We create a list of ACPI "PNP" IDs which contains _HID, _CID and CLS
entries of the respective devices. However, we squeeze them into
struct acpi_device_id which only has 9 bytes space to store the identifier
based on the ACPI spec:
"""
A _HID object evaluates to either a numeric 32-bit compressed EISA
type ID or a string. If a string, the format must be an alphanumeric
PNP or ACPI ID with no asterisk or other leading characters.
A valid PNP ID must be of the form "AAA####" where A is an uppercase
letter and # is a hex digit.
A valid ACPI ID must be of the form "NNNN####" where N is an uppercase
letter or a digit ('0'-'9') and # is a hex digit. This specification
reserves the string "ACPI" for use only with devices defined herein.
It further reserves all strings representing 4 HEX digits for
exclusive use with PCI-assigned Vendor IDs.
"""
While most people adhere to the ACPI specs, Microsoft decided that its
VM Generation Counter device [1] should only be identifiable by _CID with
an value of "VM_Gen_Counter" - longer than 9 characters.
To still allow device drivers to match identifiers that exceed the 9 byte
limit, without wasting memory for the unlikely case that you have long
identifiers, let's match only the first 9 characters of the identifier.
This patch is a prerequisite to add support for VMGenID in Linux [2].
[1] https://download.microsoft.com/download/3/1/C/31CFC307-98CA-4CA5-914C-D9772691E214/VirtualMachineGenerationID.docx
[2] https://lore.kernel.org/lkml/20220225124848.909093-1-Jason@xxxxxxxxx/
Signed-off-by: Alexander Graf <graf@xxxxxxxxxx>
---
Alternatives to the approach above would be:
1) Always set id[8] = '\0' in acpi_add_id()
2) Allocate the id in struct acpi_device_id dynamically
I'm happy to explore option 1 instead if people believe it's cleaner.
Option 2 on the other hand seems overkill for the issue at hand. We don't
have a lot of devices that exceed the 8 character threshold, so chance of
collision is quite small. On the other hand, the extra overhead of
maintaining the string allocation dynamically will quickly become a
headache.
---
drivers/acpi/bus.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
index 07f604832fd6..aba93171739f 100644
--- a/drivers/acpi/bus.c
+++ b/drivers/acpi/bus.c
@@ -829,7 +829,7 @@ static bool __acpi_match_device(struct acpi_device *device,
/* First, check the ACPI/PNP IDs provided by the caller. */
if (acpi_ids) {
for (id = acpi_ids; id->id[0] || id->cls; id++) {
- if (id->id[0] && !strcmp((char *)id->id, hwid->id))
+ if (id->id[0] && !strncmp((char *)id->id, hwid->id, ACPI_ID_LEN))
goto out_acpi_match;
if (id->cls && __acpi_match_device_cls(id, hwid))
goto out_acpi_match;
--
2.16.4
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