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

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Message-ID: <4435081.P3BKoakBJD@wuerfel>
Date:	Mon, 19 May 2014 15:08:36 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Jassi Brar <jaswinder.singh@...aro.org>
Cc:	Jassi Brar <jassisinghbrar@...il.com>,
	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 Friday 16 May 2014 19:03:25 Jassi Brar wrote:
> >> +/**
> >> + * struct mbox_controller - Controller of a class of communication chans
> >> + * @dev:             Device backing this controller
> >> + * @controller_name: Literal name of the controller.
> >> + * @ops:             Operators that work on each communication chan
> >> + * @chans:           Null terminated array of chans.
> >> + * @txdone_irq:              Indicates if the controller can report to API when
> >> + *                   the last transmitted data was read by the remote.
> >> + *                   Eg, if it has some TX ACK irq.
> >> + * @txdone_poll:     If the controller can read but not report the TX
> >> + *                   done. Ex, some register shows the TX status but
> >> + *                   no interrupt rises. Ignored if 'txdone_irq' is set.
> >> + * @txpoll_period:   If 'txdone_poll' is in effect, the API polls for
> >> + *                   last TX's status after these many millisecs
> >> + */
> >> +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 still don't understand why it matters. This data structure is internal
to the kernel, at least I don't see anything that is accessed by the
hardware here. Note that anything that allocates a mbox_controller through
kmalloc will not get the alignment from here anyway, but will get the
default slab alignment instead (which happens to also be 32 bytes on
ARM).

> 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.

I don't have a strong opinion on that.

>  Also, some platform might need to communicate with remote master
> during very early boot like for initializing system timers and clocks.
> The API isn't working then.

Do you have an example for a platform like that? I'd expect that normally
we can have a boot loader that sets up the system timer to work good
enough for us to get into normal driver initialization.

	Arnd
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