Fix compilation of demo.bin on Rocket

- Adjust memory model to fix `relocation truncated` errors
- Make isr.c shared between BIOS and demo to resolve dep on `plic_init`

Based on: https://github.com/enjoy-digital/litex/issues/1168

litex/soc/cores/cpu/rocket/core.py

@@ -118,6 +118,7 @@ class Rocket(CPU):
         flags =  "-mno-save-restore "
         flags += GCC_FLAGS[self.variant]
         flags += "-D__rocket__ "
+        flags += "-mcmodel=medany"
         return flags
 
     def __init__(self, platform, variant="standard"):

litex/soc/software/bios/Makefile

@@ -6,8 +6,7 @@ ifdef TFTP_SERVER_PORT
 CFLAGS += -DTFTP_SERVER_PORT=$(TFTP_SERVER_PORT)
 endif
 
-OBJECTS = isr.o			\
-	  boot-helper.o		\
+OBJECTS = boot-helper.o	\
 	  boot.o			\
 	  helpers.o			\
 	  cmd_bios.o		\

litex/soc/software/demo/Makefile

@@ -3,7 +3,7 @@ BUILD_DIR?=../build/
 include $(BUILD_DIR)/software/include/generated/variables.mak
 include $(SOC_DIRECTORY)/software/common.mak
 
-OBJECTS   = isr.o donut.o helloc.o main.o crt0.o
+OBJECTS   = donut.o helloc.o main.o crt0.o
 ifdef WITH_CXX
 	OBJECTS += hellocpp.o
 endif

litex/soc/software/demo/isr.c

@@ -1,29 +0,0 @@
-// This file is Copyright (c) 2020 Florent Kermarrec <florent@enjoy-digital.fr>
-// License: BSD
-
-#include <generated/csr.h>
-#include <generated/soc.h>
-#include <irq.h>
-#include <libbase/uart.h>
-
-void isr(void);
-
-#ifdef CONFIG_CPU_HAS_INTERRUPT
-
-void isr(void)
-{
-	__attribute__((unused)) unsigned int irqs;
-
-	irqs = irq_pending() & irq_getmask();
-
-#ifndef UART_POLLING
-	if(irqs & (1 << UART_INTERRUPT))
-		uart_isr();
-#endif
-}
-
-#else
-
-void isr(void){};
-
-#endif

litex/soc/software/libbase/Makefile

@@ -10,7 +10,8 @@ OBJECTS =  \
 	memtest.o  \
 	uart.o     \
 	spiflash.o \
-	i2c.o
+	i2c.o \
+	isr.o
 
 all: libbase.a