console: 16550: Prepare for skipping initialisation

On some platforms the UART might have already been initialised, for
instance by firmware running before TF-A or by a separate management
processor. In this case it would not be need to initialise it again
(doing so could create spurious characters). But more importantly this
saves us from knowing the right baudrate and the right base clock rate
for the UART. This can lead to more robust and versatile firmware builds.

Allow to skip the 16550 UART initialisation and baud rate divisor
programming, by interpreting an input clock rate of "0" to signify this
case. This will just skip the call to console_16550_core_init, but still
will register the console properly.

Users should just pass 0 as the second parameter, the baudrate (third
parameter) will then be ignored as well.

Fix copy & paste typos in comments for the console_16550_register()
function on the way.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I9f8fca5b358f878fac0f31dc411358fd160786ee

drivers/ti/uart/aarch32/16550_console.S

@@ -89,16 +89,19 @@ endfunc console_16550_core_init
 	.globl console_16550_register
 
 	/* -------------------------------------------------------
-	 * int console_stm32_register(uintptr_t baseaddr,
+	 * int console_16550_register(uintptr_t baseaddr,
 	 *     uint32_t clock, uint32_t baud,
-	 *     struct console_stm32 *console);
-	 * Function to initialize and register a new STM32
+	 *     console_16550_t *console);
+	 * Function to initialize and register a new 16550
 	 * console. Storage passed in for the console struct
 	 * *must* be persistent (i.e. not from the stack).
+	 * If r1 (UART clock) is 0, initialisation will be
+         * skipped, relying on previous code to have done
+         * this already. r2 is ignored then as well.
 	 * In: r0 - UART register base address
 	 *     r1 - UART clock in Hz
-	 *     r2 - Baud rate
-	 *     r3 - pointer to empty console_stm32 struct
+	 *     r2 - Baud rate (ignored if r1 is 0)
+	 *     r3 - pointer to empty console_16550_t struct
 	 * Out: return 1 on success, 0 on error
 	 * Clobber list : r0, r1, r2
 	 * -------------------------------------------------------
@@ -110,10 +113,15 @@ func console_16550_register
 	beq	register_fail
 	str	r0, [r4, #CONSOLE_T_16550_BASE]
 
+	/* A clock rate of zero means to skip the initialisation. */
+	cmp	r1, #0
+	beq	register_16550
+
 	bl	console_16550_core_init
 	cmp	r0, #0
 	beq	register_fail
 
+register_16550:
 	mov	r0, r4
 	pop	{r4, lr}
 	finish_console_register 16550 putc=1, getc=1, flush=1

drivers/ti/uart/aarch64/16550_console.S

@@ -92,9 +92,12 @@ endfunc console_16550_core_init
 	 * Function to initialize and register a new 16550
 	 * console. Storage passed in for the console struct
 	 * *must* be persistent (i.e. not from the stack).
+	 * If w1 (UART clock) is 0, initialisation will be
+	 * skipped, relying on previous code to have done
+	 * this already. w2 is ignored then as well.
 	 * In: x0 - UART register base address
 	 *     w1 - UART clock in Hz
-	 *     w2 - Baud rate
+	 *     w2 - Baud rate (ignored if w1 is 0)
 	 *     x3 - pointer to empty console_16550_t struct
 	 * Out: return 1 on success, 0 on error
 	 * Clobber list : x0, x1, x2, x6, x7, x14
@@ -106,9 +109,13 @@ func console_16550_register
 	cbz	x6, register_fail
 	str	x0, [x6, #CONSOLE_T_16550_BASE]
 
+	/* A clock rate of zero means to skip the initialisation. */
+	cbz	w1, register_16550
+
 	bl	console_16550_core_init
 	cbz	x0, register_fail
 
+register_16550:
 	mov	x0, x6
 	mov	x30, x7
 	finish_console_register 16550 putc=1, getc=1, flush=1

include/drivers/ti/uart/uart_16550.h

@@ -87,6 +87,11 @@ typedef struct {
  * framework. The |console| pointer must point to storage that will be valid
  * for the lifetime of the console, such as a global or static local variable.
  * Its contents will be reinitialized from scratch.
+ * When |clock| has a value of 0, the UART will *not* be initialised. This
+ * means the UART should already be enabled and the baudrate and clock setup
+ * should have been done already, either by platform specific code or by
+ * previous firmware stages. The |baud| parameter will be ignored in this
+ * case as well.
  */
 int console_16550_register(uintptr_t baseaddr, uint32_t clock, uint32_t baud,
 			   console_16550_t *console);