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Date:   Tue, 27 Sep 2022 13:46:59 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ricardo Neri <ricardo.neri@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Len Brown <len.brown@intel.com>, Mel Gorman <mgorman@suse.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Valentin Schneider <vschneid@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, "Tim C . Chen" <tim.c.chen@intel.com>
Subject: Re: [RFC PATCH 14/23] thermal: intel: hfi: Update the class of the
 current task
Message-ID: <YzLis/a3aIJnJ2ZE@hirez.programming.kicks-ass.net>
References: <20220909231205.14009-1-ricardo.neri-calderon@linux.intel.com>
 <20220909231205.14009-15-ricardo.neri-calderon@linux.intel.com>
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In-Reply-To: <20220909231205.14009-15-ricardo.neri-calderon@linux.intel.com>
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On Fri, Sep 09, 2022 at 04:11:56PM -0700, Ricardo Neri wrote:
> +union hfi_thread_feedback_char_msr {
> +	struct {
> +		u8	classid;
> +		u64	__reserved:55;
> +		u8	valid:1;
> +	} split;
> +	u64 full;
> +};

Urgh, did you perhaps mean:

	struct {
		u64	classid    :8;
		u64	__reserved :55;
		u64	valid      :1
	};

?

Because yes, GCC does fold that into a single u64, but that's
implementation defined behaviour; the C spec doesn't require one to pack
adjacent bitfields of different types together.

I layout of:

	u8	class;			// offset 0
	u64	__reserver : 55;	// offset 8
	u8	valid : 1;		// offset 16

with a total size of 24 bytes is, AFAIU, a valid result of what you
wrote.