[PATCH v2 0/3] rust, nova-core: add DeviceSize trait for SZ_* constants

[PATCH v2 0/3] rust, nova-core: add DeviceSize trait for SZ_* constants

[John Hubbard]

Changes in v2:
  * Replaced flat SZ_*_U64 constants with a DeviceSize trait that
    provides SZ_* as associated constants on u32 and u64.

  * A define_sizes! macro generates everything from one list of names.

  * Added Alignment::from_u64() so alignment values can be constructed
    from DeviceSize constants without falling back to usize variants.

  * Rebased onto drm-rust-next. No longer depends on the Blackwell
    patchset.

v1 is here:
    https://lore.kernel.org/20260310023145.120037-1-jhubbard@nvidia.com

John Hubbard (3):
  rust: sizes: add DeviceSize trait for device address space constants
  rust: ptr: add Alignment::from_u64() for DeviceSize constants
  gpu: nova-core: use DeviceSize trait for u64 size constants

 drivers/gpu/nova-core/fb.rs     |  31 ++++----
 drivers/gpu/nova-core/gsp/fw.rs |  18 ++---
 drivers/gpu/nova-core/regs.rs   |   6 +-
 rust/kernel/ptr.rs              |  35 +++++++++
 rust/kernel/sizes.rs            | 132 +++++++++++++++++++++-----------
 5 files changed, 147 insertions(+), 75 deletions(-)


base-commit: a544873ce0575b2fd8285a1364d3e09929d9a3ba
-- 
2.53.0

[PATCH v2 1/3] rust: sizes: add DeviceSize trait for device address space constants

[John Hubbard]

The SZ_* constants are usize, matching the CPU pointer width. But
device address spaces have their own widths (32-bit MMIO windows,
64-bit GPU framebuffers, etc.), so drivers end up with repeated
usize-to-u64 or usize-to-u32 conversion calls like
usize_as_u64(SZ_1M). This adds boilerplate with no safety benefit.

Add a DeviceSize trait with associated SZ_* constants, implemented for
u32 and u64. With the trait in scope, callers write u64::SZ_1M or
u32::SZ_4K to get the constant in their device's native width. All
SZ_* values fit in a u32, so both implementations are lossless. The
u32 impl has a const assert to catch any future constant that would
overflow; the u64 cast from usize is inherently lossless.

Replace the hand-written constant list with a define_sizes! macro that
generates the usize constants, the trait, and both trait impls from a
single list of names. Adding a new size or a new target type requires
changing only one place.

The trait also enables future generic APIs that accept T: DeviceSize,
so shared infrastructure can work with whatever address width the
driver declares.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/all/DGB9G697GSWO.3VBFGU5MKFPMR@kernel.org/
Link: https://lore.kernel.org/all/DGHI8WRKBQS9.38910L6FIIZTE@kernel.org/
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 rust/kernel/sizes.rs | 132 ++++++++++++++++++++++++++++---------------
 1 file changed, 88 insertions(+), 44 deletions(-)

diff --git a/rust/kernel/sizes.rs b/rust/kernel/sizes.rs
index 661e680d9330..6b11ec6d97b3 100644
--- a/rust/kernel/sizes.rs
+++ b/rust/kernel/sizes.rs
@@ -3,48 +3,92 @@
 //! Commonly used sizes.
 //!
 //! C headers: [`include/linux/sizes.h`](srctree/include/linux/sizes.h).
+//!
+//! The top-level `SZ_*` constants are [`usize`]-typed, for use in kernel page
+//! arithmetic and similar CPU-side work.
+//!
+//! The [`DeviceSize`] trait provides the same constants as associated constants
+//! on [`u32`] and [`u64`], for use in device address spaces where the address
+//! width depends on the hardware. Device drivers frequently need these constants
+//! as [`u64`] (or [`u32`]) rather than [`usize`], because device address spaces
+//! are sized independently of the CPU pointer width.
+//!
+//! ```
+//! use kernel::sizes::{DeviceSize, SZ_1M};
+//!
+//! // usize constant (CPU-side)
+//! let pages: usize = SZ_1M / kernel::page::PAGE_SIZE;
+//!
+//! // Device-side constant via the trait
+//! let heap_size: u64 = 14 * u64::SZ_1M;
+//! let small: u32 = u32::SZ_4K;
+//! ```
+
+macro_rules! define_sizes {
+    ($($name:ident),* $(,)?) => {
+        // `usize` constants, from the C `SZ_*` defines in `include/linux/sizes.h`.
+        $(
+            #[doc = concat!("`", stringify!($name), "` as a [`usize`].")]
+            pub const $name: usize = bindings::$name as usize;
+        )*
+
+        /// Size constants for device address spaces.
+        ///
+        /// Implemented for [`u32`] and [`u64`] so drivers can choose the width
+        /// that matches their hardware. All `SZ_*` values fit in a [`u32`], so
+        /// both implementations are lossless.
+        ///
+        /// ```
+        /// use kernel::sizes::DeviceSize;
+        ///
+        /// let gpu_heap: u64 = 14 * u64::SZ_1M;
+        /// let mmio_window: u32 = u32::SZ_16M;
+        /// ```
+        pub trait DeviceSize {
+            $(
+                #[doc = concat!("`", stringify!($name), "` for this type.")]
+                const $name: Self;
+            )*
+        }
+
+        impl DeviceSize for u32 {
+            $(
+                const $name: Self = {
+                    assert!(self::$name <= u32::MAX as usize);
+                    self::$name as u32
+                };
+            )*
+        }
+
+        impl DeviceSize for u64 {
+            $(
+                const $name: Self = self::$name as u64;
+            )*
+        }
+    };
+}
 
