linux-kernel - Re: [PATCH] PM / OPP: fix debugfs files for 64-bit

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Message-ID: <20151007163309.GM21513@n2100.arm.linux.org.uk>
Date:	Wed, 7 Oct 2015 17:33:09 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nishanth Menon <nm@...com>, Len Brown <len.brown@...el.com>,
	Arnd Bergmann <arnd@...db.de>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, Pavel Machek <pavel@....cz>,
	Dmitry Torokhov <dtor@...omium.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] PM / OPP: fix debugfs files for 64-bit

On Wed, Oct 07, 2015 at 04:51:49PM +0530, Viresh Kumar wrote:
> On 07-10-15, 12:07, Greg Kroah-Hartman wrote:
> > Why would you be wanting to create a "unsigned long" as an api anyway?
> > Just force it to be u64 all the time, can't you do that?
> 
> Okay, so the variable in question (lets say frequency) is an 'unsigned
> long' and that's how all the APIs of clock framework expect/define
> it.
> 
> And you are probably saying that we do this:
> 
> unsigned long freq;
> 
> debugfs_create_u64((u64 *)&freq);
> 
> Right? Or are you asking to update clock APIs to be converted to u64?

Don't you even think about doing that - that's totally broken no matter
what, and this is a good example of why casts are Bad.

debugfs_create_u64() will create a debugfs object that will want to
access a 64-bit value, but the value pointed to is only 32-bit.  The
net result is that the debugfs file ends up reading or writing the
neigbouring 32-bits, which may potentially be unaligned.

The variable pointed to for debugfs_create_u64() must be a 64-bit
value.  No casts allowed.

An alternative would be to have debugfs_create_ulong() or similar
which has accessors for standard C types.

I don't think forcing subsystems wanting to use debugfs to have to use
u{8,16,32,64} types for internal data is on - these are for things that
we want to say "we want this to be a 8, 16, 32 or 64-bit type
respectively" which is not what most subsystems need to do with
internal data.  This looks like a debugfs cockup to me. :)

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