drivers: imx: crash-console: Add a mxc_crash_console driver

This patch does two main things

- It implements the crash console UART init in assembly, as a
  hard-coded 115200 8N1 assumed from the 24 MHz clock.

  If the clock setup code has not run yet, this code can't work but,
  setting up clocks and clock-gates is way out of scope for this type of
  recovery function.

- It adds code to write a character out of the NXP UART without using any
  stack-based operations when doing so.

- Provides support for crash console in DCE or DTE mode.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>

drivers/imx/uart/imx_crash_uart.S

+/*
+ * Copyright (c) Linaro 2018 Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+#include <arch.h>
+#include <asm_macros.S>
+#include <assert_macros.S>
+#include <imx_uart.h>
+#include <platform_def.h>
+
+	.globl	imx_crash_uart_init
+	.globl	imx_crash_uart_putc
+
+	/* -----------------------------------------------
+	 * int imx_crash_uart_init(uintptr_t base_addr,
+	 * unsigned int uart_clk, unsigned int baud_rate)
+	 * Function to initialize the console without a
+	 * C Runtime to print debug information. This
+	 * function will be accessed by console_init and
+	 * crash reporting.
+	 * In: r0 - console base address
+	 *     r1 - Uart clock in Hz
+	 *     r2 - Baud rate
+	 * Out: return 1 on success else 0 on error
+	 * Clobber list : r1, r2, r3, r4
+	 * -----------------------------------------------
+	 */
+func imx_crash_uart_init
+	/* Free up r1 as a scratch reg */
+	mov     r4, r0
+	mov     r0, r1
+
+	/* Reset UART via CR2 */
+	add     r1, r4, #IMX_UART_CR2_OFFSET
+	movs    r3, #0
+	str     r3, [r4, #IMX_UART_CR2_OFFSET]
+
+	/* Wait for reset complete */
+__wait_cr2_reset:
+	ldr     r3, [r1, #0]
+	ands    r3, #IMX_UART_CR2_SRST
+	beq     __wait_cr2_reset
+
+	/* Enable UART */
+	movs    r3, #IMX_UART_CR1_UARTEN
+	mov     r1, r2
+	str     r3, [r4, #IMX_UART_CR1_OFFSET]
+
+	/*
+	 * Ignore RTC/CTS - disable reset
+	 * Magic value #16423 =>
+	 * IMX_UART_CR2_IRTS | IMX_UART_CR2_WS | IMX_UART_CR2_TXEN | IMX_UART_CR2_RXEN | IMX_UART_CR2_SRST
+	 */
+	movw    r3, #16423
+	str     r3, [r4, #IMX_UART_CR2_OFFSET]
+
+	/*
+	 * No parity, autobaud detect-old, rxdmuxsel=1 (fixed i.mx7)
+	 * Magic value => #132
+	 * IMX_UART_CR3_ADNIMP | IMX_UART_CR3_RXDMUXSEL
+	 */
+	movs    r3, #132
+	str     r3, [r4, #IMX_UART_CR3_OFFSET]
+
+	/*
+	 * Set CTS FIFO trigger to 32 bytes bits 15:10
+	 * Magic value => #32768
+	 * FIFO trigger bitmask 100000
+	 * */
+	mov     r3, #32768
+	str     r3, [r4, #IMX_UART_CR4_OFFSET]
+
+	/*
+	 * TX/RX-thresh = 2 bytes, DCE (bit6 = 0), refclk @24MHz / 4
+	 * Magic value #2562
+	 * IMX_UART_FCR_TXTL(TX_RX_THRESH) | IMX_UART_FCR_RXTL(TX_RX_THRESH) | IMX_UART_FCR_RFDIV2
+	 */
+	#ifdef IMX_UART_DTE
+	movw    r3, #2626
+	#else
+	movw    r3, #2562
+	#endif
+	str     r3, [r4, #IMX_UART_FCR_OFFSET]
+
+	/* This BIR should be set to 0x0F prior to writing the BMR */
+	movs    r3, #15
+	str     r3, [r4, #IMX_UART_BIR_OFFSET]
+
+	/* Hard-code to 115200 @ 24 MHz */
+	movs	r0, #104
+	str     r0, [r4, #IMX_UART_BMR_OFFSET]
+
+	/* Indicate success */
+	movs    r0, #1
+	bx	lr
+endfunc imx_crash_uart_init
+
+	/* --------------------------------------------------------
+	 * int imx_crash_uart_putc(int c, uintptr_t base_addr)
+	 * Function to output a character over the console. It
+	 * returns the character printed on success or -1 on error.
+	 * In : r0 - character to be printed
+	 *      r1 - console base address
+	 * Out : return -1 on error else return character.
+	 * Clobber list : r2
+	 * --------------------------------------------------------
+	 */
+func imx_crash_uart_putc
+	/* Output specified character to UART shift-register */
+	str	r0, [r1, #IMX_UART_TXD_OFFSET]
+
+        /* Wait for transmit IMX_UART_STAT2_OFFSET.IMX_UART_STAT2_TXDC == 1 */
+__putc_spin_ready:
+	ldr	r2, [r1, #IMX_UART_STAT2_OFFSET]
+	ands	r2, #IMX_UART_STAT2_TXDC
+	beq	__putc_spin_ready
+
+        /* Transmit complete do we need to fixup \n to \n\r */
+	cmp	r0, #10
+	beq	__putc_fixup_lf
+
+	/* No fixup necessary - exit here */
+	movs	r0, #0
+	bx	lr
+
+	/* Fixup \n to \n\r */
+__putc_fixup_lf:
+	movs	r0, #13
+	b	imx_crash_uart_putc
+endfunc imx_crash_uart_putc