New tutorial, raspbootin for 64 bit

6 years ago · a0b68d2db7
parent 734cb2fb1b
commit a0b68d2db7
12 changed files with 592 additions and 0 deletions
--- a/14_raspbootin64/Makefile
+++ b/14_raspbootin64/Makefile
@ -0,0 +1,43 @@
+#
+# Copyright (C) 2018 bzt (bztsrc@github)
+#
+# Permission is hereby granted, free of charge, to any person
+# obtaining a copy of this software and associated documentation
+# files (the "Software"), to deal in the Software without
+# restriction, including without limitation the rights to use, copy,
+# modify, merge, publish, distribute, sublicense, and/or sell copies
+# of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+# HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+# WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+# DEALINGS IN THE SOFTWARE.
+#
+#
+
+SRCS = $(wildcard *.c)
+OBJS = $(SRCS:.c=.o)
+CFLAGS = -Wall -O2 -ffreestanding -nostdinc -nostdlib -nostartfiles
+
+all: clean kernel8.img
+
+start.o: start.S
+	aarch64-elf-gcc $(CFLAGS) -c start.S -o start.o
+
+%.o: %.c
+	aarch64-elf-gcc $(CFLAGS) -c $< -o $@
+
+kernel8.img: start.o $(OBJS)
+	aarch64-elf-ld -nostdlib -nostartfiles start.o $(OBJS) -T link.ld -o kernel8.elf
+	aarch64-elf-objcopy -O binary kernel8.elf kernel8.img
+
+clean:
+	rm kernel8.elf *.o >/dev/null 2>/dev/null || true
--- a/14_raspbootin64/OLVASSEL.md
+++ b/14_raspbootin64/OLVASSEL.md
@ -0,0 +1,40 @@
+Oktatóanyag 15 - Raspbootin64
+=============================
+
+Mivel folyton iragatni az SD kártyát strapás, és nem tesz jót a kártyának, ezért egy olyan kernel8.img-t készítünk,
+ami a soros vonalról fogja betölteni az igazi kernel8.img-t.
+
+Ez az oktatóanyag a jól ismert [raspbootin](https://github.com/mrvn/raspbootin) átírása 64 bitre.
+A betöltőprogram egyik felét adja csak, a kernel fogadót, ami az RPi-n fut. A másik fél, a PC-n futó küldő,
+megtalálható az eredeti forrásban [raspbootcom](https://github.com/mrvn/raspbootin/raspbootcom) néven.
+Ha Windowsos gépekről is szeretnél kernelt küldeni, akkor javaslom inkább a John Cronin féle átiratot, a
+[raspbootin-server](https://github.com/jncronin/rpi-boot/blob/master/raspbootin-server.c)-t, ami natív Win32 API-t használ.
+
+Hogy az új kernelt ugyanoda tölthessük be, el kell mozdítanunk a kódunkat az útból. Ezt chain loading-nak hívják, amikor
+az első kód ugyanarra a címre tölti be a második kódot, ezért az utóbbi azt hiszi, a firmware töltötte be.
+Hogy ezt megvalósítsuk, egy alacsonyabb címre linkeljük a kódot, és mivel a GPU ettől függetlenül a 0x80000-ra tölt be,
+nekünk kell a módosított címre másolnunk magunkat. Fontos, hogy ezalatt csak relatív címzést használhatunk. Amikor
+végeztünk, a 0x80000-as címen lévő memóriának használaton kívülinek kell lennie. Ajánlott a kódunkat minimalizálni, mivel
+úgyis figyelmen kívül hagyja az újonnan betöltendő kód. Ezért kivettem az `uart_puts()` eljárást, így a teljes méret
+1024 bájt alá csökkent. Ezt a következő paranccsal ellenőriztem:
+
+```sh
+$ aarch64-elf-readelf -s kernel8.elf | grep __bss_end
+    21: 000000000007ffd0     0 NOTYPE  GLOBAL DEFAULT    4 __bss_end
+```
+
+Start
+-----
+
+Hozzáadtam egy ciklust, ami átmásolja a kódunkat arra a címre, ahová vártuk, hogy betöltődjön.
+
+Linker
+------
+
+Másik címre linkelünk ebben a példában. Hasonlóan a bss méret kiszámításához, ugyanúgy meghatározzuk a kód
+méretét is, amit másolnunk kell.
+
+Main
+----
+
+Kiírjuk, hogy 'RBIN64', majd beolvassuk az új kernelt a soros vonalról, és átadjuk rá a vezérlést.