-/// 0x00000400
-pub const SZ_1K: usize = bindings::SZ_1K as usize;
-/// 0x00000800
-pub const SZ_2K: usize = bindings::SZ_2K as usize;
-/// 0x00001000
-pub const SZ_4K: usize = bindings::SZ_4K as usize;
-/// 0x00002000
-pub const SZ_8K: usize = bindings::SZ_8K as usize;
-/// 0x00004000
-pub const SZ_16K: usize = bindings::SZ_16K as usize;
-/// 0x00008000
-pub const SZ_32K: usize = bindings::SZ_32K as usize;
-/// 0x00010000
-pub const SZ_64K: usize = bindings::SZ_64K as usize;
-/// 0x00020000
-pub const SZ_128K: usize = bindings::SZ_128K as usize;
-/// 0x00040000
-pub const SZ_256K: usize = bindings::SZ_256K as usize;
-/// 0x00080000
-pub const SZ_512K: usize = bindings::SZ_512K as usize;
-/// 0x00100000
-pub const SZ_1M: usize = bindings::SZ_1M as usize;
-/// 0x00200000
-pub const SZ_2M: usize = bindings::SZ_2M as usize;
-/// 0x00400000
-pub const SZ_4M: usize = bindings::SZ_4M as usize;
-/// 0x00800000
-pub const SZ_8M: usize = bindings::SZ_8M as usize;
-/// 0x01000000
-pub const SZ_16M: usize = bindings::SZ_16M as usize;
-/// 0x02000000
-pub const SZ_32M: usize = bindings::SZ_32M as usize;
-/// 0x04000000
-pub const SZ_64M: usize = bindings::SZ_64M as usize;
-/// 0x08000000
-pub const SZ_128M: usize = bindings::SZ_128M as usize;
-/// 0x10000000
-pub const SZ_256M: usize = bindings::SZ_256M as usize;
-/// 0x20000000
-pub const SZ_512M: usize = bindings::SZ_512M as usize;
-/// 0x40000000
-pub const SZ_1G: usize = bindings::SZ_1G as usize;
-/// 0x80000000
-pub const SZ_2G: usize = bindings::SZ_2G as usize;
+define_sizes! {
+    SZ_1K,   // 0x0000_0400
+    SZ_2K,   // 0x0000_0800
+    SZ_4K,   // 0x0000_1000
+    SZ_8K,   // 0x0000_2000
+    SZ_16K,  // 0x0000_4000
+    SZ_32K,  // 0x0000_8000
+    SZ_64K,  // 0x0001_0000
+    SZ_128K, // 0x0002_0000
+    SZ_256K, // 0x0004_0000
+    SZ_512K, // 0x0008_0000
+    SZ_1M,   // 0x0010_0000
+    SZ_2M,   // 0x0020_0000
+    SZ_4M,   // 0x0040_0000
+    SZ_8M,   // 0x0080_0000
+    SZ_16M,  // 0x0100_0000
+    SZ_32M,  // 0x0200_0000
+    SZ_64M,  // 0x0400_0000
+    SZ_128M, // 0x0800_0000
+    SZ_256M, // 0x1000_0000
+    SZ_512M, // 0x2000_0000
+    SZ_1G,   // 0x4000_0000
+    SZ_2G,   // 0x8000_0000
+}
-- 
2.53.0

[PATCH v2 2/3] rust: ptr: add Alignment::from_u64() for DeviceSize constants

[John Hubbard]

Alignment::new() takes a const usize, which means callers that work
with DeviceSize constants still need to import the usize SZ_*
variants. Add from_u64() so callers can write
Alignment::from_u64(u64::SZ_128K) and stay entirely in the
DeviceSize world.

