Nouveau - Nov 2025 - [PATCH v2 1/3] rust: helpers: Add list helpers for C linked list operations

> On Nov 25, 2025, at 9:52?AM, Alexandre Courbot <acourbot at
nvidia.com> wrote:
> 
> ?On Wed Nov 12, 2025 at 2:13 AM JST, Joel Fernandes wrote:
>> Add Rust helper functions for common C linked list operations
>> that are implemented as macros or inline functions and thus not
>> directly accessible from Rust.
>> 
>> Signed-off-by: Joel Fernandes <joelagnelf at nvidia.com>
>> ---
>> rust/helpers/helpers.c |  1 +
>> rust/helpers/list.c    | 32 ++++++++++++++++++++++++++++++++
>> 2 files changed, 33 insertions(+)
>> create mode 100644 rust/helpers/list.c
>> 
>> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
>> index 79c72762ad9c..634fa2386bbb 100644
>> --- a/rust/helpers/helpers.c
>> +++ b/rust/helpers/helpers.c
>> @@ -32,6 +32,7 @@
>> #include "io.c"
>> #include "jump_label.c"
>> #include "kunit.c"
>> +#include "list.c"
>> #include "maple_tree.c"
>> #include "mm.c"
>> #include "mutex.c"
>> diff --git a/rust/helpers/list.c b/rust/helpers/list.c
>> new file mode 100644
>> index 000000000000..fea2a18621da
>> --- /dev/null
>> +++ b/rust/helpers/list.c
>> @@ -0,0 +1,32 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +/*
>> + * Helpers for C Circular doubly linked list implementation.
>> + */
>> +
>> +#include <linux/list.h>
>> +
>> +bool rust_helper_list_empty(const struct list_head *head)
>> +{
>> +    return list_empty(head);
>> +}
>> +
>> +void rust_helper_list_del(struct list_head *entry)
>> +{
>> +    list_del(entry);
>> +}
>> +
>> +void rust_helper_INIT_LIST_HEAD(struct list_head *list)
>> +{
>> +    INIT_LIST_HEAD(list);
>> +}
>> +
>> +void rust_helper_list_add(struct list_head *new, struct list_head
*head)
>> +{
>> +    list_add(new, head);
>> +}
>> +
>> +void rust_helper_list_add_tail(struct list_head *new, struct list_head
*head)
>> +{
>> +    list_add_tail(new, head);
>> +}
> 
> Just noticed, but of these helpers only `INIT_LIST_HEAD` and
> `list_add_tail` seem to be used (in doccomments).
Correct, but it makes sense to add the most obvious/common ones (also to make it
clear that using these are supported).

Thanks.

On 11/25/25 10:16 AM, Joel Fernandes wrote:
>> On Nov 25, 2025, at 9:52?AM, Alexandre Courbot <acourbot at
nvidia.com> wrote:
>> ?On Wed Nov 12, 2025 at 2:13 AM JST, Joel Fernandes wrote:
...
>> Just noticed, but of these helpers only `INIT_LIST_HEAD` and
>> `list_add_tail` seem to be used (in doccomments).
> 
> Correct, but it makes sense to add the most obvious/common ones (also to
make it clear that using these are supported).
> 
It's important to exercise the code. Let's please add that
coverage, seeing as how it is quite easy to do, right?


thanks,
-- 
John Hubbard

On Wed Nov 26, 2025 at 3:16 AM JST, Joel Fernandes
wrote:
>
>
>> On Nov 25, 2025, at 9:52?AM, Alexandre Courbot <acourbot at
nvidia.com> wrote:
>> 
>> ?On Wed Nov 12, 2025 at 2:13 AM JST, Joel Fernandes wrote:
>>> Add Rust helper functions for common C linked list operations
>>> that are implemented as macros or inline functions and thus not
>>> directly accessible from Rust.
>>> 
>>> Signed-off-by: Joel Fernandes <joelagnelf at nvidia.com>
>>> ---
>>> rust/helpers/helpers.c |  1 +
>>> rust/helpers/list.c    | 32 ++++++++++++++++++++++++++++++++
>>> 2 files changed, 33 insertions(+)
>>> create mode 100644 rust/helpers/list.c
>>> 
>>> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
>>> index 79c72762ad9c..634fa2386bbb 100644
>>> --- a/rust/helpers/helpers.c
>>> +++ b/rust/helpers/helpers.c
>>> @@ -32,6 +32,7 @@
>>> #include "io.c"
>>> #include "jump_label.c"
>>> #include "kunit.c"
>>> +#include "list.c"
>>> #include "maple_tree.c"
>>> #include "mm.c"
>>> #include "mutex.c"
>>> diff --git a/rust/helpers/list.c b/rust/helpers/list.c
>>> new file mode 100644
>>> index 000000000000..fea2a18621da
>>> --- /dev/null
>>> +++ b/rust/helpers/list.c
>>> @@ -0,0 +1,32 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +
>>> +/*
>>> + * Helpers for C Circular doubly linked list implementation.
>>> + */
>>> +
>>> +#include <linux/list.h>
>>> +
>>> +bool rust_helper_list_empty(const struct list_head *head)
>>> +{
>>> +    return list_empty(head);
>>> +}
>>> +
>>> +void rust_helper_list_del(struct list_head *entry)
>>> +{
>>> +    list_del(entry);
>>> +}
>>> +
>>> +void rust_helper_INIT_LIST_HEAD(struct list_head *list)
>>> +{
>>> +    INIT_LIST_HEAD(list);
>>> +}
>>> +
>>> +void rust_helper_list_add(struct list_head *new, struct list_head
*head)
>>> +{
>>> +    list_add(new, head);
>>> +}
>>> +
>>> +void rust_helper_list_add_tail(struct list_head *new, struct
list_head *head)
>>> +{
>>> +    list_add_tail(new, head);
>>> +}
>> 
>> Just noticed, but of these helpers only `INIT_LIST_HEAD` and
>> `list_add_tail` seem to be used (in doccomments).
>
> Correct, but it makes sense to add the most obvious/common ones (also to
make it clear that using these are supported).
"It makes sense" is subjective, and in this case I am confident it is
not the right intuition to add dead code just for the sake of it.

Each of these helpers adds a potential breakage point from the C API
should the latter change, so we should only add them if they are indeed
necessary.

Actually, some of these helpers are not used when they could have been -
you have a `is_empty` method that rewrites the C function instead of
calling the helper. The only helpers that are unjustified as of now as
`list_add` and `list_del`, and these are easy to add when they become
necessary.

But this raises an interesting dilemma: these helpers cannot be inlined
and add the overhead of a function call. On the other hand, the
definition of `list_head` is so excessively simple that manipulating it
directly is virtually as intuitive as invoking the helper - and doesn't
bear the overhead. So should we double-down on these helpers, or just
drop them completely and re-implement the list functionality we need for
increased performance?