linux-kernel - Re: [PATCHv5 2/4] mailbox: Introduce framework for mailbox

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Message-ID: <CABb+yY2PiGpqy2uevCFexAKeS4aZxJxZHEDJTRN1SW6VVjBzGQ@mail.gmail.com>
Date:	Fri, 30 May 2014 11:01:55 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Matt Porter <mporter@...aro.org>
Cc:	Jassi Brar <jaswinder.singh@...aro.org>,
	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>
Subject: Re: [PATCHv5 2/4] mailbox: Introduce framework for mailbox

On Thu, May 29, 2014 at 9:13 PM, Matt Porter <mporter@...aro.org> wrote:
> On Fri, May 16, 2014 at 07:03:25PM +0530, Jassi Brar wrote:
>> On 15 May 2014 19:57, Arnd Bergmann <arnd@...db.de> wrote:
>> > On Thursday 15 May 2014 11:41:00 Jassi Brar wrote:
>
> ...
>
>> >> +struct mbox_controller {
>> >> +     struct device *dev;
>> >> +     struct mbox_chan_ops *ops;
>> >> +     struct mbox_chan *chans;
>> >> +     int num_chans;
>> >> +     bool txdone_irq;
>> >> +     bool txdone_poll;
>> >> +     unsigned txpoll_period;
>> >> +     struct mbox_chan *(*of_xlate)(struct mbox_controller *mbox,
>> >> +                                     const struct of_phandle_args *sp);
>> >> +     /*
>> >> +      * If the controller supports only TXDONE_BY_POLL,
>> >> +      * this timer polls all the links for txdone.
>> >> +      */
>> >> +     struct timer_list poll;
>> >> +     unsigned period;
>> >> +     /* Hook to add to the global controller list */
>> >> +     struct list_head node;
>> >> +} __aligned(32);
>> >
>> > What is the __aligned(32) for?
>> >
>> Attempt to align access to mailbox?
>>
>> I am still open to opinion about whether the mailbox ownership should
>> be exclusive or shared among clients. Need to handle async messages
>> from remote is one reason one might want more than one client to own a
>> channel. Allowing for RX via notifiers might be one option but that
>> breaks semantics of 'ownership' of a mailbox channel.
>
> This is the case we have on a new family of Broadcom SoCs. One mailbox
> channel is the "system" channel and is shared amongst many clients for
> reception of unsolicited events from the coprocessor. The same channel
> is also used commonly in drivers for debug/inspection of the M0 state.
> Functionality for clock, pmu, pinmux, and cpu idle/suspend is implemented
> via IPC with the M0 so all of those drivers need to share one mailbox.
>
OK, so you have a single mailbox channel used for communication with
the remote master.

> There's a lot of common code necessary to construct/parse IPCs for
> each of the drivers. I suppose this IPC driver/api could be the
> sole owner of that system channel. However, we'd need to develop some
> binding to represent IPC resources that devices need to operate. Coming
> into this late...I wonder if I missed something about where these vendor
> specific IPC layers should live and how, if it makes sense, they should
> be represented in DT. In our case, the IPC is an integral part of the
> hardware as it's loaded from ROM.
>
Like other platforms, the IPC protocol is going to be very specific to
Broadcom, even if the mailbox controller is some popular third party
IP like PL320. So you/Broadcom have to implement parser code for IPC
(client) above the mailbox common api and the controller driver below
it. Any resource/feature specific to your client and your controller
should be passed via some Broadcom specific DT bindings. The common
mailbox api DT bindings provide only for channel-client mapping.

 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.

-Jassi
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