Both asserts evaluate at compile time in const context, so there is
no runtime cost.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 rust/kernel/ptr.rs | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
index 5b6a382637fe..b06f6b404a46 100644
--- a/rust/kernel/ptr.rs
+++ b/rust/kernel/ptr.rs
@@ -76,6 +76,41 @@ pub const fn new_checked(align: usize) -> Option<Self> {
         }
     }
 
+    /// Creates an [`Alignment`] from a [`u64`] value.
+    ///
+    /// This is useful when the alignment comes from a [`DeviceSize`] constant
+    /// rather than a [`usize`] literal.
+    ///
+    /// A build error is triggered if `align` is not a power of two, or if it
+    /// exceeds [`usize::MAX`].
+    ///
+    /// [`DeviceSize`]: crate::sizes::DeviceSize
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::ptr::Alignment;
+    /// use kernel::sizes::DeviceSize;
+    ///
+    /// let v = Alignment::from_u64(u64::SZ_128K);
+    /// assert_eq!(v.as_usize(), 0x0002_0000);
+    /// ```
+    #[inline(always)]
+    pub const fn from_u64(align: u64) -> Self {
+        assert!(
+            align.is_power_of_two(),
+            "Provided alignment is not a power of two."
+        );
+        assert!(
+            align <= usize::MAX as u64,
+            "Provided alignment exceeds usize::MAX."
+        );
+
+        // INVARIANT: `align` is a power of two.
+        // SAFETY: `align` is a power of two, fits in usize, and thus non-zero.
+        Self(unsafe { NonZero::new_unchecked(align as usize) })
+    }
+
     /// Returns the alignment of `T`.
     ///
     /// This is equivalent to [`align_of`], but with the return value provided as an [`Alignment`].
-- 
2.53.0

[PATCH v2 3/3] gpu: nova-core: use DeviceSize trait for u64 size constants

[John Hubbard]

Replace manual usize-to-u64 conversions of SZ_* constants with the
DeviceSize trait's associated constants on u64. With the DeviceSize
trait in scope, u64::SZ_1M replaces usize_as_u64(SZ_1M) and similar.

Also switch Alignment::new::<SZ_*>() calls to Alignment::from_u64(),
which accepts u64 DeviceSize constants directly.

This removes several now-unused imports: usize_as_u64 and the SZ_*
type-level constants.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 drivers/gpu/nova-core/fb.rs     | 31 ++++++++++++++-----------------
 drivers/gpu/nova-core/gsp/fw.rs | 18 +++++++-----------
 drivers/gpu/nova-core/regs.rs   |  6 +++---
 3 files changed, 24 insertions(+), 31 deletions(-)

diff --git a/drivers/gpu/nova-core/fb.rs b/drivers/gpu/nova-core/fb.rs
index 6536d0035cb1..aa80fffc4a30 100644
--- a/drivers/gpu/nova-core/fb.rs
+++ b/drivers/gpu/nova-core/fb.rs
@@ -13,7 +13,7 @@
         Alignable,
         Alignment, //
     },
-    sizes::*,
+    sizes::DeviceSize,
     sync::aref::ARef, //
 };
 
@@ -23,10 +23,7 @@
     firmware::gsp::GspFirmware,
     gpu::Chipset,
     gsp,
-    num::{
-        usize_as_u64,
-        FromSafeCast, //
-    },
+    num::FromSafeCast,
     regs,
 };
 
@@ -126,8 +123,8 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         if f.alternate() {
             let size = self.len();
 
-            if size < usize_as_u64(SZ_1M) {
-                let size_kib = size / usize_as_u64(SZ_1K);
+            if size < u64::SZ_1M {
+                let size_kib = size / u64::SZ_1K;
                 f.write_fmt(fmt!(
                     "{:#x}..{:#x} ({} KiB)",
                     self.0.start,
@@ -135,7 +132,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                     size_kib
                 ))
             } else {
-                let size_mib = size / usize_as_u64(SZ_1M);
+                let size_mib = size / u64::SZ_1M;
                 f.write_fmt(fmt!(
                     "{:#x}..{:#x} ({} MiB)",
                     self.0.start,
@@ -185,14 +182,14 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
 
         let vga_workspace = {
             let vga_base = {
-                const NV_PRAMIN_SIZE: u64 = usize_as_u64(SZ_1M);
+                const NV_PRAMIN_SIZE: u64 = u64::SZ_1M;
                 let base = fb.end - NV_PRAMIN_SIZE;
 
                 if hal.supports_display(bar) {
                     match regs::NV_PDISP_VGA_WORKSPACE_BASE::read(bar).vga_workspace_addr() {
                         Some(addr) => {
                             if addr < base {
-                                const VBIOS_WORKSPACE_SIZE: u64 = usize_as_u64(SZ_128K);
+                                const VBIOS_WORKSPACE_SIZE: u64 = u64::SZ_128K;
 
