linux-kernel - Re: [PATCH 2/3 v6] ACPI: allow longer device IDs

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Date:   Tue, 1 Mar 2022 13:29:11 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Alexander Graf <graf@...zon.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 2/3 v6] ACPI: allow longer device IDs

On Mon, Feb 28, 2022 at 7:34 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> From: Alexander Graf <graf@...zon.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.
>
> 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.
>
> [1] https://download.microsoft.com/download/3/1/C/31CFC307-98CA-4CA5-914C-D9772691E214/VirtualMachineGenerationID.docx
>
> Signed-off-by: Alexander Graf <graf@...zon.com>
> Cc: Rafael J. Wysocki <rafael@...nel.org>
> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Cc: Hans de Goede <hdegoede@...hat.com>
> Cc: Len Brown <lenb@...nel.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Co-authored-by: Jason A. Donenfeld <Jason@...c4.com>
> [Jason: reworked commit message a bit, went with len=16 approach.]
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> Hi Rafael,
>
> This patch is directed toward you specifically. The first and last patch
> of the series this is part of have been through the ringer of review a
> bit already and do not specifically require your attention, but we wound
> up getting hung up on an ACPI ID matching API limitation. This patch
> fixes that limitation with this patch that you see here, with a trivial
> one line fix, which does require your attention.
>
> The other patches will go through my random.git tree naturally, but
> because those patches depend on this one here, in order to compile
> without warnings (and be functional at all), it would be nice if you
> would provide an "Acked-by" on it and permit me to /also/ take it
> through my random.git tree (if it looks like a correct patch to you, of
> course). This would make the merge logistics a lot easier. Plus it's a
> small +1/-1 line change.
>
> This v6 updates the commit message.

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

> Please have a look and let me know what you think.
>
> Thanks,
> Jason
>
>  include/linux/mod_devicetable.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
> index 4bb71979a8fd..5da5d990ff58 100644
> --- a/include/linux/mod_devicetable.h
> +++ b/include/linux/mod_devicetable.h
> @@ -211,7 +211,7 @@ struct css_device_id {
>         kernel_ulong_t driver_data;
>  };
>
> -#define ACPI_ID_LEN    9
> +#define ACPI_ID_LEN    16
>
>  struct acpi_device_id {
>         __u8 id[ACPI_ID_LEN];
> --
> 2.35.1
>