(u64, u64)>,
 }
 
@@ -242,7 +241,6 @@ pub(crate) fn bar1_pde_base(&self) -> u64 {
 
     /// Returns the usable FB region (base, size) for driver allocation which is
     /// already retrieved from the GSP.
-    #[expect(dead_code)]
     pub(crate) fn usable_fb_region(&self) -> Option<(u64, u64)> {
         self.usable_fb_region
     }
-- 
2.34.1