--- a/14_raspbootin64/README.md
+++ b/14_raspbootin64/README.md
@ -0,0 +1,40 @@
+Tutorial 15 - Raspbootin64
+==========================
+
+Because changing SD card is boring and also to avoid potential SD card damage, we create a kernel8.img that will
+load the real kernel8.img over serial.
+
+This tutorial is a rewrite of the well known serial boot loader, [raspbootin](https://github.com/mrvn/raspbootin) in 64 bit.
+I only provide one part of the loader, the kernel receiver, which runs on the RPi. For the other
+part, the sender, which runs on your PC see the original [raspbootcom](https://github.com/mrvn/raspbootin/raspbootcom) utility.
+If you want to send kernels from a Windows machine, I suggest to take a look at John Cronin's rewrite,
+[raspbootin-server](https://github.com/jncronin/rpi-boot/blob/master/raspbootin-server.c) which can be compiled for the Win32 API.
+
+In order to load the new kernel to the same address, we have to move ourself out of the way. It's called chain
+loading: one code loads the next code to the same position in memory, therefore the latter thinks it was loaded
+by the firmware. To implement that we use a different linking address this time, and since GPU loads us to 0x80000
+regardless, we have to copy our code to that link address. That's important that we can only use relative addresses
+while doing so. When we're done, the memory at 0x80000 must be free to use. We also should minimize the size of the
+loader, since it will be regarded by the newlt loaded code anyway. By removing `uart_puts()` I've managed to shrink the
+size below 1024 bytes. I've checked that with:
+
+```sh
+$ aarch64-elf-readelf -s kernel8.elf | grep __bss_end
+    21: 000000000007ffd0     0 NOTYPE  GLOBAL DEFAULT    4 __bss_end
+```
+
+Start
+-----
+
+Added a loop to relocate our code to the address it should have been loaded to.
+
+Linker
+------
+
+We use a different linking address this time. Similarly to bss size calculation, we calculate our code's size to
+know how many bytes we have to copy.
+
+Main
+----
+
+We print 'RBIN64', receive the new kernel over serial, and jump to it.
--- a/14_raspbootin64/gpio.h
+++ b/14_raspbootin64/gpio.h
@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+#define MMIO_BASE       0x3F000000
+
+#define GPFSEL0         ((volatile unsigned int*)(MMIO_BASE+0x00200000))
+#define GPFSEL1         ((volatile unsigned int*)(MMIO_BASE+0x00200004))
+#define GPFSEL2         ((volatile unsigned int*)(MMIO_BASE+0x00200008))
+#define GPFSEL3         ((volatile unsigned int*)(MMIO_BASE+0x0020000C))
+#define GPFSEL4         ((volatile unsigned int*)(MMIO_BASE+0x00200010))
+#define GPFSEL5         ((volatile unsigned int*)(MMIO_BASE+0x00200014))
+#define GPSET0          ((volatile unsigned int*)(MMIO_BASE+0x0020001C))
+#define GPSET1          ((volatile unsigned int*)(MMIO_BASE+0x00200020))
+#define GPCLR0          ((volatile unsigned int*)(MMIO_BASE+0x00200028))
+#define GPLEV0          ((volatile unsigned int*)(MMIO_BASE+0x00200034))
+#define GPLEV1          ((volatile unsigned int*)(MMIO_BASE+0x00200038))
+#define GPEDS0          ((volatile unsigned int*)(MMIO_BASE+0x00200040))
+#define GPEDS1          ((volatile unsigned int*)(MMIO_BASE+0x00200044))
+#define GPHEN0          ((volatile unsigned int*)(MMIO_BASE+0x00200064))
+#define GPHEN1          ((volatile unsigned int*)(MMIO_BASE+0x00200068))
+#define GPPUD           ((volatile unsigned int*)(MMIO_BASE+0x00200094))
+#define GPPUDCLK0       ((volatile unsigned int*)(MMIO_BASE+0x00200098))
+#define GPPUDCLK1       ((volatile unsigned int*)(MMIO_BASE+0x0020009C))
--- a/14_raspbootin64/kernel8.img
+++ b/14_raspbootin64/kernel8.img
--- a/14_raspbootin64/link.ld
