.. SPDX-License-Identifier: GPL-2.0+
.. Copyright (C) 2020 Sean Anderson <seanga2@gmail.com>

Maix Bit
========

Several of the Sipeed Maix series of boards cotain the Kendryte K210 processor,
a 64-bit RISC-V CPU. This processor contains several peripherals to accelerate
neural network processing and other "ai" tasks. This includes a "KPU" neural
network processor, an audio processor supporting beamforming reception, and a
digital video port supporting capture and output at VGA resolution. Other
peripherals include 8M of SRAM (accessible with and without caching); remappable
pins, including 40 GPIOs; AES, FFT, and SHA256 accelerators; a DMA controller;
and I2C, I2S, and SPI controllers. Maix peripherals vary, but include spi flash;
on-board usb-serial bridges; ports for cameras, displays, and sd cards; and
ESP32 chips. Currently, only the Sipeed Maix Bit V2.0 (bitm) is supported, but
the boards are fairly similar.

Documentation for Maix boards is available from
`Sipeed's website <http://dl.sipeed.com/MAIX/HDK/>`_.
Documentation for the Kendryte K210 is available from
`Kendryte's website <https://kendryte.com/downloads/>`_. However, hardware
details are rather lacking, so most technical reference has been taken from the
`standalone sdk <https://github.com/kendryte/kendryte-standalone-sdk>`_.

Build and boot steps
--------------------

To build u-boot, run

.. code-block:: none

    make sipeed_maix_bitm_defconfig
    make CROSS_COMPILE=<your cross compile prefix>

To flash u-boot to a maix bit, run

.. code-block:: none

    kflash -tp /dev/<your tty here> -B bit_mic u-boot-dtb.bin

Boot output should look like the following:

.. code-block:: none

    U-Boot 2020.04-rc2-00087-g2221cc09c1-dirty (Feb 28 2020 - 13:53:09 -0500)

    DRAM:  8 MiB
    In:    serial@38000000
    Out:   serial@38000000
    Err:   serial@38000000
    =>

Loading Images
^^^^^^^^^^^^^^

To load a kernel, transfer it over serial.

.. code-block:: none

    => loady 80000000 1500000
    ## Switch baudrate to 1500000 bps and press ENTER ...

    *** baud: 1500000

    *** baud: 1500000 ***
    ## Ready for binary (ymodem) download to 0x80000000 at 1500000 bps...
    C
    *** file: loader.bin
    $ sz -vv loader.bin
    Sending: loader.bin
    Bytes Sent:2478208   BPS:72937
    Sending:
    Ymodem sectors/kbytes sent:   0/ 0k
    Transfer complete

    *** exit status: 0 ***
    ## Total Size      = 0x0025d052 = 2478162 Bytes
    ## Switch baudrate to 115200 bps and press ESC ...

    *** baud: 115200

    *** baud: 115200 ***
    =>

Running Programs
^^^^^^^^^^^^^^^^

Binaries
""""""""

To run a bare binary, use the ``go`` command:

.. code-block:: none

    => loady
    ## Ready for binary (ymodem) download to 0x80000000 at 115200 bps...
    C
    *** file: ./examples/standalone/hello_world.bin
    $ sz -vv ./examples/standalone/hello_world.bin
    Sending: hello_world.bin
    Bytes Sent:   4864   BPS:649
    Sending:
    Ymodem sectors/kbytes sent:   0/ 0k
    Transfer complete

    *** exit status: 0 ***
    (CAN) packets, 5 retries
    ## Total Size      = 0x000012f8 = 4856 Bytes
    => go 80000000
    ## Starting application at 0x80000000 ...
    Example expects ABI version 9
    Actual U-Boot ABI version 9
    Hello World
    argc = 1
    argv[0] = "80000000"
    argv[1] = "<NULL>"
    Hit any key to exit ...

Legacy Images
"""""""""""""

To run legacy images, use the ``bootm`` command:

.. code-block:: none

    $ tools/mkimage -A riscv -O u-boot -T standalone -C none -a 80000000 -e 80000000 -d examples/standalone/hello_world.bin hello_world.img
    Image Name:
    Created:      Thu Mar  5 12:04:10 2020
    Image Type:   RISC-V U-Boot Standalone Program (uncompressed)
    Data Size:    4856 Bytes = 4.74 KiB = 0.00 MiB
    Load Address: 80000000
    Entry Point:  80000000

    $ picocom -b 115200 /dev/ttyUSB0i
    => loady
    ## Ready for binary (ymodem) download to 0x80000000 at 115200 bps...
    C
    *** file: hello_world.img
    $ sz -vv hello_world.img
    Sending: hello_world.img
    Bytes Sent:   4992   BPS:665
    Sending:
    Ymodem sectors/kbytes sent:   0/ 0k
    Transfer complete

    *** exit status: 0 ***
    CAN) packets, 3 retries
    ## Total Size      = 0x00001338 = 4920 Bytes
    => bootm
    ## Booting kernel from Legacy Image at 80000000 ...
       Image Name:
       Image Type:   RISC-V U-Boot Standalone Program (uncompressed)
       Data Size:    4856 Bytes = 4.7 KiB
       Load Address: 80000000
       Entry Point:  80000000
       Verifying Checksum ... OK
       Loading Standalone Program
    Example expects ABI version 9
    Actual U-Boot ABI version 9
    Hello World
    argc = 0
    argv[0] = "<NULL>"
    Hit any key to exit ...