                                 // Point workspace address to end of framebuffer.
                                 fb.end - VBIOS_WORKSPACE_SIZE
@@ -211,15 +208,15 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
         };
 
         let frts = {
-            const FRTS_DOWN_ALIGN: Alignment = Alignment::new::<SZ_128K>();
-            const FRTS_SIZE: u64 = usize_as_u64(SZ_1M);
+            const FRTS_DOWN_ALIGN: Alignment = Alignment::from_u64(u64::SZ_128K);
+            const FRTS_SIZE: u64 = u64::SZ_1M;
             let frts_base = vga_workspace.start.align_down(FRTS_DOWN_ALIGN) - FRTS_SIZE;
 
             FbRange(frts_base..frts_base + FRTS_SIZE)
         };
 
         let boot = {
-            const BOOTLOADER_DOWN_ALIGN: Alignment = Alignment::new::<SZ_4K>();
+            const BOOTLOADER_DOWN_ALIGN: Alignment = Alignment::from_u64(u64::SZ_4K);
             let bootloader_size = u64::from_safe_cast(gsp_fw.bootloader.ucode.size());
             let bootloader_base = (frts.start - bootloader_size).align_down(BOOTLOADER_DOWN_ALIGN);
 
@@ -227,7 +224,7 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
         };
 
         let elf = {
-            const ELF_DOWN_ALIGN: Alignment = Alignment::new::<SZ_64K>();
+            const ELF_DOWN_ALIGN: Alignment = Alignment::from_u64(u64::SZ_64K);
             let elf_size = u64::from_safe_cast(gsp_fw.size);
             let elf_addr = (boot.start - elf_size).align_down(ELF_DOWN_ALIGN);
 
@@ -235,7 +232,7 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
         };
 
         let wpr2_heap = {
-            const WPR2_HEAP_DOWN_ALIGN: Alignment = Alignment::new::<SZ_1M>();
+            const WPR2_HEAP_DOWN_ALIGN: Alignment = Alignment::from_u64(u64::SZ_1M);
             let wpr2_heap_size =
                 gsp::LibosParams::from_chipset(chipset).wpr_heap_size(chipset, fb.end);
             let wpr2_heap_addr = (elf.start - wpr2_heap_size).align_down(WPR2_HEAP_DOWN_ALIGN);
@@ -244,7 +241,7 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
         };
 
         let wpr2 = {
-            const WPR2_DOWN_ALIGN: Alignment = Alignment::new::<SZ_1M>();
+            const WPR2_DOWN_ALIGN: Alignment = Alignment::from_u64(u64::SZ_1M);
             let wpr2_addr = (wpr2_heap.start - u64::from_safe_cast(size_of::<gsp::GspFwWprMeta>()))
                 .align_down(WPR2_DOWN_ALIGN);
 
@@ -252,7 +249,7 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
         };
 
         let heap = {
-            const HEAP_SIZE: u64 = usize_as_u64(SZ_1M);
+            const HEAP_SIZE: u64 = u64::SZ_1M;
 
             FbRange(wpr2.start - HEAP_SIZE..wpr2.start)
         };
diff --git a/drivers/gpu/nova-core/gsp/fw.rs b/drivers/gpu/nova-core/gsp/fw.rs
index 25fca1f6db2c..454a4a92105a 100644
--- a/drivers/gpu/nova-core/gsp/fw.rs
+++ b/drivers/gpu/nova-core/gsp/fw.rs
@@ -15,10 +15,7 @@
         Alignable,
         Alignment, //
     },
-    sizes::{
-        SZ_128K,
-        SZ_1M, //
-    },
+    sizes::DeviceSize,
     transmute::{
         AsBytes,
         FromBytes, //
@@ -69,7 +66,7 @@ fn client_alloc_size() -> u64 {
     /// Returns the amount of memory to reserve for management purposes for a framebuffer of size
     /// `fb_size`.
     fn management_overhead(fb_size: u64) -> u64 {
-        let fb_size_gb = fb_size.div_ceil(u64::from_safe_cast(kernel::sizes::SZ_1G));
+        let fb_size_gb = fb_size.div_ceil(u64::SZ_1G);
 
