linux-kernel - Re: [PATCH v7 1/4] Documentation: dt: add common bindings for hwspinlock

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Date:	Wed, 11 Feb 2015 11:29:07 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Suman Anna <s-anna@...com>
Cc:	Ohad Ben-Cohen <ohad@...ery.com>, Tony Lindgren <tony@...mide.com>,
	Rob Herring <robherring2@...il.com>,
	Kumar Gala <galak@...eaurora.org>,
	Bjorn Andersson <bjorn@...o.se>,
	Josh Cartwright <joshc@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v7 1/4] Documentation: dt: add common bindings for
 hwspinlock

On Thu, Jan 29, 2015 at 03:58:42AM +0000, Suman Anna wrote:
> On 01/22/2015 12:56 PM, Mark Rutland wrote:
> > On Wed, Jan 21, 2015 at 05:56:37PM +0000, Suman Anna wrote:
> >> On 01/21/2015 06:41 AM, Ohad Ben-Cohen wrote:
> >>> On Tue, Jan 20, 2015 at 8:05 PM, Tony Lindgren <tony@...mide.com> wrote:
> >>>> How about default to Linux id space and allow overriding that with
> >>>> a module param option if needed?
> >>>
> >>> I'm not sure I'm following.
> >>>
> >>> If the main point of contention is the base_id field, I'm also fine
> >>> with removing it entirely, as I'm not aware of any actual user for it
> >>> (Suman please confirm?).
> >>
> >> Yeah, well the current implementations that I am aware of only have a
> >> single bank, so all of them would be using a value of 0. I am yet to see
> >> a platform with multiple instances where the property really makes a
> >> difference. v7 has the property mandatory, so all the implementations
> >> would need to define this value even if it is 0.
> >>
> >> regards
> >> Suman
> >>
> >>>
> >>> Mark? Rob? Will you accept Suman's patches if the base_id field is removed?
> > 
> > My concern is that the mapping of hwspinlock IDs doesn't seem to be
> > explicit in the DT on a per-context basis, which is what I'd expect.
> > 
> > e.g.
> > 
> > lck: hwspinlock-device@f00 {
> > 	...
> > 	#hwlock-cells = <1>;
> > };
> > 
> > some-other-os-interface {
> > 	...
> > 	hwlocks = <&lck 0>, <&lck 1>, <&lck 2>, <&lck 3>;
> > 	hwlock-names = "glbl", "pool0", "pool1", "pool2";
> > };
> > 
> > a-different-os-interface {
> > 	...
> > 	hwlocks = <&lck 18>, <&lck 21>, <&lck 4>, <&lck 5>;
> > 	hwlock-names = "init", "teardown", "pool0", "pool1";
> > };
> > 
> > That's the only way I would expect this to possibly remain a stable
> > over time, and it's the entire reason for #hwlock-cells, no?
> > 
> > How do you expect the other components sharing the hwspinlocks to be
> > described?
> 
> Yes indeed, this is what any of the clients will use on Linux. But
> this is not necessarily the semantics for exchanging hwlocks with the
> other processor(s) which is where the global id space comes into
> picture.

I did try to consider that above. Rather than thinking about the
numbering as "global", think of it as unique within the a given pool
shared between processors. That's what the "poolN" names are about
above.

That way you can dynamically allocate within the pool and know that
Linux and the SW on the other processors will use the same ID. You can
have pools that span multiple hwlock hardware blocks, and you can have
multiple separate pools in operation at once.

Surely that covers the cases you care about?

If using names is clunky, we could instead have a pool-hwlocks property
for that purpose.

Thanks,
Mark.
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