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

On Fri, 14 Aug 2015, Jassi Brar wrote:

On Fri, Aug 14, 2015 at 12:03 PM, Lee Jones [off-list ref] wrote:

quoted

On Thu, 13 Aug 2015, Jassi Brar wrote:

quoted

On Mon, Jul 27, 2015 at 3:14 PM, Lee Jones [off-list ref] wrote:

quoted

+
+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.

.

quoted

 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.

Correct.  That's the plan.

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.

Unfortunately it's a restriction of the hardware (or the controller as
you call it, although it's not really a controller).  There is only
one IRQ for Rx'ing and that's wired up to the A9's mailbox (Mailbox
0).  If one of the remote processors attempted to send a message
through any of the other mailboxes (other than the Mailbox 0), then no
one would hear the doorbell ring and the message would go unserviced.

quoted

quoted

quoted

+
+       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.

The only thing I'm having trouble with protecting at the moment is
other clients _also_ requesting a LOOPBACK channel.  I would like to
check which clients have already requested one/them, however that
information is not available until _after_ xlate() has been called,
which is pretty frustrating.  Perhaps I'll put a comment in instead.

quoted

quoted

 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_ :)

It's impossible to know which physical channels will be used by
clients and subsequently which physical channels to expose.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org ? Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog