Re: [PATCH v3 0/4] nvmem: fix a const-unsoundness in reg_write

On Thu Jul 16, 2026 at 3:24 PM BST, Link Mauve wrote:
> This callback used to take a mutable void * for no reason, which causes
> the compiler to be unaware that the val buffer should never be modified
> by the callback.
>
> This was found while drafting the nvmem-provider Rust abstraction.
>
> Thanks to the guidance of Andy Shevchenko, this now introduces a new
> callback and deprecates the existing one, with the goal of renaming the
> new one into the old one once no user remains in the kernel.

I wouldn't call the series "const-unsoundness fix". There's nothing wrong (in
the sense of UB) in using "void *" for constant data as long as it is not
actually mutated. Yes, it's better to use "const void *", but just call it
"migrate to const void * in reg_write" or something similar.

Best,
Gary

>
> Changes since v2:
> - Link to v2:
>   https://lore.kernel.org/rust-for-linux/aljAqBNVXV1aVJqx@luna/T/
> - Rebase on top of linux-next/master.
> - Fix logic error which made all devices read-only.
> - Remove a blanket cast of function pointers to void * in
>   apple-spmi-nvmem (patch 3).
> - Also fix constness of write buffer in firmware: meson, which is used
>   exclusively by meson-efuse (patch 4).
> - Re-review all changes to make sure the passed types are more correct
>   than they were before.
> - Actually test with COMPILE_TEST, which I forgot to enable before…
>
> Changes since v1:
> - Link to v1:
>   https://lore.kernel.org/rust-for-linux/[email protected]/
> - Removed all changes to other subsystems than nvmem.
> - Added a new reg_write_const callback instead of changing the exisitng
>   reg_write.
> - Deprecated the existing reg_write callback, it will get removed once
>   all users in the kernel will be done migrating to the new one.
>
> Link Mauve (4):
>   nvmem: core: deprecate reg_write callback and add reg_write_const
>   nvmem: make all reg_write callbacks take const void *
>   nvmem: apple-spmi: improve calling safety with wrapper functions
>   firmware: meson: pass a const buffer to meson_sm_call_write()
>
>  drivers/firmware/meson/meson_sm.c       |  2 +-
>  drivers/nvmem/apple-spmi-nvmem.c        | 18 ++++++++++++++--
>  drivers/nvmem/bcm-ocotp.c               |  8 +++----
>  drivers/nvmem/core.c                    | 28 ++++++++++++++-----------
>  drivers/nvmem/imx-ocotp-scu.c           |  6 +++---
>  drivers/nvmem/imx-ocotp.c               |  6 +++---
>  drivers/nvmem/internals.h               |  1 +
>  drivers/nvmem/lan9662-otpc.c            |  6 +++---
>  drivers/nvmem/lpc18xx_eeprom.c          |  6 +++---
>  drivers/nvmem/max77759-nvmem.c          |  4 ++--
>  drivers/nvmem/meson-efuse.c             |  8 +++----
>  drivers/nvmem/qcom-spmi-sdam.c          |  6 +++---
>  drivers/nvmem/qfprom.c                  |  6 +++---
>  drivers/nvmem/rave-sp-eeprom.c          |  6 +++---
>  drivers/nvmem/snvs_lpgpr.c              |  4 ++--
>  drivers/nvmem/sprd-efuse.c              |  6 +++---
>  drivers/nvmem/stm32-bsec-optee-ta.c     |  2 +-
>  drivers/nvmem/stm32-bsec-optee-ta.h     |  4 ++--
>  drivers/nvmem/stm32-romem.c             | 10 ++++-----
>  drivers/nvmem/zynqmp_nvmem.c            |  6 +++---
>  include/linux/firmware/meson/meson_sm.h |  2 +-
>  include/linux/nvmem-provider.h          |  6 +++++-
>  22 files changed, 87 insertions(+), 64 deletions(-)
>
>
> base-commit: b8809969e1d7a591e0f49dd464a5d04b3cf02ab1