         u64::from(bindings::GSP_FW_HEAP_PARAM_SIZE_PER_GB_FB)
             .saturating_mul(fb_size_gb)
@@ -91,9 +88,8 @@ impl LibosParams {
     const LIBOS2: LibosParams = LibosParams {
         carveout_size: num::u32_as_u64(bindings::GSP_FW_HEAP_PARAM_OS_SIZE_LIBOS2),
         allowed_heap_size: num::u32_as_u64(bindings::GSP_FW_HEAP_SIZE_OVERRIDE_LIBOS2_MIN_MB)
-            * num::usize_as_u64(SZ_1M)
-            ..num::u32_as_u64(bindings::GSP_FW_HEAP_SIZE_OVERRIDE_LIBOS2_MAX_MB)
-                * num::usize_as_u64(SZ_1M),
+            * u64::SZ_1M
+            ..num::u32_as_u64(bindings::GSP_FW_HEAP_SIZE_OVERRIDE_LIBOS2_MAX_MB) * u64::SZ_1M,
     };
 
     /// Version 3 of the GSP LIBOS (GA102+)
@@ -101,9 +97,9 @@ impl LibosParams {
         carveout_size: num::u32_as_u64(bindings::GSP_FW_HEAP_PARAM_OS_SIZE_LIBOS3_BAREMETAL),
         allowed_heap_size: num::u32_as_u64(
             bindings::GSP_FW_HEAP_SIZE_OVERRIDE_LIBOS3_BAREMETAL_MIN_MB,
-        ) * num::usize_as_u64(SZ_1M)
+        ) * u64::SZ_1M
             ..num::u32_as_u64(bindings::GSP_FW_HEAP_SIZE_OVERRIDE_LIBOS3_BAREMETAL_MAX_MB)
-                * num::usize_as_u64(SZ_1M),
+                * u64::SZ_1M,
     };
 
     /// Returns the libos parameters corresponding to `chipset`.
@@ -181,7 +177,7 @@ pub(crate) fn new(gsp_firmware: &GspFirmware, fb_layout: &FbLayout) -> Self {
             gspFwWprEnd: fb_layout
                 .vga_workspace
                 .start
-                .align_down(Alignment::new::<SZ_128K>()),
+                .align_down(Alignment::from_u64(u64::SZ_128K)),
             gspFwHeapVfPartitionCount: fb_layout.vf_partition_count,
             fbSize: fb_layout.fb.end - fb_layout.fb.start,
             vgaWorkspaceOffset: fb_layout.vga_workspace.start,
diff --git a/drivers/gpu/nova-core/regs.rs b/drivers/gpu/nova-core/regs.rs
index 53f412f0ca32..8514b4bd7279 100644
--- a/drivers/gpu/nova-core/regs.rs
+++ b/drivers/gpu/nova-core/regs.rs
@@ -9,6 +9,7 @@
 
 use kernel::{
     prelude::*,
+    sizes::DeviceSize,
     time, //
 };
 
@@ -32,7 +33,6 @@
         Architecture,
         Chipset, //
     },
-    num::FromSafeCast,
 };
 
 // PMC
@@ -129,7 +129,7 @@ impl NV_PFB_PRI_MMU_LOCAL_MEMORY_RANGE {
     /// Returns the usable framebuffer size, in bytes.
     pub(crate) fn usable_fb_size(self) -> u64 {
         let size = (u64::from(self.lower_mag()) << u64::from(self.lower_scale()))
-            * u64::from_safe_cast(kernel::sizes::SZ_1M);
+            * u64::SZ_1M;
 
         if self.ecc_mode_enabled() {
             // Remove the amount of memory reserved for ECC (one per 16 units).
@@ -218,7 +218,7 @@ pub(crate) fn completed(self) -> bool {
 impl NV_USABLE_FB_SIZE_IN_MB {
     /// Returns the usable framebuffer size, in bytes.
     pub(crate) fn usable_fb_size(self) -> u64 {
-        u64::from(self.value()) * u64::from_safe_cast(kernel::sizes::SZ_1M)
+        u64::from(self.value()) * u64::SZ_1M
     }
 }
 
-- 
2.53.0

Re: [PATCH v2 2/3] rust: ptr: add Alignment::from_u64() for DeviceSize constants

[Miguel Ojeda]

On Thu, Mar 12, 2026 at 4:15 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> +    /// A build error is triggered if `align` is not a power of two, or if it
> +    /// exceeds [`usize::MAX`].

I think this wording comes from the `new` constructor, but there we
use a const assert, not a runtime one.

So as you mention in the commit message,  it is true that if you call
it in a const context you will get it at compile-time error, but the
wording here in the docs seems to imply otherwise.

