Nouveau - Nov 2025 - [PATCH 2/5] gpu: nova-core: vbios: use FromBytes for PmuLookupTable header

Use `from_bytes_copy_prefix` to create the `PmuLookupTable` header
instead of building it ourselves from the bytes stream. This lets us
remove a few `as` conversions and array accesses.

Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
---
 drivers/gpu/nova-core/vbios.rs | 44 ++++++++++++++++++++++--------------------
 1 file changed, 23 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/nova-core/vbios.rs b/drivers/gpu/nova-core/vbios.rs
index 46da51b9f6b0..b6c20627a5e3 100644
--- a/drivers/gpu/nova-core/vbios.rs
+++ b/drivers/gpu/nova-core/vbios.rs
@@ -10,6 +10,7 @@
 use kernel::error::Result;
 use kernel::prelude::*;
 use kernel::ptr::{Alignable, Alignment};
+use kernel::transmute::FromBytes;
 use kernel::types::ARef;
 
 /// The offset of the VBIOS ROM in the BAR0 space.
@@ -866,29 +867,36 @@ fn new(data: &[u8]) -> Result<Self> {
     }
 }
 
+#[repr(C)]
+struct PmuLookupTableHeader {
+    version: u8,
+    header_len: u8,
+    entry_len: u8,
+    entry_count: u8,
+}
+
+// SAFETY: all bit patterns are valid for `PmuLookupTableHeader`.
+unsafe impl FromBytes for PmuLookupTableHeader {}
+
 /// The [`PmuLookupTableEntry`] structure is used to find the
[`PmuLookupTableEntry`] for a given
 /// application ID.
 ///
 /// The table of entries is pointed to by the falcon data pointer in the BIT
table, and is used to
 /// locate the Falcon Ucode.
-#[expect(dead_code)]
 struct PmuLookupTable {
-    version: u8,
-    header_len: u8,
-    entry_len: u8,
-    entry_count: u8,
+    header: PmuLookupTableHeader,
     table_data: KVec<u8>,
 }
 
 impl PmuLookupTable {
     fn new(dev: &device::Device, data: &[u8]) -> Result<Self>
{
-        if data.len() < 4 {
-            return Err(EINVAL);
-        }
+        let header = PmuLookupTableHeader::from_bytes_copy_prefix(data)
+            .ok_or(EINVAL)?
+            .0;
 
-        let header_len = usize::from(data[1]);
-        let entry_len = usize::from(data[2]);
-        let entry_count = usize::from(data[3]);
+        let header_len = usize::from(header.header_len);
+        let entry_len = usize::from(header.entry_len);
+        let entry_count = usize::from(header.entry_count);
 
         let required_bytes = header_len + (entry_count * entry_len);
 
@@ -909,27 +917,21 @@ fn new(dev: &device::Device, data: &[u8]) ->
Result<Self> {
             dev_dbg!(dev, "PMU entry: {:02x?}\n",
&data[i..][..entry_len]);
         }
 
-        Ok(PmuLookupTable {
-            version: data[0],
-            header_len: header_len as u8,
-            entry_len: entry_len as u8,
-            entry_count: entry_count as u8,
-            table_data,
-        })
+        Ok(PmuLookupTable { header, table_data })
     }
 
     fn lookup_index(&self, idx: u8) -> Result<PmuLookupTableEntry>
{
-        if idx >= self.entry_count {
+        if idx >= self.header.entry_count {
             return Err(EINVAL);
         }
 
-        let index = (usize::from(idx)) * usize::from(self.entry_len);
+        let index = (usize::from(idx)) * usize::from(self.header.entry_len);
         PmuLookupTableEntry::new(&self.table_data[index..])
     }
 
     // find entry by type value
     fn find_entry_by_type(&self, entry_type: u8) ->
Result<PmuLookupTableEntry> {
-        for i in 0..self.entry_count {
+        for i in 0..self.header.entry_count {
             let entry = self.lookup_index(i)?;
             if entry.application_id == entry_type {
                 return Ok(entry);

-- 
2.51.0

Hi Alex,
Nice improvement, a nit:

On Wed, Oct 29, 2025 at 12:07:37AM +0900, Alexandre Courbot wrote:
[..]
>  impl PmuLookupTable {
>      fn new(dev: &device::Device, data: &[u8]) ->
Result<Self> {
> -        if data.len() < 4 {
> -            return Err(EINVAL);
> -        }
> +        let header = PmuLookupTableHeader::from_bytes_copy_prefix(data)
> +            .ok_or(EINVAL)?
> +            .0;
Can we change to the following, it is easier to read than using `.0` IMO.

 let (header, _rest) = PmuLookupTableHeader::from_bytes_copy_prefix(data)
      .ok_or(EINVAL)?;

(and similarly in the other patches).

thanks,

 - Joel