linux-kernel - [PATCH v2 7/7] gpu: nova-core: justify remaining uses of `as`

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Message-Id: <20251027-nova-as-v2-7-a26bd1d067a4@nvidia.com>
Date: Mon, 27 Oct 2025 21:54:47 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Alice Ryhl <aliceryhl@...gle.com>, David Airlie <airlied@...il.com>, 
 Simona Vetter <simona@...ll.ch>, 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>, 
 Trevor Gross <tmgross@...ch.edu>
Cc: John Hubbard <jhubbard@...dia.com>, 
 Alistair Popple <apopple@...dia.com>, 
 Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi <ttabi@...dia.com>, 
 Edwin Peer <epeer@...dia.com>, nouveau@...ts.freedesktop.org, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 rust-for-linux@...r.kernel.org, Danilo Krummrich <dakr@...nel.org>, 
 Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v2 7/7] gpu: nova-core: justify remaining uses of `as`

There are a few remaining cases where we *do* want to use `as`,
because we specifically want to strip the data that does not fit into
the destination type. Comment these uses to clear confusion about the
intent.

Acked-by: Danilo Krummrich <dakr@...nel.org>
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
 drivers/gpu/nova-core/falcon.rs         | 4 ++++
 drivers/gpu/nova-core/fb/hal/ga100.rs   | 4 ++++
 drivers/gpu/nova-core/firmware/fwsec.rs | 2 ++
 3 files changed, 10 insertions(+)

diff --git a/drivers/gpu/nova-core/falcon.rs b/drivers/gpu/nova-core/falcon.rs
index a44df1ac8873..7f6c7091c9c3 100644
--- a/drivers/gpu/nova-core/falcon.rs
+++ b/drivers/gpu/nova-core/falcon.rs
@@ -479,9 +479,13 @@ fn dma_wr<F: FalconFirmware<Target = E>>(
         // Set up the base source DMA address.
 
         regs::NV_PFALCON_FALCON_DMATRFBASE::default()
+            // CAST: `as u32` is used on purpose since we do want to strip the upper bits, which
+            // will be written to `NV_PFALCON_FALCON_DMATRFBASE1`.
             .set_base((dma_start >> 8) as u32)
             .write(bar, &E::ID);
         regs::NV_PFALCON_FALCON_DMATRFBASE1::default()
+            // CAST: `as u16` is used on purpose since the remaining bits are guaranteed to fit
+            // within a `u16`.
             .set_base((dma_start >> 40) as u16)
             .write(bar, &E::ID);
 
diff --git a/drivers/gpu/nova-core/fb/hal/ga100.rs b/drivers/gpu/nova-core/fb/hal/ga100.rs
index 871c42bf033a..b9389fa382e5 100644
--- a/drivers/gpu/nova-core/fb/hal/ga100.rs
+++ b/drivers/gpu/nova-core/fb/hal/ga100.rs
@@ -18,9 +18,13 @@ pub(super) fn read_sysmem_flush_page_ga100(bar: &Bar0) -> u64 {
 
 pub(super) fn write_sysmem_flush_page_ga100(bar: &Bar0, addr: u64) {
     regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR_HI::default()
+        // CAST: `as u32` is used on purpose since the remaining bits are guaranteed to fit within
+        // a `u32`.
         .set_adr_63_40((addr >> FLUSH_SYSMEM_ADDR_SHIFT_HI) as u32)
         .write(bar);
     regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR::default()
+        // CAST: `as u32` is used on purpose since we want to strip the upper bits that have been
+        // written to `NV_PFB_NISO_FLUSH_SYSMEM_ADDR_HI`.
         .set_adr_39_08((addr >> FLUSH_SYSMEM_ADDR_SHIFT) as u32)
         .write(bar);
 }
diff --git a/drivers/gpu/nova-core/firmware/fwsec.rs b/drivers/gpu/nova-core/firmware/fwsec.rs
index ed82e74ccdc9..ecef4cc78942 100644
--- a/drivers/gpu/nova-core/firmware/fwsec.rs
+++ b/drivers/gpu/nova-core/firmware/fwsec.rs
@@ -161,6 +161,7 @@ unsafe fn transmute<'a, 'b, T: Sized + FromBytes>(
     if offset + size_of::<T>() > fw.size() {
         return Err(EINVAL);
     }
+    // CAST: `usize` has the same size as pointers.
     if (fw.start_ptr() as usize + offset) % align_of::<T>() != 0 {
         return Err(EINVAL);
     }
@@ -184,6 +185,7 @@ unsafe fn transmute_mut<'a, 'b, T: Sized + FromBytes>(
     if offset + size_of::<T>() > fw.size() {
         return Err(EINVAL);
     }
+    // CAST: `usize` has the same size as pointers.
     if (fw.start_ptr_mut() as usize + offset) % align_of::<T>() != 0 {
         return Err(EINVAL);
     }

-- 
2.51.0