Run an arbitrary SPI flash command.

        This function uses the "USR_COMMAND" functionality in the ESP
        SPI hardware, rather than the precanned commands supported by
        hardware. So the value of spiflash_command is an actual command
        byte, sent over the wire.

        After writing command byte, writes 'data' to MOSI and then
        reads back 'read_bits' of reply on MISO. Result is a number.

def run_spiflash_command(self, spiflash_command, data=b"", read_bits=0):
        """Run an arbitrary SPI flash command.

        This function uses the "USR_COMMAND" functionality in the ESP
        SPI hardware, rather than the precanned commands supported by
        hardware. So the value of spiflash_command is an actual command
        byte, sent over the wire.

        After writing command byte, writes 'data' to MOSI and then
        reads back 'read_bits' of reply on MISO. Result is a number.
        """

        # SPI_USR register flags
        SPI_USR_COMMAND = (1 << 31)
        SPI_USR_MISO    = (1 << 28)
        SPI_USR_MOSI    = (1 << 27)

        # SPI registers, base address differs ESP32 vs 8266
        base = self.SPI_REG_BASE
        SPI_CMD_REG       = base + 0x00
        SPI_USR_REG       = base + 0x1C
        SPI_USR1_REG      = base + 0x20
        SPI_USR2_REG      = base + 0x24
        SPI_W0_REG        = base + self.SPI_W0_OFFS

        # following two registers are ESP32 only
        if self.SPI_HAS_MOSI_DLEN_REG:
            # ESP32 has a more sophisticated wayto set up "user" commands
            def set_data_lengths(mosi_bits, miso_bits):
                SPI_MOSI_DLEN_REG = base + 0x28
                SPI_MISO_DLEN_REG = base + 0x2C
                if mosi_bits > 0:
                    self.write_reg(SPI_MOSI_DLEN_REG, mosi_bits - 1)
                if miso_bits > 0:
                    self.write_reg(SPI_MISO_DLEN_REG, miso_bits - 1)
        else:

            def set_data_lengths(mosi_bits, miso_bits):
                SPI_DATA_LEN_REG = SPI_USR1_REG
                SPI_MOSI_BITLEN_S = 17
                SPI_MISO_BITLEN_S = 8
                mosi_mask = 0 if (mosi_bits == 0) else (mosi_bits - 1)
                miso_mask = 0 if (miso_bits == 0) else (miso_bits - 1)
                self.write_reg(SPI_DATA_LEN_REG,
                               (miso_mask << SPI_MISO_BITLEN_S) | (
                                   mosi_mask << SPI_MOSI_BITLEN_S))

        # SPI peripheral "command" bitmasks for SPI_CMD_REG
        SPI_CMD_USR  = (1 << 18)

        # shift values
        SPI_USR2_DLEN_SHIFT = 28

        if read_bits > 32:
            raise FatalError("Reading more than 32 bits back from a SPI flash operation is unsupported")
        if len(data) > 64:
            raise FatalError("Writing more than 64 bytes of data with one SPI command is unsupported")

        data_bits = len(data) * 8
        old_spi_usr = self.read_reg(SPI_USR_REG)
        old_spi_usr2 = self.read_reg(SPI_USR2_REG)
        flags = SPI_USR_COMMAND
        if read_bits > 0:
            flags |= SPI_USR_MISO
        if data_bits > 0:
            flags |= SPI_USR_MOSI
        set_data_lengths(data_bits, read_bits)
        self.write_reg(SPI_USR_REG, flags)
        self.write_reg(SPI_USR2_REG,
                       (7 << SPI_USR2_DLEN_SHIFT) | spiflash_command)
        if data_bits == 0:
            self.write_reg(SPI_W0_REG, 0)  # clear data register before we read it
        else:
            data = pad_to(data, 4, b'\00')  # pad to 32-bit multiple
            words = struct.unpack("I" * (len(data) // 4), data)
            next_reg = SPI_W0_REG
            for word in words:
                self.write_reg(next_reg, word)
                next_reg += 4
        self.write_reg(SPI_CMD_REG, SPI_CMD_USR)

        def wait_done():
            for _ in range(10):
                if (self.read_reg(SPI_CMD_REG) & SPI_CMD_USR) == 0:
                    return
            raise FatalError("SPI command did not complete in time")
        wait_done()

        status = self.read_reg(SPI_W0_REG)
        # restore some SPI controller registers
        self.write_reg(SPI_USR_REG, old_spi_usr)
        self.write_reg(SPI_USR2_REG, old_spi_usr2)
        return status