From 83f3f8ac40b47e220954e620a5ecbe41f54f4dc7 Mon Sep 17 00:00:00 2001
From: Martin Holst Swende <martin@swende.se>
Date: Fri, 17 Jul 2015 00:01:58 +0200
Subject: [PATCH] Potential fix for 0-length usb packets seen on OSX

---
 armsrc/lfops.c   |  6 +-----
 common/usb_cdc.c | 16 ++++++++++++++++
 common/usb_cdc.h |  1 +
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/armsrc/lfops.c b/armsrc/lfops.c
index 188d7280..733bc953 100644
--- a/armsrc/lfops.c
+++ b/armsrc/lfops.c
@@ -399,14 +399,10 @@ void SimulateTagLowFrequency(int period, int gap, int ledcontrol)
  #define OPEN_COIL()		HIGH(GPIO_SSC_DOUT)
 
 	i = 0;
-	byte_t rx[sizeof(UsbCommand)]; // Storage for usb_read call in loop
 	for(;;) {
 		//wait until SSC_CLK goes HIGH
 		while(!(AT91C_BASE_PIOA->PIO_PDSR & GPIO_SSC_CLK)) {
-			// Craig Young - Adding a usb_read() here to avoid abort on empty UsbCommand
-			// My OS X client does this preventing simulation.
-			// Performance hit should be non-existent since the read is only performed if usb_poll is true
-			if(BUTTON_PRESS() || (usb_poll() && usb_read(rx,sizeof(UsbCommand)))) {
+			if(BUTTON_PRESS() || (usb_poll_validate_length() )) {
 				DbpString("Stopped");
 				return;
 			}
diff --git a/common/usb_cdc.c b/common/usb_cdc.c
index ccbb3c50..3c6e9282 100644
--- a/common/usb_cdc.c
+++ b/common/usb_cdc.c
@@ -293,6 +293,22 @@ bool usb_poll()
   return (pUdp->UDP_CSR[AT91C_EP_OUT] & btReceiveBank);
 }
 
+/**
+	In github PR #129, some users appears to get a false positive from
+	usb_poll, which returns true, but the usb_read operation
+	still returns 0.
+	This check is basically the same as above, but also checks
+	that the length available to read is non-zero, thus hopefully fixes the
+	bug.
+**/
+bool usb_poll_validate_length()
+{
+
+	if (!usb_check()) return false;
+	if (!(pUdp->UDP_CSR[AT91C_EP_OUT] & btReceiveBank)) return false;
+	return (pUdp->UDP_CSR[AT91C_EP_OUT] >> 16) >  0;
+}
+
 //*----------------------------------------------------------------------------
 //* \fn    usb_read
 //* \brief Read available data from Endpoint OUT
diff --git a/common/usb_cdc.h b/common/usb_cdc.h
index 59e73a47..c42da8db 100644
--- a/common/usb_cdc.h
+++ b/common/usb_cdc.h
@@ -41,6 +41,7 @@ void usb_disable();
 void usb_enable();
 bool usb_check();
 bool usb_poll();
+bool usb_poll_validate_length();
 uint32_t usb_read(byte_t* data, size_t len);
 uint32_t usb_write(const byte_t* data, const size_t len);
 
-- 
2.39.2