+++ b/14_raspbootin64/link.ld
@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+SECTIONS
+{
+    . = 0x80000 - 1024;
+    PROVIDE(_code = .);
+    .text : { KEEP(*(.text.boot)) *(.text .text.* .gnu.linkonce.t*) }
+    .rodata : { *(.rodata .rodata.* .gnu.linkonce.r*) }
+    PROVIDE(_data = .);
+    .data : { *(.data .data.* .gnu.linkonce.d*) }
+    .bss (NOLOAD) : {
+        . = ALIGN(16);
+        __bss_start = .;
+        *(.bss .bss.*)
+        *(COMMON)
+        __bss_end = .;
+    }
+    _end = .;
+
+   /DISCARD/ : { *(.comment) *(.gnu*) *(.note*) *(.eh_frame*) }
+}
+__bss_size = (__bss_end - __bss_start)>>3;
+__loader_size = (_end - _code)>>3;
--- a/14_raspbootin64/main.c
+++ b/14_raspbootin64/main.c
@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+#include "uart.h"
+
+// import our start address from linker
+extern char _start;
+
+void main()
+{
+    int size=0;
+    char *kernel=(char*)&_start;
+
+    // set up serial console
+    uart_init();
+
+    // say hello. To reduce loader size I removed uart_puts()
+again:
+    uart_send('R');
+    uart_send('B');
+    uart_send('I');
+    uart_send('N');
+    uart_send('6');
+    uart_send('4');
+    uart_send('\r');
+    uart_send('\n');
+    // notify raspbootcom to send the kernel
+    uart_send(3);
+    uart_send(3);
+    uart_send(3);
+
+    // read the kernel's size
+    size=uart_getc();
+    size|=uart_getc()<<8;
+    size|=uart_getc()<<16;
+    size|=uart_getc()<<24;
+
+    // send negative or positive acknowledge
+    if(size<64 || size>1024*1024) {
+        // size error
+        uart_send('S');
+        uart_send('E');
+        goto again;
+    }
+    uart_send('O');
+    uart_send('K');
+
+    // read the kernel
+    while(size--) *kernel++ = uart_getc();
+
+    // jump to the new kernel
+    asm volatile ("b 0x80000");
+}
--- a/14_raspbootin64/mbox.c
+++ b/14_raspbootin64/mbox.c
@ -0,0 +1,63 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+#include "gpio.h"
+
+/* mailbox message buffer */
+volatile unsigned int  __attribute__((aligned(16))) mbox[8];
+
+#define VIDEOCORE_MBOX  (MMIO_BASE+0x0000B880)
+#define MBOX_READ       ((volatile unsigned int*)(VIDEOCORE_MBOX+0x0))
+#define MBOX_POLL       ((volatile unsigned int*)(VIDEOCORE_MBOX+0x10))
+#define MBOX_SENDER     ((volatile unsigned int*)(VIDEOCORE_MBOX+0x14))
+#define MBOX_STATUS     ((volatile unsigned int*)(VIDEOCORE_MBOX+0x18))
+#define MBOX_CONFIG     ((volatile unsigned int*)(VIDEOCORE_MBOX+0x1C))
+#define MBOX_WRITE      ((volatile unsigned int*)(VIDEOCORE_MBOX+0x20))
+#define MBOX_RESPONSE   0x80000000
+#define MBOX_FULL       0x80000000
+#define MBOX_EMPTY      0x40000000
+
+/**
+ * Make a mailbox call. Returns 0 on failure, non-zero on success
+ */
+int mbox_call(unsigned char ch)
+{
+    unsigned int r;
+    /* wait until we can write to the mailbox */
+    do{asm volatile("nop");}while(*MBOX_STATUS & MBOX_FULL);
+    /* write the address of our message to the mailbox with channel identifier */
+    *MBOX_WRITE = (((unsigned int)((unsigned long)&mbox)&~0xF) | (ch&0xF));
+    /* now wait for the response */
+    while(1) {
+        /* is there a response? */
+        do{asm volatile("nop");}while(*MBOX_STATUS & MBOX_EMPTY);
+        r=*MBOX_READ;
+        /* is it a response to our message? */
+        if((unsigned char)(r&0xF)==ch && (r&~0xF)==(unsigned int)((unsigned long)&mbox))
+            /* is it a valid successful response? */
+            return mbox[1]==MBOX_RESPONSE;
+    }
+    return 0;
+}
--- a/14_raspbootin64/mbox.h
+++ b/14_raspbootin64/mbox.h
@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+/* a properly aligned buffer */
+extern volatile unsigned int mbox[36];
