From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1756429AbeAROVc (ORCPT <rfc822;w@1wt.eu>);
        Thu, 18 Jan 2018 09:21:32 -0500
Received: from foss.arm.com ([217.140.101.70]:56176 "EHLO foss.arm.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1756267AbeAROVa (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 18 Jan 2018 09:21:30 -0500
Date: Thu, 18 Jan 2018 14:21:26 +0000
From: Dave Martin <Dave.Martin@arm.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>, mark.rutland@arm.com,
        marc.zyngier@arm.com, catalin.marinas@arm.com, will.deacon@arm.com,
        linux-kernel@vger.kernel.org, james.morse@arm.com,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 0/2] arm64: Run enable method for errata work arounds
 on late CPUs
Message-ID: <20180118142125.GM22781@e103592.cambridge.arm.com>
References: <20180117174220.7959-1-suzuki.poulose@arm.com>
 <20180118114540.GL22781@e103592.cambridge.arm.com>
 <5c1945a8-a66d-9306-1c6d-3c1c8febcab2@arm.com>
 <851887b9-974a-6f20-920b-d9149d8122fe@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <851887b9-974a-6f20-920b-d9149d8122fe@arm.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Jan 18, 2018 at 12:08:43PM +0000, Robin Murphy wrote:
> On 18/01/18 12:00, Robin Murphy wrote:
> [...]
> >>+struct enable_arg {
> >>+    int (*enable)(struct arm64_cpu_capabilities const *);
> >>+    struct arm64_cpu_capabilities const *cap;
> >>+};
> >>+
> >>+static int __enable_cpu_capability(void *arg)
> >>+{
> >>+    struct enable_arg const *e = arg;
> >>+
> >>+    return e->enable(e->cap);
> >>+}
> >
> >AFAICS, you shouldn't even need the intermediate struct - if you were
> >instead to call stop_machine(&caps->enable, ...), the wrapper could be:
> >
> >     <type> **fn = arg;
> >     *fn(container_of(fn, struct arm64_cpu_capabilities, enable));
> >
> >(cheaty pseudocode because there's no way I'm going to write a
> >pointer-to-function-pointer type correctly in an email client...)
> >
> >That's certainly a fair bit simpler in terms of diffstat; whether it's
> >really as intuitive as I think it is is perhaps another matter, though.
> 
> Ah, right, but then you'd be back to casting away const, and at that point
> it makes no sense to do the container_of dance instead of just passing the
> struct pointer itself around...
> 
> I shall now excuse myself from this discussion, as I'm clearly not helping
> :)
> 
> Robin.

That's what I was about to say... but neat trick.

However, it does concentrate the type fudge in one place and keeps the
arm64_cpu_capabilities::enable() prototype correct, so it's still better
than the original.


Thinking about it, the following is probably clearer and no worse:

static int __enable_cpu_capability(void *arg)
{
	struct arm64_cpu_capabilities const *cap = arg;

	return cap->enable(cap);
}

...

stop_machine(__enable_cpu_capability, (void *)caps, cpu_online_mask);


In your version, the argument would be (void *)&caps->enable, which is
really just a proxy for (void *)caps, unless I missed something.


What do you think Suzuki?  I can respin my patch if you fancy picking it
up.  Either way, it's not urgent.

Cheers
---Dave