linux-kernel - Re: [PATCH v5 2/3] ACPI: allow longer device IDs

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Date:   Tue, 1 Mar 2022 11:31:51 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Alexander Graf <graf@...zon.com>, Len Brown <lenb@...nel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v5 2/3] ACPI: allow longer device IDs

Hi,

On 2/28/22 19:19, Rafael J. Wysocki wrote:
> +Mika, Andy and Hans in case they have something to add

Thanks, I don't really have anything to add to the discussion
on v6 of this patch.

Regards,

Hans



> 
> On Mon, Feb 28, 2022 at 12:27 AM Jason A. Donenfeld <Jason@...c4.com> wrote:
>>
>> Hey again,
>>
>> On Sun, Feb 27, 2022 at 1:43 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>>>
>>> Hi Alex,
>>>
>>> On Sun, Feb 27, 2022 at 12:42:03PM +0100, Alexander Graf wrote:
>>>>> 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
>>>>
>>>>
>>>> This is only true for 64bit systems where padding automatically bloated
>>>> to 9 byte array to 16. I still believe the patch is fine as it is, but
>>>> there will be minor .rodata overhead on 32bit targets which you may want
>>>> to quantify in the patch description.
>>>
>>> Good point. So I just tried this out with a 32-bit i686 kernel and the
>>> results are the same again for the size of vmlinux. I then ran `objdump
>>> --headers` and looked at the size of the .rodata section, where it's
>>> also the same. I'm not quite sure what to make of this, as it's not what
>>> I was expecting, but I think I tested it right. So maybe we're lucky
>>> here?
>>
>> I tried a little harder to get _some_ difference on 32-bit, and
>> managed to get one by doing i386_defconfig and then switching off
>> modules to make all M into Y, and then compared sizes:
>>
>> vmlinux: 25590780 -> 25598972, so a 0.032% increase.
>> bzImage: 8698944 -> 8699424, so a 0.0055% increase.
>>
>> So it does increase, ever so slightly, but a) on 32-bit, and b) a
>> super, super tiny amount.
>>
>> In other words, I still think this patch is very much a-okay. But very
>> eager to hear from Rafael on the approach.
> 
> Increasing the ACPI_ID_LEN value is fine with me, but the patch
> changelog is not entirely accurate.
> 
> The ACPI subsystem uses struct acpi_device_id mostly (if not only) for
> device ID matching and it is generally used for creating lists of ACPI
> device IDs in drivers (and allow/deny lists etc).  The device IDs
> extracted from the ACPI tables can be longer than ACPI_ID_LEN.
> 
> This means that drivers cannot match device IDs longer than 8
> characters (excluding the terminating 0), because the IDs in the lists
> used by them for ID matching cannot be longer than this and not
> because the ACPI subsystem is limited by that value.
>