[Xen-devel] [PATCH v2 2/5] arm/early-printk: allow skipping of UART init

[Andre Przywara <andre.przywara@xxxxxxxxxxx>]

While it seems obvious to initialize the UART before using it, chances
are that some firmware code or the bootloader already did this.
So it may actually be a good idea to skip the initialization, in fact
this fixes early printk on my TC2 Versatile Express.
So provide an option in xen/arch/arm/Rules.mk to only initialize the
UART when needed.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxxx>
---
 docs/misc/arm/early-printk.txt | 4 ++++
 xen/arch/arm/Rules.mk          | 2 ++
 xen/arch/arm/arm32/head.S      | 2 ++
 xen/arch/arm/arm64/head.S      | 2 ++
 4 files changed, 10 insertions(+)

diff --git a/docs/misc/arm/early-printk.txt b/docs/misc/arm/early-printk.txt
index 10c3053..965add5 100644
--- a/docs/misc/arm/early-printk.txt
+++ b/docs/misc/arm/early-printk.txt
@@ -14,5 +14,9 @@ where mach is the name of the machine:
 
 The baud rate is hardcoded in xen/arch/arm/Rules.mk,
 see there when adding support for new machines.
+If not explicitly requested with "EARLY_PRINTK_INIT_UART := y" in Rules.mk,
+the code will not try to initialize the UART, so that bootloader or
+firmware settings can be used for maximum compatibility. The baud rate
+parameter is ignored in this case.
 
 By default early printk is disabled.
diff --git a/xen/arch/arm/Rules.mk b/xen/arch/arm/Rules.mk
index b4d6907..fdcf73e 100644
--- a/xen/arch/arm/Rules.mk
+++ b/xen/arch/arm/Rules.mk
@@ -49,6 +49,7 @@ EARLY_PRINTK_BAUD := 38400
 endif
 ifeq ($(CONFIG_EARLY_PRINTK), exynos5250)
 EARLY_PRINTK_INC := exynos4210
+EARLY_PRINTK_INIT_UART := y
 EARLY_PRINTK_BAUD := 115200
 endif
 
@@ -57,6 +58,7 @@ EARLY_PRINTK := y
 endif
 
 CFLAGS-$(EARLY_PRINTK) += -DEARLY_PRINTK
+CFLAGS-$(EARLY_PRINTK_INIT_UART) += -DEARLY_PRINTK_INIT_UART
 CFLAGS-$(EARLY_PRINTK) += -DEARLY_PRINTK_INC=\"debug-$(EARLY_PRINTK_INC).inc\"
 CFLAGS-$(EARLY_PRINTK) += -DEARLY_PRINTK_BAUD=$(EARLY_PRINTK_BAUD)
 endif
diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index ec7f640..0588d54 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -369,7 +369,9 @@ fail:   PRINT("- Boot failed -\r\n")
  * r11: Early UART base address
  * Clobbers r0-r2 */
 init_uart:
+#ifdef EARLY_PRINTK_INIT_UART
         early_uart_init r11, r1, r2
+#endif
         adr   r0, 1f
         b     puts                  /* Jump to puts */
 1:      .asciz "- UART enabled -\r\n"
diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 8955946..21b7e4d 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -116,7 +116,9 @@ boot_cpu:
 #ifdef EARLY_PRINTK
         ldr   x23, =EARLY_UART_BASE_ADDRESS /* x23 := UART base address */
         cbnz  x22, 1f
+#ifdef EARLY_PRINTK_INIT_UART
         bl    init_uart                 /* CPU 0 sets up the UART too */
+#endif
 1:      PRINT("- CPU ")
         mov   x0, x22
         bl    putn
-- 
1.7.12.1


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