linux-kernel - Re: [PATCH 0/2] arm: add a /proc/cpuinfo platform extension

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Date:	Wed, 24 Mar 2010 12:01:20 +1300
From:	Ryan Mallon <ryan@...ewatersys.com>
To:	H Hartley Sweeten <hartleys@...ionengravers.com>
CC:	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] arm: add a /proc/cpuinfo platform extension

H Hartley Sweeten wrote:
> On Tuesday, March 23, 2010 3:02 PM, Ryan Mallon wrote:
>> H Hartley Sweeten wrote:
>>> On Tuesday, March 23, 2010 1:30 PM, Ryan Mallon wrote:
>>>> H Hartley Sweeten wrote:
>>> I can't see any reason why adding additional fields will break
>>> user space, as long as an existing heading in the output is not
>>> duplicated.  Even that "really" shouldn't break anything since
>>> any application parsing this file has to do it sequentially and
>>> the new headings are located at the end of the file.
>> I'm really not sure. There may be some crappy userspace tools out there
>> which will break. I don't really mind either way if the info goes in
>> /proc/cpuinfo, or some new /proc/archinfo, just as long as it doesn't
>> break userspace in some way.
> 
> I did a grep of glibc's source (cvs-2.9, the only one currently on my
> system) to see what it tries to parse out of /proc/cpuinfo.  These are
> the only ones I could find:

According to Google Codesearch there are a few utilities around which
read /proc/cpuinfo. I honestly don't know if any of them will break,
though I suspect not. I think that seemingly innocent userspace API
changes like this have broken things in the past though.

>>>> The other problem I see is that you have a single callback for registering
>>>> the arch specific information. In you ep93xx example, each of the ep93xx
>>>> boards must add:
>>>>
>>>>  .arch_cpuinfo = ep93xx_cpuinfo,
>>>>
>>>> If one of the boards has some additional information to make available,
>>>> it would need to reimplement the entire callback, which gets messy.
>>> Not necessarily.
>>>
>>> If a board, such as the ts72xx, wanted to add additional information
>>> it just has to register it's private callback then call the ep93xx core
>>> supplied callback at the desired point in it's private one.
>>>
>>> The ts72xx currently does this exact thing with the .map_io callback.
>>> It supplies it's own private one to map the external FPGA.  It first calls
>>> the ep93xx core to map the ahb/apb space then it does an iotable_init to
>>> map the FPGA.
>> Okay, fair point. I still don't like having the seq_file callback being
>> in machine_desc. It means that all of the board files have to be edited
>> to add the callback. It should be something which happens automagically
>> in the platform core. Perhaps using a weak function for the callback, or
>> a #define check.
> 
> The callback is copied from the machine_desc in arch/arm/kernel/setup.c
> just like the other architecture-specific pointers.  If an architecture
> does not have any additional data to dump they don't have to provide the
> callback.  If it's not provided, the main seq_file callback (c_show in
> arch/arm/kernel/setup.c) will not call it.  See Patch 1/3 of the series.

My point was that you end up adding to the machine_desc for _every_
board on the platform. In the ep93xx patch example the callback is the
same in every case. It doesn't belong in machine_desc, which is the
descriptor for a specific machine or board, it belongs to the platform
(ie ep93xx), so it should be hidden from the board specific files unless
they are providing extensions to the core arch info.

~Ryan

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