Messages in this thread | | | From | Andy Shevchenko <> | Date | Mon, 28 Feb 2022 22:46:07 +0200 | Subject | Re: [PATCH 2/3 v6] ACPI: allow longer device IDs |
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On Mon, Feb 28, 2022 at 9:28 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > From: Alexander Graf <graf@amazon.com> > > We create a list of ACPI "PNP" IDs which contains _HID, _CID, and CLS > entries of the respective devices. However, when making structs for > matching, we squeeze those IDs into acpi_device_id, which only has 9 > bytes space to store the identifier. The subsystem actually captures the > full length of the IDs, and the modalias has the full length, but this > struct we use for matching is limited. It originally had 16 bytes, but > was changed to only have 9 in 6543becf26ff ("mod/file2alias: make > modalias generation safe for cross compiling"), presumably on the theory > that it would match the ACPI spec so it didn't matter.
> Unfortunately, while most people adhere to the ACPI specs, Microsoft > decided that its VM Generation Counter device [1] should only be > identifiable by _CID with a value of "VM_Gen_Counter", which is longer > than 9 characters.
Then why do we not see the ECR from somebody to update the spec or to fix MS' abuse of it? I believe _this_ should be the prerequisite to the proposed change.
> To allow device drivers to match identifiers that exceed the 9 byte > limit, this simply ups the length to 16, just like it was before the > aforementioned commit. Empirical testing indicates that this > doesn't actually increase vmlinux size on 64-bit, because the ulong in > the same struct caused there to be 7 bytes of padding anyway, and when > doing a s/M/Y/g i386_defconfig build, the bzImage only increased by > 0.0055%, so negligible. > > This patch is a prerequisite to add support for VMGenID in Linux, the > subsequent patch in this series. It has been confirmed to also work on > the udev/modalias side in userspace.
-- With Best Regards, Andy Shevchenko
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