From mboxrd@z Thu Jan  1 00:00:00 1970
From: Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of
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Date: Wed, 27 Jan 2016 14:11:42 -0800
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To: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Cc: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>, Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>, Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>, Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>, rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>, Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
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On Wed, Jan 27, 2016 at 09:34:35PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 27, 2016, at 12:37 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:
> 
> > On Wed, 27 Jan 2016, Thomas Gleixner wrote:
> > 
> >> On Wed, 27 Jan 2016, Mathieu Desnoyers wrote:
> >> > ----- On Jan 27, 2016, at 12:22 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:
> >> > Sounds fair. What is the recommended typing for "ptr" then ?
> >> > uint32_t ** or uint32_t * ?
> >> > 
> >> > It would be expected to pass a "uint32_t *" for the set
> >> > operation, but the "get" operation requires a "uint32_t **".
> >> 
> >> Well, you can't change the types depending on the opcode, so you need to stick
> >> with **.
> > 
> > Alternatively you make it:
> > 
> >  (opcode, *newptr, **oldptr, flags);
> 
> I'm tempted to stick to (opcode, **ptr, flags), because
> other syscalls that have "*newptr", "**oldptr"
> typically have them because they save the current state
> into oldptr, and set the new state, which is really
> not the case here. To eliminate any risk of confusion,
> I am tempted to keep a single "**ptr".
> 
> Unless someone has a better idea...

Either that or you could define it as "void *" and interpret it based on
flags, but that seems unfortunate; let's not imitate ioctl-style
typeless parameters.  I'd stick with the double pointer and the current
behavior.