From b18332cb4da89a2f533d46f8ea54c46dcdd774fb Mon Sep 17 00:00:00 2001
From: Kevin Bracey <kevin.bracey@arm.com>
Date: Thu, 28 Sep 2017 10:44:43 +0300
Subject: [PATCH 1/2] Avoid wait_ms() spin

System's wait_ms() spins to achieve a precise delay - we don't want this.
Call Thread::wait directly.
---
 drivers/UARTSerial.cpp | 16 ++++++++++++++++
 drivers/UARTSerial.h   |  2 ++
 2 files changed, 18 insertions(+)

diff --git a/drivers/UARTSerial.cpp b/drivers/UARTSerial.cpp
index 96cdc831d8..5cad4ffc82 100644
--- a/drivers/UARTSerial.cpp
+++ b/drivers/UARTSerial.cpp
@@ -19,7 +19,12 @@
 #include <errno.h>
 #include "UARTSerial.h"
 #include "platform/mbed_poll.h"
+
+#if MBED_CONF_RTOS_PRESENT
+#include "rtos/Thread.h"
+#else
 #include "platform/mbed_wait_api.h"
+#endif
 
 namespace mbed {
 
@@ -277,6 +282,17 @@ void UARTSerial::tx_irq(void)
     }
 }
 
+void UARTSerial::wait_ms(uint32_t millisec)
+{
+    /* wait_ms implementation for RTOS spins until exact microseconds - we
+     * want to just sleep until next tick.
+     */
+#if MBED_CONF_RTOS_PRESENT
+    rtos::Thread::wait(millisec);
+#else
+    ::wait_ms(millisec);
+#endif
+}
 } //namespace mbed
 
 #endif //(DEVICE_SERIAL && DEVICE_INTERRUPTIN)
diff --git a/drivers/UARTSerial.h b/drivers/UARTSerial.h
index ebb5779c1c..df07f2e075 100644
--- a/drivers/UARTSerial.h
+++ b/drivers/UARTSerial.h
@@ -160,6 +160,8 @@ public:
 
 private:
 
+    void wait_ms(uint32_t millisec);
+
     /** SerialBase lock override */
     virtual void lock(void);
 

From de4ced33a6c6f181081ddde7f3258410cc4c47ac Mon Sep 17 00:00:00 2001
From: Kevin Bracey <kevin.bracey@arm.com>
Date: Mon, 2 Oct 2017 09:59:24 +0300
Subject: [PATCH 2/2] Make poll() use wait(1) rather than yield()

Spinning while polling is overly CPU intensive, and inconsistent with
the current blocking behaviour of UARTSerial.

Change to use Thread::wait(1) to match UARTSerial.
---
 platform/mbed_poll.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/platform/mbed_poll.cpp b/platform/mbed_poll.cpp
index 28492010d4..c2a08fa3dd 100644
--- a/platform/mbed_poll.cpp
+++ b/platform/mbed_poll.cpp
@@ -66,7 +66,7 @@ int poll(pollfh fhs[], unsigned nfhs, int timeout)
 #ifdef MBED_CONF_RTOS_PRESENT
         // TODO - proper blocking
         // wait for condition variable, wait queue whatever here
-        rtos::Thread::yield();
+        rtos::Thread::wait(1);
 #endif
     }
     return count;