rackmon2: Fix large timeouts observed on wedge400

Summary:
The current write method goes in 4 steps.
1. disableRead
2. write request
3. wait for LSR assert.
4. enableRead
5. Read response
Normally, a response would arrive after #4 thus,
marking the transaction as successful.
But occationally (1/10 times) the response
arrives before #4 thus causing the device to
completely miss the response -- resulting in a timeout.

This change, replaces this with
1. Keeping read enabled throughout.
2. ignore LSR changes (Do not do #3).
This change completely eliminates timeouts on wedge400+ with
an FTDI USB device which do not need this sequencing.

But we would still need this functionality for wedge100 where we use
the native aspeed device (Pending a verification on wedge100. If it
is not needed even on that platform, that functionality can be
completely stripped.)

Imported from: D35445732

Reviewed By: jasmeetbagga

Differential Revision: D36388930

fbshipit-source-id: 3bfe0aa1e633732fecd48a14b478da01dc0a8f8b

fboss/platform/rackmon/UARTDevice.cpp

@@ -74,7 +74,7 @@ void UARTDevice::setAttribute(bool readEnable, int baudrate) {
   }
 }
 
-void UARTDevice::waitWrite() {
+void AspeedRS485Device::waitWrite() {
   int loops;
   for (loops = 0; loops < 100; loops++) {
     int lsr;
@@ -89,7 +89,7 @@ void UARTDevice::waitWrite() {
   }
 }
 
-void UARTDevice::write(const uint8_t* buf, size_t len) {
+void AspeedRS485Device::write(const uint8_t* buf, size_t len) {
   // Write with read disabled. Hence we need to do this
   // as fast as possible. So, muck around with the priorities
   // to make sure we get there.

fboss/platform/rackmon/UARTDevice.h

@@ -11,7 +11,6 @@ class UARTDevice : public Device {
   int baudrate_ = -1;
 
  protected:
-  void waitWrite() override;
   virtual void setAttribute(bool readEnable, int baudrate);
 
   void readEnable() {
@@ -37,26 +36,24 @@ class UARTDevice : public Device {
   }
 
   void open() override;
-  void write(const uint8_t* buf, size_t len) override;
-
-  void write(const std::vector<uint8_t>& buf) {
-    write(buf.data(), buf.size());
-  }
 };
 
 class AspeedRS485Device : public UARTDevice {
+ protected:
+  void waitWrite() override;
+
  public:
   AspeedRS485Device(const std::string& device, int baudrate)
       : UARTDevice(device, baudrate) {}
   void open() override;
+  void write(const uint8_t* buf, size_t len) override;
 };
 
 class LocalEchoUARTDevice : public UARTDevice {
  public:
   LocalEchoUARTDevice(const std::string& device, int baudrate)
       : UARTDevice(device, baudrate) {}
   void write(const uint8_t* buf, size_t len) override;
-  void waitWrite() override {}
 };
 
 } // namespace rackmon