[Xen-devel] [PATCH] xen/arm: avoid lost characters with early_printk

[Julien Grall <julien.grall@xxxxxxxxxx>]

Introduce the function early_flush to wait until the UART has finished to
transfer the character.
When early printk is enabled, it's possible to loose the last characters if:
    - the UART is initialized by the UART driver
    - Xen hang

This is result to a truncated message. To be sure that the message is fully
transfered by early_printk, add a call to early_flush. This will avoid lost
characters.

Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
---
 xen/arch/arm/arm32/debug.S  |    7 +++++++
 xen/arch/arm/arm64/debug.S  |    7 +++++++
 xen/arch/arm/early_printk.c |    7 +++++++
 3 files changed, 21 insertions(+)

diff --git a/xen/arch/arm/arm32/debug.S b/xen/arch/arm/arm32/debug.S
index 1bfbfc1..92f5724 100644
--- a/xen/arch/arm/arm32/debug.S
+++ b/xen/arch/arm/arm32/debug.S
@@ -32,6 +32,13 @@ early_putch:
         early_uart_transmit r1, r0
         mov   pc, lr
 
+.globl early_flush
+/* Flush the UART - this function is called by C */
+early_flush:
+        ldr   r1, =FIXMAP_ADDR(FIXMAP_CONSOLE)  /* r1 := VA UART base address 
*/
+        early_uart_ready r1, r2
+        mov   pc, lr
+
 /*
  * Local variables:
  * mode: ASM
diff --git a/xen/arch/arm/arm64/debug.S b/xen/arch/arm/arm64/debug.S
index 38b7c74..c7b5e6c 100644
--- a/xen/arch/arm/arm64/debug.S
+++ b/xen/arch/arm/arm64/debug.S
@@ -32,6 +32,13 @@ early_putch:
         early_uart_transmit x15, w0
         ret
 
+.globl early_flush
+/* Flush the UART - this function is called by C */
+early_flush:
+        ldr   x15, =FIXMAP_ADDR(FIXMAP_CONSOLE)  /* x15 := VA UART base 
address */
+        early_uart_ready x15, 1
+        ret
+
 /*
  * Local variables:
  * mode: ASM
diff --git a/xen/arch/arm/early_printk.c b/xen/arch/arm/early_printk.c
index 5950323..ca151a5 100644
--- a/xen/arch/arm/early_printk.c
+++ b/xen/arch/arm/early_printk.c
@@ -16,6 +16,7 @@
 #include <asm/early_printk.h>
 
 void early_putch(char c);
+void early_flush(void);
 
 /* Early printk buffer */
 static char __initdata buf[512];
@@ -34,6 +35,12 @@ static void __init early_vprintk(const char *fmt, va_list 
args)
 {
     vsnprintf(buf, sizeof(buf), fmt, args);
     early_puts(buf);
+
+    /*
+     * Wait the UART has finished to transfer all characters before
+     * to continue. This will avoid lost characters if Xen abort.
+     */
+    early_flush();
 }
 
 void __init early_printk(const char *fmt, ...)
-- 
1.7.10.4


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