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Date: Tue, 11 Nov 2025 17:34:48 +0000
From: Dave Martin <Dave.Martin@arm.com>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: Fenghua Yu <fenghuay@nvidia.com>,
	Reinette Chatre <reinette.chatre@intel.com>,
	Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>,
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	x86@kernel.org, linux-kernel@vger.kernel.org,
	patches@lists.linux.dev
Subject: Re: [PATCH v13 12/32] x86,fs/resctrl: Support binary fixed point
 event counters
Message-ID: <aRNzuMtoBT90CEAs@e133380.arm.com>
References: <20251029162118.40604-1-tony.luck@intel.com>
 <20251029162118.40604-13-tony.luck@intel.com>
 <aQtiSmZ14b+U/J4U@e133380.arm.com>
 <aRIYZOm6ZtpQFr4m@agluck-desk3>
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In-Reply-To: <aRIYZOm6ZtpQFr4m@agluck-desk3>

Hi Tony,

On Mon, Nov 10, 2025 at 08:52:52AM -0800, Luck, Tony wrote:
> On Wed, Nov 05, 2025 at 02:42:18PM +0000, Dave Martin wrote:
> > > +static void print_event_value(struct seq_file *m, unsigned int binary_bits, u64 val)
> > > +{
> > > +	unsigned long long frac;
> > > +	char buf[10];
> > 
> > In place of the magic number 10, how about
> > decplaces[MAX_BINARY_BITS] + 1 ?
> > 
> > (I think the compiler should accept that as an initialiser if the array
> > is const.)
> 
> The compiler (gcc 15.2.1) accepts without any warnings. But generates
> different code.
> 
> sparse complains:
> fs/resctrl/ctrlmondata.c:640:45: warning: Variable length array is used.

Hmmm.  Shame.

(Of course, this is only a warning.  sparse may not know how to
determine that the resulting buffer is limited to a sane size, but
looking at the code makes it pretty obvious.  Perhaps best avoided,
though.)

> I may change the hard coded constant to 21 (guaranteed to be big enough
> for a "long long" plus terminating NUL byte.)

I guess.  We may be able to sidestep this, though (see my other reply
about getting rid of buf[] altogether.)

Cheers
---Dave