Re: [PATCH] ACPI: utils: Ignore leading root scope prefix in string _UID match

On Mon, Aug 31, 2026 at 8:02 PM Mario Limonciello
<[email protected]> wrote:
>
> Firmware may express an ACPI namespace path used as a _UID with or
> without the leading root scope character ('\'). For example, an AMD
> IVRS IVHD ACPI HID device entry may carry a character UID of
> "\_SB.MHSP" while the corresponding device's _UID evaluates to
> "_SB.MHSP" (or vice versa).  This semantic difference is due to how
> Windows PnP enumerates and uses devices.

Can you please elaborate a bit more?

I would expect _UID to return the string without the leading
backslash, so where does the other one come from, exactly?

> acpi_str_uid_match() compared the two strings verbatim, so such
> entries failed to match on the UID even though they refer to the same
> object. In the AMD IOMMU case (get_acpihid_device_id()) this caused
> the exact HID+UID match to be missed and the code to fall through to
> the HID-only path, spuriously raising a FW_BUG.
>
> Skip a single leading '\' on either string before comparing so that
> paths that differ only by the root scope prefix are treated as a
> match. The integer _UID path is unaffected.

But this sort of assumes that the string returned by _UID will always
be a namespace path, but is that the case really?

> Signed-off-by: Mario Limonciello <[email protected]>
> ---
>  include/acpi/acpi_bus.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/include/acpi/acpi_bus.h b/include/acpi/acpi_bus.h
> index 1a45e0d521d8e..e0bec35953ef1 100644
> --- a/include/acpi/acpi_bus.h
> +++ b/include/acpi/acpi_bus.h
> @@ -835,7 +835,15 @@ static inline bool acpi_str_uid_match(struct acpi_device *adev, const char *uid2
>  {
>         const char *uid1 = acpi_device_uid(adev);
>
> -       return uid1 && uid2 && !strcmp(uid1, uid2);
> +       if (!uid1 || !uid2)
> +               return false;
> +
> +       if (*uid1 == '\\')
> +               uid1++;
> +       if (*uid2 == '\\')
> +               uid2++;
> +
> +       return !strcmp(uid1, uid2);
>  }
>
>  static inline bool acpi_int_uid_match(struct acpi_device *adev, u64 uid2)
> --
> 2.43.0
>