linux-kernel - Re: [PATCH] ACPI: bus: Match first 9 bytes of device IDs

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Date:   Fri, 25 Feb 2022 18:21:45 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Alexander Graf <graf@...zon.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Woodhouse, David" <dwmw@...zon.co.uk>
Subject: Re: [PATCH] ACPI: bus: Match first 9 bytes of device IDs

On Fri, Feb 25, 2022 at 6:13 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> The device subsystem side of things already deals with this properly:
> the modalias of the QEMU vmgenid device comes up as
> 'acpi:QEMUVGID:VM_GEN_COUNTER', which means it already captures the
> entire string, and exposes it in the correct way (modulo the all caps)

Ahh, so the userspace side of this won't work right. Shucks. That's what
I was concerned about.

> I don't like this hack. If we are going to accept the fact that CIDs
> could be arbitrary length strings, we should handle them properly.
>
> So what we need is a way for a module to describe its compatibility
> with such a _CID, which shouldn't be that complicated.

Can't we do something more boring and just...

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
@@ -210,9 +210,9 @@ struct css_device_id {
 	__u8 type; /* subchannel type */
 	kernel_ulong_t driver_data;
 };
 
-#define ACPI_ID_LEN	9
+#define ACPI_ID_LEN	16
 
 struct acpi_device_id {
 	__u8 id[ACPI_ID_LEN];
 	kernel_ulong_t driver_data;


As you can see from the context, those additional 7 bytes were being
wasted on padding anyway inside the acpi_device_id struct, so it's
basically free, it would seem. This seems like the least convoluted way
of solving this issue? If we ever encounter _more_ ACPI devices with
weird names, we could revisit a fancy dynamic solution, but for now, why
don't we keep it simple?

Jason