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.

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.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
---
v2:
 * Add kdoc
 * Make sure the string is at least two characters long to normalize
---
 include/acpi/acpi_bus.h | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/acpi/acpi_bus.h b/include/acpi/acpi_bus.h
index a10a591c18b23..cfc3c8a21cb8e 100644
--- a/include/acpi/acpi_bus.h
+++ b/include/acpi/acpi_bus.h
@@ -826,11 +826,29 @@ static inline bool acpi_dev_hid_match(struct acpi_device *adev, const char *hid2
 	return hid1 && hid2 && !strcmp(hid1, hid2);
 }
 
+/**
+ * acpi_str_uid_match - Compare ACPI string _UID values
+ * @adev: ACPI device providing the primary _UID
+ * @uid2: Secondary _UID string to check against
+ *
+ * Returns true when both _UID strings match after optionally removing a
+ * leading root scope character ('\') when the string contains additional
+ * characters. Returns false if either _UID is missing or differs after the
+ * normalization.
+ */
 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[1])
+		uid1++;
+	if (*uid2 == '\\' && uid2[1])
+		uid2++;
+
+	return !strcmp(uid1, uid2);
 }
 
 static inline bool acpi_int_uid_match(struct acpi_device *adev, u64 uid2)
-- 
2.43.0

On Tue, Sep 8, 2026 at 6:32 AM Mario Limonciello
<mario.limonciello@amd.com> 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.
>
> 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.
>
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> ---
> v2:
>  * Add kdoc
>  * Make sure the string is at least two characters long to normalize
> ---
>  include/acpi/acpi_bus.h | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/include/acpi/acpi_bus.h b/include/acpi/acpi_bus.h
> index a10a591c18b23..cfc3c8a21cb8e 100644
> --- a/include/acpi/acpi_bus.h
> +++ b/include/acpi/acpi_bus.h
> @@ -826,11 +826,29 @@ static inline bool acpi_dev_hid_match(struct acpi_device *adev, const char *hid2
>         return hid1 && hid2 && !strcmp(hid1, hid2);
>  }
>
> +/**
> + * acpi_str_uid_match - Compare ACPI string _UID values
> + * @adev: ACPI device providing the primary _UID
> + * @uid2: Secondary _UID string to check against
> + *
> + * Returns true when both _UID strings match after optionally removing a
> + * leading root scope character ('\') when the string contains additional
> + * characters. Returns false if either _UID is missing or differs after the
> + * normalization.
> + */

I'd rather update the existing kerneldoc comments of
acpi_dev_uid_match() than add a new one here.

You can say something like this in there:

"If both the UID in @adev and @uid2 are strings, they are compared
after optionally skipping a leading backslash ('\') when the given
string contains additional characters."

>  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[1])
> +               uid1++;
> +       if (*uid2 == '\\' && uid2[1])
> +               uid2++;
> +
> +       return !strcmp(uid1, uid2);
>  }
>
>  static inline bool acpi_int_uid_match(struct acpi_device *adev, u64 uid2)
> --
> 2.43.0
>