Cheers,
Miguel

Re: [PATCH v2 2/3] rust: ptr: add Alignment::from_u64() for DeviceSize constants

[John Hubbard]

On 3/11/26 11:16 PM, Miguel Ojeda wrote:
> On Thu, Mar 12, 2026 at 4:15 AM John Hubbard <jhubbard@nvidia.com> wrote:
>>
>> +    /// A build error is triggered if `align` is not a power of two, or if it
>> +    /// exceeds [`usize::MAX`].
> 
> I think this wording comes from the `new` constructor, but there we

Yes it does--guilty as charged, haha. :)

> use a const assert, not a runtime one.
> 
> So as you mention in the commit message,  it is true that if you call
> it in a const context you will get it at compile-time error, but the
> wording here in the docs seems to imply otherwise.
> 

Good catch, thanks.


thanks,
-- 
John Hubbard

Re: [PATCH v2 3/3] gpu: nova-core: use DeviceSize trait for u64 size constants

[Alexandre Courbot]

On Thu Mar 12, 2026 at 12:15 PM JST, John Hubbard wrote:
<snip>
> @@ -211,15 +208,15 @@ pub(crate) fn new(chipset: Chipset, bar: &Bar0, gsp_fw: &GspFirmware) -> Result<
>          };
>  
>          let frts = {
> -            const FRTS_DOWN_ALIGN: Alignment = Alignment::new::<SZ_128K>();
> -            const FRTS_SIZE: u64 = usize_as_u64(SZ_1M);
> +            const FRTS_DOWN_ALIGN: Alignment = Alignment::from_u64(u64::SZ_128K);

Replacing the call to `new` by `from_u64` seems to be unnecessary -
`Alignment` works with `usize`, which `SZ_128K` already is. By using
`from_u64` we convert `SZ_128K` into a `u64` in the `define_sizes`
macro, only to convert it back into a `usize`.

All the uses of `from_u64` follow this pattern, so I'd say we can just
drop patch 2.

> +            const FRTS_SIZE: u64 = u64::SZ_1M;

This, on the other hand, is really useful and much better than using
`usize_as_u64`. Actually I'm wondering whether we should not have
`DeviceSize` implemented for `usize` as well to provide better scope and
consistency (in which case it should just be renamed `Size`?) and sunset
the original definitions.

Re: [PATCH v2 2/3] rust: ptr: add Alignment::from_u64() for DeviceSize constants

[Gary Guo]

On Thu Mar 12, 2026 at 3:15 AM GMT, John Hubbard wrote:
> Alignment::new() takes a const usize, which means callers that work
> with DeviceSize constants still need to import the usize SZ_*
> variants. Add from_u64() so callers can write
> Alignment::from_u64(u64::SZ_128K) and stay entirely in the
> DeviceSize world.
>
> Both asserts evaluate at compile time in const context, so there is
> no runtime cost.
>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  rust/kernel/ptr.rs | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
>
> diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
> index 5b6a382637fe..b06f6b404a46 100644
> --- a/rust/kernel/ptr.rs
> +++ b/rust/kernel/ptr.rs
> @@ -76,6 +76,41 @@ pub const fn new_checked(align: usize) -> Option<Self> {
>          }
>      }
>  
> +    /// Creates an [`Alignment`] from a [`u64`] value.
> +    ///
> +    /// This is useful when the alignment comes from a [`DeviceSize`] constant
> +    /// rather than a [`usize`] literal.
> +    ///
> +    /// A build error is triggered if `align` is not a power of two, or if it
> +    /// exceeds [`usize::MAX`].
> +    ///
> +    /// [`DeviceSize`]: crate::sizes::DeviceSize
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::ptr::Alignment;
> +    /// use kernel::sizes::DeviceSize;
> +    ///
> +    /// let v = Alignment::from_u64(u64::SZ_128K);
> +    /// assert_eq!(v.as_usize(), 0x0002_0000);
> +    /// ```
> +    #[inline(always)]
> +    pub const fn from_u64(align: u64) -> Self {
> +        assert!(
> +            align.is_power_of_two(),
> +            "Provided alignment is not a power of two."
> +        );

This should be build_assert! to match the doc.

Best,
Gary

> +        assert!(
> +            align <= usize::MAX as u64,
> +            "Provided alignment exceeds usize::MAX."
> +        );
> +
> +        // INVARIANT: `align` is a power of two.
> +        // SAFETY: `align` is a power of two, fits in usize, and thus non-zero.
> +        Self(unsafe { NonZero::new_unchecked(align as usize) })
> +    }
> +
>      /// Returns the alignment of `T`.
>      ///
>      /// This is equivalent to [`align_of`], but with the return value provided as an [`Alignment`].

Re: [PATCH v2 1/3] rust: sizes: add DeviceSize trait for device address space constants

[Alexandre Courbot]

General note: `make rustfmt` reformats code from the series.

