doc: riscv: Add documentation for Sipeed Maix Bit

This patch adds documentation for the Sipeed Maix bit, and more generally
for the Kendryte K210 processor.

Signed-off-by: Sean Anderson <seanga2@gmail.com>

3 files changed, 308 insertions, 0 deletions

diff --git a/doc/board/index.rst b/doc/board/index.rst
index 19733a6398..0a15899180 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -18,6 +18,7 @@ Board-specific doc
    renesas/index
    rockchip/index
    sifive/index
+   sipeed/index
    st/index
    tbs/index
    toradex/index
diff --git a/doc/board/sipeed/index.rst b/doc/board/sipeed/index.rst
new file mode 100644
index 0000000000..3518e2d8f4
--- /dev/null
+++ b/doc/board/sipeed/index.rst
@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Sipeed
+======
+
+.. toctree::
+   :maxdepth: 2
+
+   maix
diff --git a/doc/board/sipeed/maix.rst b/doc/board/sipeed/maix.rst
new file mode 100644
index 0000000000..06e0008b9f
--- /dev/null
+++ b/doc/board/sipeed/maix.rst
@@ -0,0 +1,298 @@
+.. 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
+========== ========= ===========