Subject: Re: linux-next: build failure after merge of the rust tree

From: "Alexandre Courbot" <[email-masked]>
Date: Mon, 17 Nov 2025 20:19:38 +0900

Hi Stephen,
On Mon Nov 17, 2025 at 5:04 PM JST, Stephen Rothwell wrote:
> Hi all,
>
> After merging the rust tree, today's linux-next build (x86_64
> allmodconfig) failed like this:
>
> error[E0277]: `Chipset` doesn't implement `core::fmt::Display`
>    --> drivers/gpu/nova-core/gpu.rs:233:13
>     |
> 233 |             self.chipset,
>     |             ^^^^^^^^^^^^ `Chipset` cannot be formatted with the default formatter
>     |
>     = help: the trait `core::fmt::Display` is not implemented for `Chipset`
>     = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
>     = note: this error originates in the macro `$crate::format_args` which comes from the expansion of the macro `write` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> error[E0277]: `Revision` doesn't implement `core::fmt::Display`
>    --> drivers/gpu/nova-core/gpu.rs:235:13
>     |
> 235 |             self.revision
>     |             ^^^^^^^^^^^^^ `Revision` cannot be formatted with the default formatter
>     |
>     = help: the trait `core::fmt::Display` is not implemented for `Revision`
>     = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
>     = note: this error originates in the macro `$crate::format_args` which comes from the expansion of the macro `write` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> error[E0277]: the trait bound `MsgFunction: kernel::fmt::Display` is not satisfied
>    --> drivers/gpu/nova-core/gsp/cmdq.rs:532:9
>     |
> 532 | /         dev_dbg!(
> 533 | |             &self.dev,
> 534 | |             "GSP RPC: send: seq# {}, function={}, length=0x{:x}\n",
>     | |                                               -- required by a bound introduced by this call
> 535 | |             self.seq,
> 536 | |             M::FUNCTION,
> 537 | |             dst.header.length(),
> 538 | |         );
>     | |_________^ the trait `kernel::fmt::Display` is not implemented for `MsgFunction`
>     |
>     = help: the following other types implement trait `kernel::fmt::Display`:
>               &T
>               Arc<T>
>               Arguments<'_>
>               BStr
>               Box<T, A>
>               CStr
>               Chipset
>               Class
>             and 22 others
>     = note: required for `kernel::fmt::Adapter<&MsgFunction>` to implement `core::fmt::Display`
> note: required by a bound in `core::fmt::rt::Argument::<'_>::new_display`
>    --> /usr/lib/rustlib/src/rust/library/core/src/fmt/rt.rs:113:27
>     |
> 113 |     pub fn new_display<T: Display>(x: &T) -> Argument<'_> {
>     |                           ^^^^^^^ required by this bound in `Argument::<'_>::new_display`
>     = note: this error originates in the macro `::core::format_args` which comes from the expansion of the macro `dev_dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> error: aborting due to 3 previous errors

I could work around this with the following patch, but it looks terribly
wrong. In gpu.rs we need to implement core::fmt::Display (because of the
use of `write!`), but in fw.rs we need to implement
kernel::fmt::Display. It is not clear to me how the two interfaces are
supposed to interact.

Tamir, could you advise on the correct thing to do here?

diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index 19755af6ad04..de924ffc0823 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -115,7 +115,7 @@ pub(crate) fn arch(&self) -> Architecture {
 // Hence, replace with something like strum_macros derive(Display).
 //
 // For now, redirect to fmt::Debug for convenience.
-impl fmt::Display for Chipset {
+impl core::fmt::Display for Chipset {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{self:?}")
     }
@@ -169,7 +169,7 @@ fn from(boot0: regs::NV_PMC_BOOT_42) -> Self {
     }
 }

-impl fmt::Display for Revision {
+impl core::fmt::Display for Revision {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{:x}.{:x}", self.major, self.minor)
     }
diff --git a/drivers/gpu/nova-core/gsp/fw.rs b/drivers/gpu/nova-core/gsp/fw.rs
index 8deec5e0a1d4..53afdbfa9554 100644
--- a/drivers/gpu/nova-core/gsp/fw.rs
+++ b/drivers/gpu/nova-core/gsp/fw.rs
@@ -7,12 +7,12 @@
 use r570_144 as bindings;

 use core::{
-    fmt,
     ops::Range, //
 };

 use kernel::{
     dma::CoherentAllocation,
+    fmt,
     prelude::*,
     ptr::{
         Alignable,