On Thu Mar 12, 2026 at 12:15 PM JST, John Hubbard wrote:
> The SZ_* constants are usize, matching the CPU pointer width. But
> device address spaces have their own widths (32-bit MMIO windows,
> 64-bit GPU framebuffers, etc.), so drivers end up with repeated
> usize-to-u64 or usize-to-u32 conversion calls like
> usize_as_u64(SZ_1M). This adds boilerplate with no safety benefit.

`usize_as_u64` is Nova-only at the moment, so it might not be
well understood in this context.

>
> Add a DeviceSize trait with associated SZ_* constants, implemented for
> u32 and u64. With the trait in scope, callers write u64::SZ_1M or
> u32::SZ_4K to get the constant in their device's native width. All
> SZ_* values fit in a u32, so both implementations are lossless. The
> u32 impl has a const assert to catch any future constant that would
> overflow; the u64 cast from usize is inherently lossless.
>
> Replace the hand-written constant list with a define_sizes! macro that
> generates the usize constants, the trait, and both trait impls from a
> single list of names. Adding a new size or a new target type requires
> changing only one place.
>
> The trait also enables future generic APIs that accept T: DeviceSize,
> so shared infrastructure can work with whatever address width the
> driver declares.
>
> Suggested-by: Danilo Krummrich <dakr@kernel.org>
> Link: https://lore.kernel.org/all/DGB9G697GSWO.3VBFGU5MKFPMR@kernel.org/
> Link: https://lore.kernel.org/all/DGHI8WRKBQS9.38910L6FIIZTE@kernel.org/
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  rust/kernel/sizes.rs | 132 ++++++++++++++++++++++++++++---------------
>  1 file changed, 88 insertions(+), 44 deletions(-)
>
> diff --git a/rust/kernel/sizes.rs b/rust/kernel/sizes.rs
> index 661e680d9330..6b11ec6d97b3 100644
> --- a/rust/kernel/sizes.rs
> +++ b/rust/kernel/sizes.rs
> @@ -3,48 +3,92 @@
>  //! Commonly used sizes.
>  //!
>  //! C headers: [`include/linux/sizes.h`](srctree/include/linux/sizes.h).
> +//!
> +//! The top-level `SZ_*` constants are [`usize`]-typed, for use in kernel page
> +//! arithmetic and similar CPU-side work.
> +//!
> +//! The [`DeviceSize`] trait provides the same constants as associated constants
> +//! on [`u32`] and [`u64`], for use in device address spaces where the address
> +//! width depends on the hardware. Device drivers frequently need these constants
> +//! as [`u64`] (or [`u32`]) rather than [`usize`], because device address spaces
> +//! are sized independently of the CPU pointer width.
> +//!

There should be a `# Examples` here.

> +//! ```
> +//! use kernel::sizes::{DeviceSize, SZ_1M};
> +//!
> +//! // usize constant (CPU-side)
> +//! let pages: usize = SZ_1M / kernel::page::PAGE_SIZE;

Maybe `num_pages_in_1m` to be more precise.

> +//!
> +//! // Device-side constant via the trait
> +//! let heap_size: u64 = 14 * u64::SZ_1M;
> +//! let small: u32 = u32::SZ_4K;
> +//! ```
> +
> +macro_rules! define_sizes {
> +    ($($name:ident),* $(,)?) => {
> +        // `usize` constants, from the C `SZ_*` defines in `include/linux/sizes.h`.
> +        $(
> +            #[doc = concat!("`", stringify!($name), "` as a [`usize`].")]

All the information in this doccomment (the value and the type) is
already in the declaration below, which appears both in the
rust-analyzer and the HTML doc.

The original doccomments OTOH were useful and they showed the
hexadecimal value, allowing the reader to understand at a peek which bit
was affected. Right now we don't have this information as the value is
displayed in decimal in the documentation. So I think we should just
keep the original doccomments without any extra ornament.

> +            pub const $name: usize = bindings::$name as usize;
> +        )*
> +
> +        /// Size constants for device address spaces.
> +        ///
> +        /// Implemented for [`u32`] and [`u64`] so drivers can choose the width
> +        /// that matches their hardware. All `SZ_*` values fit in a [`u32`], so
> +        /// both implementations are lossless.
> +        ///
> +        /// ```
> +        /// use kernel::sizes::DeviceSize;
> +        ///
> +        /// let gpu_heap: u64 = 14 * u64::SZ_1M;
> +        /// let mmio_window: u32 = u32::SZ_16M;
> +        /// ```
> +        pub trait DeviceSize {
> +            $(
> +                #[doc = concat!("`", stringify!($name), "` for this type.")]
> +                const $name: Self;
> +            )*
> +        }
> +
> +        impl DeviceSize for u32 {
> +            $(
> +                const $name: Self = {
> +                    assert!(self::$name <= u32::MAX as usize);
> +                    self::$name as u32
> +                };
> +            )*
> +        }
> +
> +        impl DeviceSize for u64 {
> +            $(
> +                const $name: Self = self::$name as u64;

I know 64-bit is that largest architecture so far, but in order to
protect for an eventual future where larger sizes exist (and for
consistency), let's have the defensive `assert` here as well.

