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    SubjectRe: [PATCH v2 2/6] Documentation: devicetree: add bindings to support ARM MHU doorbells
    On Wed, May 24, 2017 at 3:46 PM, Sudeep Holla <sudeep.holla@arm.com> wrote:
    > The ARM MHU has mechanism to assert interrupt signals to facilitate
    > inter-processor message based communication. It drives the signal using
    > a 32-bit register, with all 32-bits logically ORed together. It also
    > enables software to set, clear and check the status of each of the bits
    > of this register independently. Each bit of the register can be
    > associated with a type of event that can contribute to raising the
    > interrupt thereby allowing it to be used as independent doorbells.
    >
    > Since the first version of this binding can't support doorbells,
    > this patch extends the existing binding to support them.
    >
    > Cc: Alexey Klimov <alexey.klimov@arm.com>
    > Cc: Jassi Brar <jaswinder.singh@linaro.org>
    > Cc: Rob Herring <robh+dt@kernel.org>
    > Cc: devicetree@vger.kernel.org
    > Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
    > ---
    > .../devicetree/bindings/mailbox/arm-mhu.txt | 46 ++++++++++++++++++++--
    > 1 file changed, 43 insertions(+), 3 deletions(-)
    >
    > diff --git a/Documentation/devicetree/bindings/mailbox/arm-mhu.txt b/Documentation/devicetree/bindings/mailbox/arm-mhu.txt
    > index 4971f03f0b33..bd9a3a267caf 100644
    > --- a/Documentation/devicetree/bindings/mailbox/arm-mhu.txt
    > +++ b/Documentation/devicetree/bindings/mailbox/arm-mhu.txt
    > @@ -10,21 +10,42 @@ STAT register and the remote clears it after having read the data.
    > The last channel is specified to be a 'Secure' resource, hence can't be
    > used by Linux running NS.
    >
    > +The MHU drives the interrupt signal using a 32-bit register, with all
    > +32-bits logically ORed together. It provides a set of registers to
    > +enable software to set, clear and check the status of each of the bits
    > +of this register independently. The use of 32 bits per interrupt line
    > +enables software to provide more information about the source of the
    > +interrupt. For example, each bit of the register can be associated with
    > +a type of event that can contribute to raising the interrupt. Each of
    > +the 32-bits can be used as "doorbell" to alert the remote processor.
    > +
    > Mailbox Device Node:
    > ====================
    >
    > Required properties:
    > --------------------
    > -- compatible: Shall be "arm,mhu" & "arm,primecell"
    > +- compatible: Shall be "arm,primecell" and one of the below:
    > + "arm,mhu" - if the controller doesn't support
    > + doorbell model
    > + "arm,mhu-doorbell" - if the controller supports
    > + doorbell model
    > - reg: Contains the mailbox register address range (base
    > address and length)
    > -- #mbox-cells Shall be 1 - the index of the channel needed.
    > +- #mbox-cells Shall be 1 - the index of the channel needed when
    > + compatible is "arm,mhu"
    > + Shall be 2 - the index of the channel needed, and
    > + the index of the doorbell bit with the channel when
    > + compatible is "arm,mhu-doorbell"
    > - interrupts: Contains the interrupt information corresponding to
    > - each of the 3 links of MHU.
    > + each of the 3 physical channels of MHU namely low
    > + priority non-secure, high priority non-secure and
    > + secure channels.
    >
    > Example:
    > --------
    >
    > +1. Controller which doesn't support doorbells
    > +
    > mhu: mailbox@2b1f0000 {
    > #mbox-cells = <1>;
    > compatible = "arm,mhu", "arm,primecell";
    > @@ -41,3 +62,22 @@ used by Linux running NS.
    > reg = <0 0x2e000000 0x4000>;
    > mboxes = <&mhu 1>; /* HP-NonSecure */
    > };
    > +
    > +2. Controller which supports doorbells
    > +
    > + mhu: mailbox@2b1f0000 {
    > + #mbox-cells = <2>;
    > + compatible = "arm,mhu-doorbell", "arm,primecell";
    > + reg = <0 0x2b1f0000 0x1000>;
    > + interrupts = <0 36 4>, /* LP-NonSecure */
    > + <0 35 4>, /* HP-NonSecure */
    > + <0 37 4>; /* Secure */
    > + clocks = <&clock 0 2 1>;
    > + clock-names = "apb_pclk";
    > + };
    > +
    > + mhu_client: scb@2e000000 {
    > + compatible = "arm,scpi";
    > + reg = <0 0x2e000000 0x200>;
    > + mboxes = <&mhu 1 4>; /* HP-NonSecure 5th doorbell bit */
    > + };
    >
    Every MHU controller can by driven as "arm,mhu-doorbell" or "arm,mhu"
    equally fine. So you are basically smuggling a s/w feature into DT.

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