From 4a784b998a62555cf67811929a653f4eb369f168 Mon Sep 17 00:00:00 2001
From: tmk <hasu@tmk-kbd.com>
Date: Sun, 6 Feb 2022 16:40:22 +0900
Subject: [PATCH] ibmpc: Naked ISR with Fixed register:r2 #717

---
 converter/ibmpc_usb/Makefile |  3 +++
 tmk_core/protocol/ibmpc.cpp  | 36 ++++++++++++++++++++++++++++--------
 tmk_core/protocol/ibmpc.hpp  |  1 -
 3 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/converter/ibmpc_usb/Makefile b/converter/ibmpc_usb/Makefile
index 3cb7a7600a..fa6182bc93 100644
--- a/converter/ibmpc_usb/Makefile
+++ b/converter/ibmpc_usb/Makefile
@@ -88,6 +88,9 @@ IBMPC_MOUSE_ENABLE ?= yes	# enable mouse support(+2000)
 # Optimize size but this may cause error "relocation truncated to fit"
 #EXTRALDFLAGS = -Wl,--relax
 
+# Fixed register r2 for reading data line
+EXTRAFLAGS ?= -ffixed-r2
+
 
 #
 # Keymap file
diff --git a/tmk_core/protocol/ibmpc.cpp b/tmk_core/protocol/ibmpc.cpp
index e9af15c653..ca13a2c28d 100644
--- a/tmk_core/protocol/ibmpc.cpp
+++ b/tmk_core/protocol/ibmpc.cpp
@@ -206,8 +206,7 @@ void IBMPC::host_isr_clear(void)
 
 void IBMPC::isr(void)
 {
-    uint8_t dbit;
-    dbit = IBMPC_DATA_PIN&(1<<data_bit);
+    volatile register uint8_t STORED_PIN asm("r2");
 
     // Timeout check
     uint8_t t;
@@ -231,7 +230,7 @@ void IBMPC::isr(void)
     }
 
     isr_state = isr_state>>1;
-    if (dbit) isr_state |= 0x8000;
+    if (STORED_PIN & data_mask) isr_state |= 0x8000;
 
     // isr_state: state of receiving data from keyboard
     //
@@ -392,17 +391,38 @@ void IBMPC::host_set_led(uint8_t led)
 }
 
 
-// NOTE: With this ISR data line should be read within 5us after clock falling edge.
-// Confirmed that ATmega32u4 can read data line in 2.5us from interrupt after
-// ISR prologue pushs r18, r19, r20, r21, r24, r25 r30 and r31 with GCC 5.4.0
-ISR(IBMPC_INT_VECT)
+// This generates prologue/epilogue code just like one for not-naked ISR
+extern "C" void ibmpc_isr(void) __attribute__ ((signal,__INTR_ATTRS));
+void ibmpc_isr(void)
 {
     IBMPC::interface0.isr();
 }
 
+// This reads data line and store in r2 immediately without prologue code
+ISR(IBMPC_INT_VECT, ISR_NAKED)
+{
+    asm volatile (
+        "in     r2,     %[pin]"         "\n\t"
+        "rjmp   ibmpc_isr"              "\n\t"
+        :
+        : [pin] "I" (_SFR_IO_ADDR(IBMPC_DATA_PIN))
+    );
+}
+
 #if defined(IBMPC_CLOCK_BIT1) && defined(IBMPC_DATA_BIT1) && defined(IBMPC_INT_VECT1)
-ISR(IBMPC_INT_VECT1)
+extern "C" void ibmpc_isr1(void) __attribute__ ((signal,__INTR_ATTRS));
+void ibmpc_isr1(void)
 {
     IBMPC::interface1.isr();
 }
+
+ISR(IBMPC_INT_VECT1, ISR_NAKED)
+{
+    asm volatile (
+        "in     r2,     %[pin]"         "\n\t"
+        "rjmp   ibmpc_isr1"             "\n\t"
+        :
+        : [pin] "I" (_SFR_IO_ADDR(IBMPC_DATA_PIN))
+    );
+}
 #endif
diff --git a/tmk_core/protocol/ibmpc.hpp b/tmk_core/protocol/ibmpc.hpp
index d1765e7f09..522df35a68 100644
--- a/tmk_core/protocol/ibmpc.hpp
+++ b/tmk_core/protocol/ibmpc.hpp
@@ -90,7 +90,6 @@ POSSIBILITY OF SUCH DAMAGE.
 #define IBMPC_LED_NUM_LOCK    1
 #define IBMPC_LED_CAPS_LOCK   2
 
-
 class IBMPC
 {
     public: