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

[Sudeep Holla <sudeep.holla@arm.com>]

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.

--
Regards,
Sudeep