linux-kernel - Re: [PATCH 1/9] sysfs: add sysfs_remove_bin_file

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Date:   Wed, 6 Oct 2021 23:55:38 -0700
From:   Zev Weiss <zev@...ilderbeest.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     openbmc@...ts.ozlabs.org, Jeremy Kerr <jk@...econstruct.com.au>,
        Joel Stanley <joel@....id.au>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] sysfs: add sysfs_remove_bin_file_self() function

On Wed, Oct 06, 2021 at 11:12:37PM PDT, Greg Kroah-Hartman wrote:
>On Wed, Oct 06, 2021 at 10:58:59PM -0700, Zev Weiss wrote:
>> On Wed, Oct 06, 2021 at 10:23:33PM PDT, Greg Kroah-Hartman wrote:
>> > On Wed, Oct 06, 2021 at 05:09:46PM -0700, Zev Weiss wrote:
>> > > This is simply the bin_attribute analog to sysfs_remove_file_self().
>> >
>> > No, no binary sysfs file should be triggering a remove.
>> >
>> > binary sysfs files are "pass-through-only" from userspace to hardware,
>> > the kernel should not be even knowing what is read/written to them.
>> >
>> > What do you think this is needed for?
>> >
>>
>> So, I initially set out to be able to activate/deactivate specific DT nodes
>> at runtime by using the device-tree "reserved" status as defined in the spec
>> (but not currently used anywhere in the kernel) to mean essentially "create
>> a device for this but don't bind a driver to it" (leaving it to userspace to
>> invoke bind/unbind or similar), and added initial support for the specific
>> driver I'm concerned with at the moment (aspeed-smc) -- that was the
>> previous patch series linked in the cover letter of this one.
>>
>> In the discussion of that series, Rob suggested as an alternate approach:
>>
>> > Another possibility is making 'status' writeable from userspace. It is
>> > just a sysfs file.
>>
>> That seemed sort of appealing to me, and this seemed like the most obvious
>> way to go about implementing it.  Given that DT properties are binary
>> attributes, I gather you'd consider that a non-starter though?
>
>Why would a text attribute of "status" be a binary sysfs file?  That
>feels really wrong as again, binary sysfs files are not supposed to be
>parsed or handled by the kernel at all, they are only a pass-through.
>

Well, at present all device tree properties are binary sysfs files 
regardless of type, and from a brief git history check it appears 
they've been that way since DT sysfs support was introduced in commit 
75b57ecf9d1d ("of: Make device nodes kobjects so they show up in 
sysfs").

On the surface it seems like it would make sense for string properties 
like status to be text files instead of binary, but (a) looking at some 
of the discussion that preceded that patch, it sounds like there may be 
some ambiguity regarding what the "true" types of different properties 
actually are [0], and (b) changing the contents of those files from e.g.  
"okay\0" to "okay\n" seems likely to lead to broken userspace, so I'd 
guess it's probably moot anyway.

[0] https://lore.kernel.org/lkml/1363801579.17680.3.camel@pasglop/


Zev