Add usable_vram field to FbLayout to store the usable VRAM region for
driver allocations. This is populated after GSP boot with the region
extracted from GSP's fbRegionInfoParams.

FbLayout is now a two-phase structure:
1. new() computes firmware layout from hardware
2. set_usable_vram() populates usable region from GSP

The new usable_vram field represents the actual usable VRAM region
(~23.7GB on a 24GB GPU GA102 Ampere GPU).

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 drivers/gpu/nova-core/fb.rs | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/nova-core/fb.rs b/drivers/gpu/nova-core/fb.rs
index c62abcaed547..779447952b19 100644
--- a/drivers/gpu/nova-core/fb.rs
+++ b/drivers/gpu/nova-core/fb.rs
@@ -97,6 +97,10 @@ pub(crate) fn unregister(&self, bar: &Bar0) {
 /// Layout of the GPU framebuffer memory.
 ///
 /// Contains ranges of GPU memory reserved for a given purpose during the GSP boot process.
+///
+/// This structure is populated in 2 steps:
+/// 1. [`FbLayout::new()`] computes firmware layout from hardware.
+/// 2. [`FbLayout::set_usable_vram()`] populates usable region from GSP response.
 #[derive(Debug)]
 pub(crate) struct FbLayout {
     /// Range of the framebuffer. Starts at `0`.
@@ -111,10 +115,14 @@ pub(crate) struct FbLayout {
     pub(crate) elf: Range<u64>,
     /// WPR2 heap.
     pub(crate) wpr2_heap: Range<u64>,
-    /// WPR2 region range, starting with an instance of `GspFwWprMeta`.
+    /// WPR2 region range, starting with an instance of [`GspFwWprMeta`].
     pub(crate) wpr2: Range<u64>,
+    /// Non-WPR heap carved before WPR2, used by GSP firmware.
     pub(crate) heap: Range<u64>,
     pub(crate) vf_partition_count: u8,
+    /// Usable VRAM region for driver allocations (from GSP `fbRegionInfoParams`).
+    /// Initially [`None`], populated after GSP boot with usable region info.
+    pub(crate) usable_vram: Option<Range<u64>>,
 }
 
 impl FbLayout {
@@ -212,6 +220,19 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
             wpr2,
             heap,
             vf_partition_count: 0,
+            usable_vram: None,
         })
     }
+
+    /// Set the usable VRAM region from GSP response.
+    ///
+    /// Called after GSP boot with the first usable region extracted from
+    /// GSP's `fbRegionInfoParams`. Usable regions are those that:
+    /// - Are not reserved for firmware internal use.
+    /// - Are not protected (hardware-enforced access restrictions).
+    /// - Support compression (can use GPU memory compression for bandwidth).
+    /// - Support ISO (isochronous memory for display requiring guaranteed bandwidth).
+    pub(crate) fn set_usable_vram(&mut self, base: u64, size: u64) {
+        self.usable_vram = Some(base..base.saturating_add(size));
+    }
 }
-- 
2.34.1