Date:   Wed, 31 Jul 2019 02:31:48 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Morten Borup Petersen <morten_bp@...e.dk>
Cc:     Tushar Khandelwal <tushar.khandelwal@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "tushar.2nov@...il.com" <tushar.2nov@...il.com>,
        "nd@....com" <nd@....com>,
        Morten Borup Petersen <morten.petersen@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Devicetree List <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/4] mailbox: arm_mhuv2: add device tree binding documentation

On Sun, Jul 28, 2019 at 4:28 PM Morten Borup Petersen <morten_bp@...e.dk> wrote:
>
>
>
> On 7/25/19 7:49 AM, Jassi Brar wrote:
> > On Sun, Jul 21, 2019 at 4:58 PM Jassi Brar <jassisinghbrar@...il.com> wrote:
> >>
> >> On Wed, Jul 17, 2019 at 2:26 PM Tushar Khandelwal
> >> <tushar.khandelwal@....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@...00000 {
> >>> +               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@...00000 {
> >>> +               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@...00000 {
> >>> +               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@...00000 {
> >>> +       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@...00000 {
> >        .....
> >        mboxes = <&mbox 0x0 0x0 0x0 0x1>
> > }
> >
> > mhu_client2: proto2@...00000 {
> >        .....
> >        mboxes = <&mbox 0x0 0x0 0x0 0x6>
> > }
> >
> > Cheers!
> >
>
> As mentioned in the response to your initial comment, the driver does
> not currently support mixing protocols.
>
Thanks for acknowledging that limitation. But lets also address it.

> 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).
>
We can not change DT bindings again when we feel like updating the driver.
The bindings should precisely and completely define the h/w, not what
mode we currently implement.
It is not for pure idealism, it actually makes the code simpler and futureproof.

> 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.
>
I don't see why the driver should worry -- the channel carries 32-bit
message or some random value just to trigger an interrupt is purely
upto the client driver.

> 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.
>
Not sure about that, that would be implementing virtual channels. Each
window carries one signal, and that is the minimum bandwidth assigned
to a channel.

Thanks

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