Message-ID: <20251203055923.1247681-8-jhubbard@nvidia.com>
Date: Tue,  2 Dec 2025 21:58:59 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Alexandre Courbot <acourbot@...dia.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Timur Tabi <ttabi@...dia.com>,
	Alistair Popple <apopple@...dia.com>,
	Edwin Peer <epeer@...dia.com>,
	Zhi Wang <zhiw@...dia.com>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	nouveau@...ts.freedesktop.org,
	rust-for-linux@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	John Hubbard <jhubbard@...dia.com>
Subject: [PATCH 07/31] gpu: nova-core: set DMA mask width based on GPU architecture

This removes a "TODO" item in the code, which was hardcoded to work on
Ampere and Ada GPUs. Hopper/Blackwell+ have a larger width, so do an
early read of boot42, in order to pick the correct value.

Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 drivers/gpu/nova-core/driver.rs | 33 +++++++++++++++++----------------
 drivers/gpu/nova-core/gpu.rs    | 29 ++++++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
index d91bbc50cde7..3179a4d47af4 100644
--- a/drivers/gpu/nova-core/driver.rs
+++ b/drivers/gpu/nova-core/driver.rs
@@ -4,8 +4,10 @@
     auxiliary,
     c_str,
     device::Core,
-    dma::Device,
-    dma::DmaMask,
+    dma::{
+        Device,
+        DmaMask, //
+    },
     pci,
     pci::{
         Class,
@@ -17,7 +19,10 @@
     sync::Arc, //
 };
 
-use crate::gpu::Gpu;
+use crate::gpu::{
+    read_architecture,
+    Gpu, //
+};
 
 #[pin_data]
 pub(crate) struct NovaCore {
@@ -28,14 +33,6 @@ pub(crate) struct NovaCore {
 
 const BAR0_SIZE: usize = SZ_16M;
 
-// For now we only support Ampere which can use up to 47-bit DMA addresses.
-//
-// TODO: Add an abstraction for this to support newer GPUs which may support
-// larger DMA addresses. Limiting these GPUs to smaller address widths won't
-// have any adverse affects, unless installed on systems which require larger
-// DMA addresses. These systems should be quite rare.
-const GPU_DMA_BITS: u32 = 47;
-
 pub(crate) type Bar0 = pci::Bar<BAR0_SIZE>;
 
 kernel::pci_device_table!(
@@ -73,11 +70,6 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self
         pdev.enable_device_mem()?;
         pdev.set_master();
 
-        // SAFETY: No concurrent DMA allocations or mappings can be made because
-        // the device is still being probed and therefore isn't being used by
-        // other threads of execution.
-        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::new::<GPU_DMA_BITS>())? };
-
         let devres_bar = Arc::pin_init(
             pdev.iomap_region_sized::<BAR0_SIZE>(0, c_str!("nova-core/bar0")),
             GFP_KERNEL,
@@ -88,6 +80,15 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self
         let bar_clone = Arc::clone(&devres_bar);
         let bar = bar_clone.access(pdev.as_ref())?;
 
+        // Read the GPU architecture early to determine the correct DMA address width.
+        // Hopper/Blackwell+ support 52-bit DMA addresses, earlier architectures use 47-bit.
+        let arch = read_architecture(bar)?;
+
+        // SAFETY: No concurrent DMA allocations or mappings can be made because
+        // the device is still being probed and therefore isn't being used by
+        // other threads of execution.
+        unsafe { pdev.dma_set_mask_and_coherent(DmaMask::try_new(arch.dma_addr_bits())?)? };
+
         let this = KBox::pin_init(
             try_pin_init!(Self {
                 gpu <- Gpu::new(pdev, devres_bar, bar),
diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index c21ce91924f5..624bbc2a54e8 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -102,7 +102,7 @@ fn try_from(value: u32) -> Result<Self, Self::Error> {
 });
 
 impl Chipset {
-    pub(crate) fn arch(&self) -> Architecture {
+    pub(crate) const fn arch(&self) -> Architecture {
         match self {
             Self::TU102 | Self::TU104 | Self::TU106 | Self::TU117 | Self::TU116 => {
                 Architecture::Turing
@@ -155,6 +155,19 @@ pub(crate) enum Architecture {
     Blackwell = 0x1b,
 }
 
+impl Architecture {
+    /// Returns the number of DMA address bits supported by this architecture.
+    ///
+    /// Hopper and Blackwell support 52-bit DMA addresses, while earlier architectures
+    /// (Turing, Ampere, Ada) support 47-bit DMA addresses.
+    pub(crate) const fn dma_addr_bits(&self) -> u32 {
+        match self {
+            Self::Turing | Self::Ampere | Self::Ada => 47,
+            Self::Hopper | Self::Blackwell => 52,
+        }
+    }
+}
+
 impl TryFrom<u8> for Architecture {
     type Error = Error;
 
@@ -203,6 +216,20 @@ pub(crate) struct Spec {
     revision: Revision,
 }
 
+/// Reads the GPU architecture from BAR0 registers.
+///
+/// This is a lightweight check used early in probe to determine the correct DMA address width
+/// before the full [`Spec`] is constructed.
+pub(crate) fn read_architecture(bar: &Bar0) -> Result<Architecture> {
+    let boot0 = regs::NV_PMC_BOOT_0::read(bar);
+
+    if boot0.is_older_than_fermi() {
+        return Err(ENODEV);
+    }
+
+    regs::NV_PMC_BOOT_42::read(bar).architecture()
+}
+
 impl Spec {
     fn new(dev: &device::Device, bar: &Bar0) -> Result<Spec> {
         // Some brief notes about boot0 and boot42, in chronological order:
-- 
2.52.0

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