linux-kernel - Re: [PATCH v2 2/6] mailbox: dt-bindings: Add shared [driver <=> device tree] defines

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Date:	Wed, 12 Aug 2015 09:53:09 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Jassi Brar <jassisinghbrar@...il.com>
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 2/6] mailbox: dt-bindings: Add shared [driver <=>
 device tree] defines

On Mon, 10 Aug 2015, Jassi Brar wrote:

> On Mon, Aug 10, 2015 at 1:54 PM, Lee Jones <lee.jones@...aro.org> wrote:
> > On Mon, 10 Aug 2015, Jassi Brar wrote:
> >> On Mon, Jul 27, 2015 at 3:14 PM, Lee Jones <lee.jones@...aro.org> wrote:
> >> > This header is currently only used for defines pertaining to data
> >> > direction i.e. Rx, Tx or Loopback.
> >> >
> >> > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> >> > ---
> >> >  include/dt-bindings/mailbox/mailbox.h | 14 ++++++++++++++
> >> >  1 file changed, 14 insertions(+)
> >> >  create mode 100644 include/dt-bindings/mailbox/mailbox.h
> >> >
> >> > diff --git a/include/dt-bindings/mailbox/mailbox.h b/include/dt-bindings/mailbox/mailbox.h
> >> > new file mode 100644
> >> > index 0000000..82e929a
> >> > --- /dev/null
> >> > +++ b/include/dt-bindings/mailbox/mailbox.h
> >> > @@ -0,0 +1,14 @@
> >> > +/*
> >> > + * This program is free software; you can redistribute it and/or modify
> >> > + * it under the terms of the GNU General Public License version 2 as
> >> > + * published by the Free Software Foundation.
> >> > + */
> >> > +
> >> > +#ifndef __MAILBOX_CONTROLLER_DT_BINDINGS_H
> >> > +#define __MAILBOX_CONTROLLER_DT_BINDINGS_H
> >> > +
> >> > +#define MBOX_TX                0x1
> >> > +#define MBOX_RX                0x2
> >> > +#define MBOX_LOOPBACK  (MBOX_RX | MBOX_TX)
> >> > +
> >> Not sure I understand 'loopback'.  Does it mean h/w has some
> >> 'loopback' mode for testing purposes? Or it simply means the
> >> controller can send as well as receive messages?
> >
> > 'loopback' allows firmware to conduct some early function tests.
> > However, channels are simplex, so we provide protection against
> > multiple allocation of single channel.  By allocating a LOOPBACK
> > channel we over-ride this protection and allow a single channel to be
> > allocated twice, once for Rx and the other for Tx.
> >
> So basically hardware is half-duplex, not simplex. I think maybe you
> should simply allow for RX and TX always. It should work. Just
> handover any received data before send_data (reflecting the h/w
> limitation). That way you don't need any such special flag.

Unfortunately no, that's not correct.  Only Mailbox 0 is half-duplex.
The others are simplex (Rx only).  Ideally I'd like to keep the
LOOPBACK flag, as it's easier to figure out if what someone is
attempting to do is actually valid.  It is only ever used for
'loopback' testing anyway, so it's a perfectly reasonable
description of the channel configuration.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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