Date:	Thu, 5 Jun 2014 07:12:05 -0400
From:	Matt Porter <mporter@...aro.org>
To:	Jassi Brar <jaswinder.singh@...aro.org>
Cc:	Sudeep Holla <sudeep.holla@....com>,
	Jassi Brar <jassisinghbrar@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Anna, Suman" <s-anna@...com>,
	Loic Pallardy <loic.pallardy@...com>,
	LeyFoon Tan <lftan.linux@...il.com>,
	Craig McGeachie <slapdau@...oo.com.au>,
	Courtney Cavin <courtney.cavin@...ymobile.com>,
	Rob Herring <robherring2@...il.com>,
	Josh Cartwright <joshc@...eaurora.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Kumar Gala <galak@...eaurora.org>,
	"ks.giri@...sung.com" <ks.giri@...sung.com>,
	Devicetree List <devicetree@...r.kernel.org>
Subject: Re: [PATCHv5 2/4] mailbox: Introduce framework for mailbox

On Tue, Jun 03, 2014 at 03:51:55PM +0530, Jassi Brar wrote:
> On 3 June 2014 15:05, Sudeep Holla <sudeep.holla@....com> wrote:
> > Hi Jassi,
> >
> > On Mon, Jun 2, 2014 at 6:11 PM, Jassi Brar <jassisinghbrar@...il.com> wrote:
> >> On Mon, Jun 2, 2014 at 8:44 PM, Matt Porter <mporter@...aro.org> wrote:
> >>> On Fri, May 30, 2014 at 11:01:55AM +0530, Jassi Brar wrote:
> >>>
> >>>>  Being more specific to your platform, I think you need some server
> >>>> code (mailbox's client) that every driver (like clock, pmu, pinmux
> >>>> etc) registers with to send messages to remote and receive commands
> >>>> from remote ... perhaps by registering some filter to sort out
> >>>> messages for each driver.
> >>>
> >>> Right, and here's where you hit on the problem. This server you mention
> >>> is not a piece of hardware, it would be a software construct. As such, it
> >>> doesn't fit into the DT binding as it exists. It's probably best to
> >>> illustrate in DT syntax.
> >>>
> >>> If I were to represent the hardware relationship in the DT binding now
> >>> it would look like this:
> >>>
> >>> ---
> >>> cpm: mailbox@...dbeef {
> >>>         compatible = "brcm,bcm-cpm-mailbox";
> >>>         reg = <...>;
> >>>         #mbox-cells <1>;
> >>>         interrupts = <...>;
> >>> };
> >>>
> >>> /* clock complex */
> >>> ccu {
> >>>         compatible = "brcm,bcm-foo-ccu";
> >>>         mbox = <&cpm CPM_SYSTEM_CHANNEL>;
> >>>         mbox-names = "system";
> >>>         /* leaving out other mailboxes for brevity */
> >>>         #clock-cells <1>;
> >>>         clock-output-names = "bar",
> >>>                              "baz";
> >>> };
> >>>
> >>> pmu {
> >>>         compatible = "brcm,bcm-foo-pmu"
> >>>         mbox = <&cpm CPM_SYSTEM_CHANNEL>;
> >>>         mbox-names = "system";
> >>> };
> >>>
> >>> pinmux {
> >>>         compatible = "brcm,bcm-foo-pinctrl";
> >>>         mbox = <&cpm CPM_SYSTEM_CHANNEL>;
> >>>         mbox-names = "system";
> >>> };
> >>> ---
> >> Yeah, I too don't think its a good idea.
> >>
> >>
> >>> What we would need to do is completely ignore this information in each
> >>> of the of the client drivers associated with the clock, pmu, and pinmux
> >>> devices. This IPC server would need to be instantiated and get the
> >>> mailbox information from some source. mbox_request_channel() only works
> >>> when the client has an of_node with the mbox-names property present.
> >>> Let's say we follow this model and represent it in DT:
> >>>
> >>> --
> >>> cpm: mailbox@...dbeef {
> >>>         compatible = "brcm,bcm-cpm-mailbox";
> >>>         reg = <...>;
> >>>         #mbox-cells <1>;
> >>>         interrupts = <...>;
> >>> };
> >>>
> >>> cpm_ipc {
> >>>         compatible = "brcm,bcm-cpm-ipc";
> >>>         mbox = <&cpm CPM_SYSTEM_CHANNEL>;
> >>>         mbox-names = "system";
> >>>         /* leaving out other mailboxes for brevity */
> >>> };
> >>> ---
> >>>
> >>> This would allow an ipc driver to exclusively own this system channel,
> >>> but now we've invented a binding that doesn't reflect the hardware at
> >>> all. It's describing software so I don't believe the DT maintainers will
> >>> allow this type of construct.
> >>>
> >> Must the server node specify MMIO and an IRQ, to be acceptable? Like ...
> >>
> >> cpm_ipc : cpm@...dbeef {
> >>          compatible = "brcm,bcm-cpm-ipc";
> >>        /*  reg = <0xdeadbeef 0x100>; */
> >>        /*  interrupts = <0 123 4>;  */
> >>          mbox = <&cpm CPM_SYSTEM_CHANNEL>;
> >>          mbox-names = "system";
> >> };
> >>
> >> cpm_ipc already specifies a hardware resource (mbox) that its driver
> >> needs, I think that should be enough reason. If it were some purely
> >> soft property for the driver like
> >>       mode = "poll";  //or "irq"
> >> then the node wouldn't be justified because that is the job of a
> >> build-time config or run-time module option.
> >>
> >
> > Like Matt, I am also in similar situation where there's a lot of common
> > code necessary to construct/parse IPCs for each of the drivers using the
> > mailbox.
> >
> > As per your suggestion if we have single DT node to specify both the
> > controller and the client, we might still have to pollute this node with
> > software specific compatibles.
> >
> I am afraid you misunderstood me. I don't suggest single node for
> mailbox controller and client, and IIUC, neither did Matt. Please note
> the controller is cpm and client is cpm_ipc.

Correct, I had separate controller and consumer nodes as written
above...to match the binding.

> BTW, here we at least have a hardware resource to specify in the DT
> node, there are examples in kernel where the DT nodes are purely
> virtual. For ex, grep for "linux,spdif-dit". So I think we should be
> ok.
> 

There's a bit of a difference between my concern over a virtual node and
this example you've cited. In the dummy spdif transmitter, it's defining
a virtual device that plugs in for a codec, a hardware concept well
defined in the audio bindings. I agree that there are many examples of
this type of virtual node, including dummy phys, but in all cases they
are stubbing out a real hardware concept.

I find it to be distinctly different to create a node that doesn't
represent the hardware's use of mailboxes. I'd be happy if a DT
maintainer could say that this is acceptable though. ;)

-Matt
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