linux-kernel - Re: [PATCH v2 0/2] arm64: Run enable method for errata work arounds on late CPUs

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Message-ID: <20180118142125.GM22781@e103592.cambridge.arm.com>
Date:   Thu, 18 Jan 2018 14:21:26 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>, mark.rutland@....com,
        marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, james.morse@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 0/2] arm64: Run enable method for errata work arounds
 on late CPUs

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