Over- and Under-clocking
------------------------

To change the clock speed of the K210, you will need to enable
``CONFIG_CLK_K210_SET_RATE`` and edit the board's device tree. To do this, add a
section to ``arch/riscv/arch/riscv/dts/k210-maix-bit.dts`` like the following:

.. code-block:: none

    &sysclk {
	assigned-clocks = <&sysclk K210_CLK_PLL0>;
	assigned-clock-rates = <800000000>;
    };

There are three PLLs on the K210: PLL0 is the parent of most of the components,
including the CPU and RAM. PLL1 is the parent of the neural network coprocessor.
PLL2 is the parent of the sound processing devices. Note that child clocks of
PLL0 and PLL2 run at *half* the speed of the PLLs. For example, if PLL0 is
running at 800 MHz, then the CPU will run at 400 MHz. This is the example given
above. The CPU can be overclocked to around 600 MHz, and underclocked to 26 MHz.

It is possible to set PLL2's parent to PLL0. The plls are more accurate when
converting between similar frequencies. This makes it easier to get an accurate
frequency for I2S. As an example, consider sampling an I2S device at 44.1 kHz.
On this device, the I2S serial clock runs at 64 times the sample rate.
Therefore, we would like to run PLL2 at an even multiple of 2.8224 MHz. If
PLL2's parent is IN0, we could use a frequency of 390 MHz (the same as the CPU's
default speed).  Dividing by 138 yields a serial clock of about 2.8261 MHz. This
results in a sample rate of 44.158 kHz---around 50 Hz or .1% too fast. If,
instead, we set PLL2's parent to PLL1 running at 390 MHz, and request a rate of
2.8224 * 136 = 383.8464 MHz, the achieved rate is 383.90625 MHz. Dividing by 136
yields a serial clock of about 2.8228 MHz. This results in a sample rate of
44.107 kHz---just 7 Hz or .02% too fast. This configuration is shown in the
following example:

.. code-block:: none

    &sysclk {
	assigned-clocks = <&sysclk K210_CLK_PLL1>, <&sysclk K210_CLK_PLL2>;
	assigned-clock-parents = <0>, <&sysclk K210_CLK_PLL1>;
	assigned-clock-rates = <390000000>, <383846400>;
    };

There are a couple of quirks to the PLLs. First, there are more frequency ratios
just above and below 1.0, but there is a small gap around 1.0. To be explicit,
if the input frequency is 100 MHz, it would be impossible to have an output of
99 or 101 MHz. In addition, there is a maximum frequency for the internal VCO,
so higher input/output frequencies will be less accurate than lower ones.

Technical Details
-----------------

Boot Sequence
^^^^^^^^^^^^^

1. ``RESET`` pin is deasserted.
2. Both harts begin executing at ``0x00001000``.
3. Both harts jump to firmware at ``0x88000000``.
4. One hart is chosen as a boot hart.
5. Firmware reads value of pin ``IO_16`` (ISP).

   * If the pin is low, enter ISP mode. This mode allows loading data to ram,
     writing it to flash, and booting from specific addresses.
   * If the pin is high, continue boot.
6. Firmware reads the next stage from flash (SPI3) to address ``0x80000000``.

   * If byte 0 is 1, the next stage is decrypted using the built-in AES
     accelerator and the one-time programmable, 128-bit AES key.
   * Bytes 1 to 4 hold the length of the next stage.
   * The SHA-256 sum of the next stage is automatically calculated, and verified
     against the 32 bytes following the next stage.
7. The boot hart sends an IPI to the other hart telling it to jump to the next
   stage.
8. The boot hart jumps to ``0x80000000``.

Memory Map
^^^^^^^^^^

========== ========= ===========
Address    Size      Description
========== ========= ===========
0x00000000 0x1000    debug
0x00001000 0x1000    rom
0x02000000 0xC000    clint
0x0C000000 0x4000000 plic
0x38000000 0x1000    uarths
0x38001000 0x1000    gpiohs
0x40000000 0x400000  sram0 (non-cached)
0x40400000 0x200000  sram1 (non-cached)
0x40600000 0x200000  airam (non-cached)
0x40800000 0xC00000  kpu
0x42000000 0x400000  fft
0x50000000 0x1000    dmac
0x50200000 0x200000  apb0
0x50200000 0x80      gpio
0x50210000 0x100     uart0
0x50220000 0x100     uart1
0x50230000 0x100     uart2
0x50240000 0x100     spi slave
0x50250000 0x200     i2s0
0x50250200 0x200     apu
0x50260000 0x200     i2s1
0x50270000 0x200     i2s2
0x50280000 0x100     i2c0
0x50290000 0x100     i2c1
0x502A0000 0x100     i2c2
0x502B0000 0x100     fpioa
0x502C0000 0x100     sha256
0x502D0000 0x100     timer0
0x502E0000 0x100     timer1
0x502F0000 0x100     timer2
0x50400000 0x200000  apb1
0x50400000 0x100     wdt0
0x50410000 0x100     wdt1
0x50420000 0x100     otp control
0x50430000 0x100     dvp
0x50440000 0x100     sysctl
0x50450000 0x100     aes
0x50460000 0x100     rtc
0x52000000 0x4000000 apb2
0x52000000 0x100     spi0
0x53000000 0x100     spi1
0x54000000 0x200     spi3
0x80000000 0x400000  sram0 (cached)
0x80400000 0x200000  sram1 (cached)
0x80600000 0x200000  airam (cached)
0x88000000 0x20000   otp
0x88000000 0xC200    firmware
0x8801C000 0x1000    riscv priv spec 1.9 config
0x8801D000 0x2000    flattened device tree (contains only addresses and
                     interrupts)
0x8801f000 0x1000    credits
========== ========= ===========
