[PATCH] arm64: wait for transmit completion before next character transmission

[Pratyush Anand]

Previous transmission must be completed before next character to be
transmitted, otherwise TX buffer may saturate and we will not see all
the characters on screen.

Signed-off-by: Pratyush Anand <panand at redhat.com>
---
 purgatory/arch/arm64/purgatory-arm64.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/purgatory/arch/arm64/purgatory-arm64.c b/purgatory/arch/arm64/purgatory-arm64.c
index 25960c30bd05..3a1c9243bfa2 100644
--- a/purgatory/arch/arm64/purgatory-arm64.c
+++ b/purgatory/arch/arm64/purgatory-arm64.c
@@ -11,15 +11,37 @@ extern uint32_t *arm64_sink;
 extern void (*arm64_kernel_entry)(uint64_t);
 extern uint64_t arm64_dtb_addr;
 
+static void wait_for_xmit_complete(void)
+{
+	volatile uint32_t status;
+	volatile uint32_t *status_reg;
+
+	/*
+	 * Since most of the UART with ARM platform has LSR register at
+	 * offset 0x14 and should have value as 0x60 for TX empty, so we
+	 * have hardcoded these values. Can modify in future if need
+	 * arises.
+	 */
+	status_reg = (volatile uint32_t *)((uint64_t)arm64_sink + 0x14);
+	while (1) {
+		status = *status_reg;
+		if ((status & 0x60) == 0x60)
+			break;
+	}
+}
+
 void putchar(int ch)
 {
 	if (!arm64_sink)
 		return;
 
+	wait_for_xmit_complete();
 	*arm64_sink = ch;
 
-	if (ch == '\n')
+	if (ch == '\n') {
+		wait_for_xmit_complete();
 		*arm64_sink = '\r';
+	}
 }
 
 void setup_arch(void)
-- 
2.1.0