> +            )*
> +        }
> +    };
> +}
>  
> -/// 0x00000400
> -pub const SZ_1K: usize = bindings::SZ_1K as usize;
> -/// 0x00000800
> -pub const SZ_2K: usize = bindings::SZ_2K as usize;
> -/// 0x00001000
> -pub const SZ_4K: usize = bindings::SZ_4K as usize;
> -/// 0x00002000
> -pub const SZ_8K: usize = bindings::SZ_8K as usize;
> -/// 0x00004000
> -pub const SZ_16K: usize = bindings::SZ_16K as usize;
> -/// 0x00008000
> -pub const SZ_32K: usize = bindings::SZ_32K as usize;
> -/// 0x00010000
> -pub const SZ_64K: usize = bindings::SZ_64K as usize;
> -/// 0x00020000
> -pub const SZ_128K: usize = bindings::SZ_128K as usize;
> -/// 0x00040000
> -pub const SZ_256K: usize = bindings::SZ_256K as usize;
> -/// 0x00080000
> -pub const SZ_512K: usize = bindings::SZ_512K as usize;
> -/// 0x00100000
> -pub const SZ_1M: usize = bindings::SZ_1M as usize;
> -/// 0x00200000
> -pub const SZ_2M: usize = bindings::SZ_2M as usize;
> -/// 0x00400000
> -pub const SZ_4M: usize = bindings::SZ_4M as usize;
> -/// 0x00800000
> -pub const SZ_8M: usize = bindings::SZ_8M as usize;
> -/// 0x01000000
> -pub const SZ_16M: usize = bindings::SZ_16M as usize;
> -/// 0x02000000
> -pub const SZ_32M: usize = bindings::SZ_32M as usize;
> -/// 0x04000000
> -pub const SZ_64M: usize = bindings::SZ_64M as usize;
> -/// 0x08000000
> -pub const SZ_128M: usize = bindings::SZ_128M as usize;
> -/// 0x10000000
> -pub const SZ_256M: usize = bindings::SZ_256M as usize;
> -/// 0x20000000
> -pub const SZ_512M: usize = bindings::SZ_512M as usize;
> -/// 0x40000000
> -pub const SZ_1G: usize = bindings::SZ_1G as usize;
> -/// 0x80000000
> -pub const SZ_2G: usize = bindings::SZ_2G as usize;
> +define_sizes! {
> +    SZ_1K,   // 0x0000_0400
> +    SZ_2K,   // 0x0000_0800
> +    SZ_4K,   // 0x0000_1000
> +    SZ_8K,   // 0x0000_2000
> +    SZ_16K,  // 0x0000_4000
> +    SZ_32K,  // 0x0000_8000
> +    SZ_64K,  // 0x0001_0000
> +    SZ_128K, // 0x0002_0000
> +    SZ_256K, // 0x0004_0000
> +    SZ_512K, // 0x0008_0000
> +    SZ_1M,   // 0x0010_0000
> +    SZ_2M,   // 0x0020_0000
> +    SZ_4M,   // 0x0040_0000
> +    SZ_8M,   // 0x0080_0000
> +    SZ_16M,  // 0x0100_0000
> +    SZ_32M,  // 0x0200_0000
> +    SZ_64M,  // 0x0400_0000
> +    SZ_128M, // 0x0800_0000
> +    SZ_256M, // 0x1000_0000
> +    SZ_512M, // 0x2000_0000
> +    SZ_1G,   // 0x4000_0000
> +    SZ_2G,   // 0x8000_0000
> +}

This macro looks like it has its arguments backwards. `SZ_*` is a fixed
list of values from the C headers that is stable and never changes.
OTOH, the types on which we want to implement `DeviceSize` may vary (as
I suggested in patch 3 to also support `usize`). So I think the macro
invocation should be like:

  define_sizes!(usize, u32, u64);

This would make it easier to integrate the doccomment fix I suggested
above. The only drawback is that generating the module-level usize
consts would need an internal macro rule with the SZ_* list, but that
doesn't add much complexity and makes more sense that way IMHO - you
would just be moving the list from the macro call site to the macro
internals.