Re: [PATCH 1/4] mailbox: arm_mhuv2: add device tree binding documentation

[Tushar Khandelwal <Tushar.Khandelwal@arm.com>]

On 02/08/2019, 11:59, "Sudeep Holla" <sudeep.holla@arm.com> wrote:

    On Sun, Jul 21, 2019 at 04:58:04PM -0500, Jassi Brar wrote:
    > On Wed, Jul 17, 2019 at 2:26 PM Tushar Khandelwal
    > <tushar.khandelwal@arm.com> wrote:
    >
    > > diff --git a/Documentation/devicetree/bindings/mailbox/arm,mhuv2.txt b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.txt
    > > new file mode 100644
    > > index 000000000000..3a05593414bc
    > > --- /dev/null
    > > +++ b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.txt
    > > @@ -0,0 +1,108 @@
    > > +Arm MHUv2 Mailbox Driver
    > > +========================
    > > +
    > > +The Arm Message-Handling-Unit (MHU) Version 2 is a mailbox controller that has
    > > +between 1 and 124 channel windows to provide unidirectional communication with
    > > +remote processor(s).
    > > +
    > > +Given the unidirectional nature of the device, an MHUv2 mailbox may only be
    > > +written to or read from. If a pair of MHU devices is implemented between two
    > > +processing elements to provide bidirectional communication, these must be
    > > +specified as two separate mailboxes.
    > > +
    > > +A device tree node for an Arm MHUv2 device must specify either a receiver frame
    > > +or a sender frame, indicating which end of the unidirectional MHU device which
    > > +the device node entry describes.
    > > +
    > > +An MHU device must be specified with a transport protocol. The transport
    > > +protocol of an MHU device determines the method of data transmission as well as
    > > +the number of provided mailboxes.
    > > +Following are the possible transport protocol types:
    > > +- Single-word: An MHU device implements as many mailboxes as it
    > > +               provides channel windows. Data is transmitted through
    > > +               the MHU registers.
    > > +- Multi-word:  An MHU device implements a single mailbox. All channel windows
    > > +               will be used during transmission. Data is transmitted through
    > > +               the MHU registers.
    > > +- Doorbell:    An MHU device implements as many mailboxes as there are flag
    > > +               bits available in its channel windows. Optionally, data may
    > > +               be transmitted through a shared memory region, wherein the MHU
    > > +               is used strictly as an interrupt generation mechanism.
    > > +
    > > +Mailbox Device Node:
    > > +====================
    > > +
    > > +Required properties:
    > > +--------------------
    > > +- compatible:  Shall be "arm,mhuv2" & "arm,primecell"
    > > +- reg:         Contains the mailbox register address range (base
    > > +               address and length)
    > > +- #mbox-cells  Shall be 1 - the index of the channel needed.
    > > +- mhu-frame    Frame type of the device.
    > > +               Shall be either "sender" or "receiver"
    > > +- mhu-protocol Transport protocol of the device. Shall be one of the
    > > +               following: "single-word", "multi-word", "doorbell"
    > > +
    > > +Required properties (receiver frame):
    > > +-------------------------------------
    > > +- interrupts:  Contains the interrupt information corresponding to the
    > > +               combined interrupt of the receiver frame
    > > +
    > > +Example:
    > > +--------
    > > +
    > > +       mbox_mw_tx: mhu@10000000 {
    > > +               compatible = "arm,mhuv2","arm,primecell";
    > > +               reg = <0x10000000 0x1000>;
    > > +               clocks = <&refclk100mhz>;
    > > +               clock-names = "apb_pclk";
    > > +               #mbox-cells = <1>;
    > > +               mhu-protocol = "multi-word";
    > > +               mhu-frame = "sender";
    > > +       };
    > > +
    > > +       mbox_sw_tx: mhu@10000000 {
    > > +               compatible = "arm,mhuv2","arm,primecell";
    > > +               reg = <0x11000000 0x1000>;
    > > +               clocks = <&refclk100mhz>;
    > > +               clock-names = "apb_pclk";
    > > +               #mbox-cells = <1>;
    > > +               mhu-protocol = "single-word";
    > > +               mhu-frame = "sender";
    > > +       };
    > > +
    > > +       mbox_db_rx: mhu@10000000 {
    > > +               compatible = "arm,mhuv2","arm,primecell";
    > > +               reg = <0x12000000 0x1000>;
    > > +               clocks = <&refclk100mhz>;
    > > +               clock-names = "apb_pclk";
    > > +               #mbox-cells = <1>;
    > > +               interrupts = <0 45 4>;
    > > +               interrupt-names = "mhu_rx";
    > > +               mhu-protocol = "doorbell";
    > > +               mhu-frame = "receiver";
    > > +       };
    > > +
    > > +       mhu_client: scb@2e000000 {
    > > +       compatible = "fujitsu,mb86s70-scb-1.0";
    > > +       reg = <0 0x2e000000 0x4000>;
    > > +       mboxes =
    > > +               // For multi-word frames, client may only instantiate a single
    > > +               // mailbox for a mailbox controller
    > > +               <&mbox_mw_tx 0>,
    > > +
    > > +               // For single-word frames, client may instantiate as many
    > > +               // mailboxes as there are channel windows in the MHU
    > > +                <&mbox_sw_tx 0>,
    > > +                <&mbox_sw_tx 1>,
    > > +                <&mbox_sw_tx 2>,
    > > +                <&mbox_sw_tx 3>,
    > > +
    > > +               // For doorbell frames, client may instantiate as many mailboxes
    > > +               // as there are bits available in the combined number of channel
    > > +               // windows ((channel windows * 32) mailboxes)
    > > +                <mbox_db_rx 0>,
    > > +                <mbox_db_rx 1>,
    > > +                ...
    > > +                <mbox_db_rx 17>;
    > > +       };
    >
    > If the mhuv2 instance implements, say, 3 channel windows between
    > sender (linux) and receiver (firmware), and Linux runs two protocols
    > each requiring 1 and 2-word sized messages respectively. The hardware
    > supports that by assigning windows [0] and [1,2] to each protocol.
    > However, I don't think the driver can support that. Or does it?
    >

    FWIW, the IP is designed to cover wide range of usecase from IoT to servers
    with variable window length. I don't see the need to complicate the driver
    supporting mix-n-match at the cost of latency. Each platform choose one
    transport protocol for all it's use.

The driver design is to address the most probable scenarios and not all.
Single-word : Client gets one 32-bit window
Doorbell : Client gets 32 data pointers (arm_message)
Multi-word: Client gets all channels available in the platform.

--Tushar
    --
    Regards,
    Sudeep


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