linux-kernel - Re: [PATCH v2 3/6] mailbox: Add support for ST's Mailbox IP

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Date:	Fri, 14 Aug 2015 13:09:13 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernel@...inux.com, Devicetree List <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 3/6] mailbox: Add support for ST's Mailbox IP

On Fri, Aug 14, 2015 at 12:03 PM, Lee Jones <lee.jones@...aro.org> wrote:
> On Thu, 13 Aug 2015, Jassi Brar wrote:
>> On Mon, Jul 27, 2015 at 3:14 PM, Lee Jones <lee.jones@...aro.org> wrote:
>>
>> > +
>> > +static bool sti_mbox_tx_is_ready(struct mbox_chan *chan)
>> > +{
>> > +       struct sti_channel *chan_info = chan->con_priv;
>> > +       struct sti_mbox_device *mdev = chan_info->mdev;
>> > +       unsigned int instance = chan_info->instance;
>> > +       unsigned int channel = chan_info->channel;
>> > +       void __iomem *base = MBOX_BASE(mdev, instance);
>> > +
>> > +       if (!(chan_info->direction & MBOX_TX))
>> > +               return false;
>> >
>> Here the 'direction' is gotten via DT node of the client i.e, you
>> expect consumer drivers to tell the provider what its limitations are?
>>
>> IMO if some physical channel can't do TX then that should be either
>> hardcoded inside the controller driver or learnt via DT node of the
>> _controller_.
>
> That's a fair point.
.

>  I need to create a new property similar to the
> already existing 'read-only'.  I guess 'tx-only' is equivalent.
>
Just to be clear, if you must have such a property it should come from
the _controller_ node.

However at one point you said,  "Only the A9 (Mbox 0) can Rx."
  Which sounds like the 'simplex' constraint is not coming from the
mailbox controller but from the remote endpoints that don't RX+TX
except for one of them. That makes more sense than a controller with
differently capable physical channels. If that is indeed the
situation, then the controller is actually 'duplex' and there should
be no tx-only/rx-only property anywhere. Everything automatically
falls into place because client drivers are written for specific
targets and, unless you write some code, there can be no TX call to a
remote that doesn't listen.

>> > +
>> > +       for (i = 0; i < mbox->num_chans; i++) {
>> > +               chan_info = mbox->chans[i].con_priv;
>> > +
>> > +               /* Is requested channel free? */
>> > +               if (direction != MBOX_LOOPBACK &&
>> >
>> Consider this example when 2 clients ask for same physical channel but
>> in different modes.
>>            mboxes = <&mboxA 0 1 MBOX_TX>;
>>            mboxes = <&mboxA 0 1 MBOX_LOOPBACK>;
>>
>> You happily assign 2 virtual channels backed by one physical channel
>> {mboxA, 0, 1}. The 2 clients think they can freely do startup(),
>> shutdown() and send_data() on the channels. But obviously we are
>> screwed with races like
>>    client1.startup()
>>     -> client2.startup()
>>         -> client2.send_data()
>>             -> client2.shutdown()
>>                 -> client1.send_data()  XXXX
>
> Good catch and a fair point.  As you say, it's unlikely to happen, but
> I would like to prevent it in any case.
>
No, such races are a practical problem. We must own them.

I was talking about problems that arise because someone wrote bad
DT... those are not 'practical' problems because there are too many
ways to screw up with bad DT properties that if we try to check for
them we'll go insane.

>>  Now you can shove in some more checks to 'fix' the race OR you can
>> simply expose only physical channels.
>
> We can't expose all of the channels.  There are too many and would
> take up too much *unused* memory.
>
I am aware of that. I said expose _only_ physical channels, not _all_ :)

Cheers!
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