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SubjectRe: [PATCH 1/4] mailbox: arm_mhuv2: add device tree binding documentation
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On 7/25/19 7:49 AM, Jassi Brar wrote:
> On Sun, Jul 21, 2019 at 4:58 PM Jassi Brar <jassisinghbrar@gmail.com> 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?
>>
> Thinking about it, IMO, the mbox-cell should carry a 128 (4x32) bit
> mask specifying the set of windows (corresponding to the bits set in
> the mask) associated with the channel.
> And the controller driver should see any channel as associated with
> variable number of windows 'N', where N is [0,124]
>
> mhu_client1: proto1@2e000000 {
> .....
> mboxes = <&mbox 0x0 0x0 0x0 0x1>
> }
>
> mhu_client2: proto2@2f000000 {
> .....
> mboxes = <&mbox 0x0 0x0 0x0 0x6>
> }
>
> Cheers!
>

As mentioned in the response to your initial comment, the driver does
not currently support mixing protocols.
If mixing protocols is to be supported in the future, then this seems
like a suitable way of specifying which channels are associated with
which mailboxes (especially for mixing single- and multi-word modes).
However, there still is an issue in that both single-word and doorbell
requires only 1 channel window - and as such, the transport protocol
cannot be deduced from merely the number of masked channel windows.
Furthermore, for doorbell, a mbox may be registered for _each_ available
bit within a channel window (further complicating things if we were to
include mixing protocols in this initial driver version), making
assigning channel windows to mailboxes semantically different from when
assigning to single- or multi-word.

Thanks,
Morten
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