plat: rpi4: Skip UART initialisation

So far we have seen two different clock setups for the Raspberry Pi 4
board, with the VPU clock divider being different. This was handled by
reading the divider register and adjusting the base clock rate
accordingly.
Recently a new GPU firmware version appeared that changed the clock rate
*again*, though this time at a higher level, so the VPU rate (and the
apparent PLLC parent clock) did not seem to change, judging by reading
the clock registers.
So rather than playing cat and mouse with the GPU firmware or going
further down the rabbit hole of exploring the whole clock tree, let's
just skip the baud rate programming altogether. This works because the
GPU firmware actually sets up and programs the debug UART already, so
we can just use it.

Pass 0 as the base clock rate to let the console driver skip the setup,
also remove the no longer needed clock code.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Ica88a3f3c9c11059357c1e6dd8f7a4d9b1f98fd7

plat/rpi/rpi4/aarch64/plat_helpers.S

@@ -136,8 +136,8 @@ endfunc platform_mem_init
 	 */
 func plat_crash_console_init
 	mov_imm	x0, PLAT_RPI3_UART_BASE
-	mov_imm	x1, PLAT_RPI4_VPU_CLK_RATE
-	mov_imm	x2, PLAT_RPI3_UART_BAUDRATE
+	mov	x1, xzr
+	mov	x2, xzr
 	b	console_16550_core_init
 endfunc plat_crash_console_init
 

plat/rpi/rpi4/include/rpi_hw.h

@@ -58,13 +58,6 @@
  */
 #define RPI3_PM_RSTS_WRCFG_HALT		U(0x00000555)
 
-/*
- * Clock controller
- */
-#define RPI4_IO_CLOCK_OFFSET		ULL(0x00101000)
-#define RPI4_CLOCK_BASE			(RPI_IO_BASE + RPI4_IO_CLOCK_OFFSET)
-#define RPI4_VPU_CLOCK_DIVIDER		ULL(0x0000000c)
-
 /*
  * Hardware random number generator.
  */
@@ -88,7 +81,6 @@
  */
 #define RPI3_IO_MINI_UART_OFFSET	ULL(0x00215040)
 #define RPI3_MINI_UART_BASE		(RPI_IO_BASE + RPI3_IO_MINI_UART_OFFSET)
-#define PLAT_RPI4_VPU_CLK_RATE		ULL(1000000000)
 
 /*
  * GPIO controller

plat/rpi/rpi4/rpi4_bl31_setup.c

@@ -119,8 +119,6 @@ void bl31_early_platform_setup2(u_register_t arg0, u_register_t arg1,
 				u_register_t arg2, u_register_t arg3)
 
 {
-	uint32_t div_reg;
-
 	/*
 	 * LOCAL_CONTROL:
 	 * Bit 9 clear: Increment by 1 (vs. 2).
@@ -136,16 +134,12 @@ void bl31_early_platform_setup2(u_register_t arg0, u_register_t arg1,
 
 	/*
 	 * Initialize the console to provide early debug support.
-	 * Different GPU firmware revisions set up the VPU divider differently,
-	 * so read the actual divider register to learn the UART base clock
-	 * rate. The divider is encoded as a 12.12 fixed point number, but we
-	 * just care about the integer part of it.
+	 * We rely on the GPU firmware to have initialised the UART correctly,
+	 * as the baud base clock rate differs across GPU firmware revisions.
+	 * Providing a base clock of 0 lets the 16550 UART init routine skip
+	 * the initial enablement and baud rate setup.
 	 */
-	div_reg = mmio_read_32(RPI4_CLOCK_BASE + RPI4_VPU_CLOCK_DIVIDER);
-	div_reg = (div_reg >> 12) & 0xfff;
-	if (div_reg == 0)
-		div_reg = 1;
-	rpi3_console_init(PLAT_RPI4_VPU_CLK_RATE / div_reg);
+	rpi3_console_init(0);
 
 	bl33_image_ep_info.pc = plat_get_ns_image_entrypoint();
 	bl33_image_ep_info.spsr = rpi3_get_spsr_for_bl33_entry();