linux-kernel - Re: [PATCH v1 1/9] of: Add NVIDIA Tegra XUSB mailbox binding

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Message-ID: <53AB5473.3050204@wwwdotorg.org>
Date:	Wed, 25 Jun 2014 17:00:03 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Andrew Bresticker <abrestic@...omium.org>
CC:	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-usb@...r.kernel.org,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Linus Walleij <linus.walleij@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mathias Nyman <mathias.nyman@...el.com>,
	Grant Likely <grant.likely@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Kishon Vijay Abraham I <kishon@...com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v1 1/9] of: Add NVIDIA Tegra XUSB mailbox binding

On 06/25/2014 04:37 PM, Andrew Bresticker wrote:
> On Wed, Jun 25, 2014 at 2:42 PM, Stephen Warren <swarren@...dotorg.org> wrote:
>> On 06/18/2014 12:16 AM, Andrew Bresticker wrote:
>>> Add device-tree bindings for the Tegra XUSB mailbox which will be used
>>> for communication between the Tegra XHCI controller and the host.
>>
>> Sorry for the slow review.
>>
>>> diff --git a/Documentation/devicetree/bindings/mailbox/nvidia,tegra124-xusb-mbox.txt b/Documentation/devicetree/bindings/mailbox/nvidia,tegra124-xusb-mbox.txt
>>
>>> +NVIDIA Tegra XUSB mailbox
>>> +=========================
>>> +
>>> +The Tegra XUSB mailbox is used by the Tegra XHCI controller's firmware to
>>> +communicate with the host.
>>
>> Isn't the mailbox an internal implementation detail of the XUSB controller.
>>
>> In other words, I'd naively think that there isn't a standalone generic
>> mailbox that can be used by anything, but we just happen to want to use
>> for XUSB. Rather, there's an XUSB controller, and part of the interface
>> to that controller is a mailbox.
> 
> Yes, the mailbox isn't an actual piece of hardware but rather the
> interface through which the XUSB host and AP communicate.
> 
>> As such, I don't think we want a standalone mailbox node in DT. Rather,
>> we should add the required reg and interrupt values into the XUSB DT node.
>>
>> The driver for that XUSB HW module can either:
>>
>> a) Register as both a mailbox driver and an EHCI driver.
>>
>> b) Spawn a child device to instantiate the mailbox driver.
>>
>> Perhaps (b) could be assisted by using the MFD framework?
> 
> So in the RFC series I did something like (a) where the XUSB host
> handled the mailbox interrupt with both the PHY and host could
> registering notifiers to handle the messages.  It was suggested by
> Arnd though that I make a separate mailbox driver.  Instead of having
> a both a host and mailbox node, I could have a single XUSB host node
> and have the mailbox driver bind to that - thoughts?

Yes, that sounds like what I meant by (b) above. I don't think you can
actually have 2 drivers bind to the same DT node though, so it'd have to
work something like:

* XUSB host node causes a platform device to be instantiated
* XUSB host driver probe()s against that
* XUSB host driver's probe() creates a platform device for the mailbox
* XUSB mailbox driver probe()s against that.

Or, perhaps go completely MFD, and have 2 child devices (XUSB host and
XUSB mailbox) instantiated by the MFD parent, which is what is in the DT.
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