linux-kernel - Re: [PATCH] asm-generic: Remove asm/setup.h from the UABI.

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Message-ID: <mhng-1f6be9fb-cb82-4a66-b23b-59b0c0d33b42@palmerdabbelt-glaptop>
Date:   Fri, 23 Apr 2021 07:36:45 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject:     Re: [PATCH] asm-generic: Remove asm/setup.h from the UABI.

On Fri, 23 Apr 2021 02:09:46 PDT (-0700), Arnd Bergmann wrote:
> On Fri, Apr 23, 2021 at 4:57 AM Palmer Dabbelt <palmer@...belt.com> wrote:
>>
>> From: Palmer Dabbelt <palmerdabbelt@...gle.com>
>>
>> I honestly have no idea if this is sane.
>>
>> This all came up in the context of increasing COMMAND_LINE_SIZE in the
>> RISC-V port.  In theory that's a UABI break, as COMMAND_LINE_SIZE is the
>> maximum length of /proc/cmdline and userspace could staticly rely on
>> that to be correct.
>>
>> Usually I wouldn't mess around with changing this sort of thing, but
>> PowerPC increased it with a5980d064fe2 ("powerpc: Bump COMMAND_LINE_SIZE
>> to 2048").  There are also a handful of examples of COMMAND_LINE_SIZE
>> increasing, but they're from before the UAPI split so I'm not quite sure
>> what that means: e5a6a1c90948 ("powerpc: derive COMMAND_LINE_SIZE from
>> asm-generic"), 684d2fd48e71 ("[S390] kernel: Append scpdata to kernel
>> boot command line"), 22242681cff5 ("MIPS: Extend COMMAND_LINE_SIZE"),
>> and 2b74b85693c7 ("sh: Derive COMMAND_LINE_SIZE from
>> asm-generic/setup.h.").
>>
>> It seems to me like COMMAND_LINE_SIZE really just shouldn't have been
>> part of the UABI to begin with, and userspace should be able to handle
>> /proc/cmdline of whatever length it turns out to be.  I don't see any
>> references to COMMAND_LINE_SIZE anywhere but Linux via a quick Google
>> search, but that's not really enough to consider it unused on my end.
>>
>> I couldn't think of a better way to ask about this then just sending the
>> patch.
>
> I think removing asm/setup.h from the uapi headers makes sense,
> but then we should do it consistently for all architectures as far
> as possible.
>
> Most architectures either use the generic file or they provide their
> own one-line version, so if we move them back, I would do it
> for all.

Ya, makes sense.  I just wanted to see if anyone had a reason for things 
being this way before I chased everything around.

> The architectures that have additional contents in this file
> are alpha, arm, and ia64. We I would leave those unchanged
> in that case.