+
+#define MBOX_REQUEST    0
+
+/* channels */
+#define MBOX_CH_POWER   0
+#define MBOX_CH_FB      1
+#define MBOX_CH_VUART   2
+#define MBOX_CH_VCHIQ   3
+#define MBOX_CH_LEDS    4
+#define MBOX_CH_BTNS    5
+#define MBOX_CH_TOUCH   6
+#define MBOX_CH_COUNT   7
+#define MBOX_CH_PROP    8
+
+/* tags */
+#define MBOX_TAG_GETSERIAL      0x10004
+#define MBOX_TAG_SETCLKRATE     0x38002
+#define MBOX_TAG_LAST           0
+
+int mbox_call(unsigned char ch);
--- a/14_raspbootin64/start.S
+++ b/14_raspbootin64/start.S
@ -0,0 +1,64 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+.section ".text.boot"
+
+.global _start
+
+_start:
+    // read cpu id, stop slave cores
+    mrs     x1, mpidr_el1
+    and     x1, x1, #3
+    cbz     x1, 2f
+    // cpu id > 0, stop
+1:  wfe
+    b       1b
+2:  // cpu id == 0
+
+    // relocate our code from load address to link address
+    ldr     x1, =0x80000
+    ldr     x2, =_start
+    ldr     w3, =__loader_size
+1:  ldr     x4, [x1], #8
+    str     x4, [x2], #8
+    sub     w3, w3, #1
+    cbnz    w3, 1b
+
+    // set stack before our code
+    ldr     x1, =_start
+    mov     sp, x1
+
+    // clear bss
+    ldr     x1, =__bss_start
+    ldr     w2, =__bss_size
+3:  cbz     w2, 4f
+    str     xzr, [x1], #8
+    sub     w2, w2, #1
+    cbnz    w2, 3b
+
+    // jump to C code, should not return
+4:  bl      main
+    // for failsafe, halt this core too
+    b       1b
--- a/14_raspbootin64/uart.c
+++ b/14_raspbootin64/uart.c
@ -0,0 +1,99 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+#include "gpio.h"
+#include "mbox.h"
+
+/* PL011 UART registers */
+#define UART0_DR        ((volatile unsigned int*)(MMIO_BASE+0x00201000))
+#define UART0_FR        ((volatile unsigned int*)(MMIO_BASE+0x00201018))
+#define UART0_IBRD      ((volatile unsigned int*)(MMIO_BASE+0x00201024))
+#define UART0_FBRD      ((volatile unsigned int*)(MMIO_BASE+0x00201028))
+#define UART0_LCRH      ((volatile unsigned int*)(MMIO_BASE+0x0020102C))
+#define UART0_CR        ((volatile unsigned int*)(MMIO_BASE+0x00201030))
+#define UART0_IMSC      ((volatile unsigned int*)(MMIO_BASE+0x00201038))
+#define UART0_ICR       ((volatile unsigned int*)(MMIO_BASE+0x00201044))
+
+/**
+ * Set baud rate and characteristics (115200 8N1) and map to GPIO
+ */
+void uart_init()
+{
+    register unsigned int r;
+
+    /* initialize UART */
+    *UART0_CR = 0;         // turn off UART0
+
+    /* set up clock for consistent divisor values */
+    mbox[0] = 8*4;
+    mbox[1] = MBOX_REQUEST;
+    mbox[2] = MBOX_TAG_SETCLKRATE; // set clock rate
+    mbox[3] = 12;
+    mbox[4] = 8;
+    mbox[5] = 2;           // UART clock
+    mbox[6] = 4000000;     // 4Mhz
+    mbox[7] = MBOX_TAG_LAST;
+    mbox_call(MBOX_CH_PROP);
+
+    /* map UART0 to GPIO pins */
+    r=*GPFSEL1;
+    r&=~((7<<12)|(7<<15)); // gpio14, gpio15
+    r|=(4<<12)|(4<<15);    // alt0
+    *GPFSEL1 = r;
+    *GPPUD = 0;            // enable pins 14 and 15
+    r=150; while(r--) { asm volatile("nop"); }
+    *GPPUDCLK0 = (1<<14)|(1<<15);
+    r=150; while(r--) { asm volatile("nop"); }
+    *GPPUDCLK0 = 0;        // flush GPIO setup
+
+    *UART0_ICR = 0x7FF;    // clear interrupts
+    *UART0_IBRD = 2;       // 115200 baud
+    *UART0_FBRD = 0xB;
+    *UART0_LCRH = 0b11<<5; // 8n1
+    *UART0_CR = 0x301;     // enable Tx, Rx, FIFO
+}
+
+/**
+ * Send a character
+ */
+void uart_send(unsigned int c) {
+    /* wait until we can send */
+    do{asm volatile("nop");}while(*UART0_FR&0x20);
+    /* write the character to the buffer */
+    *UART0_DR=c;
+}
+
+/**
+ * Receive a character
+ */
+char uart_getc() {
+    char r;
+    /* wait until something is in the buffer */
+    do{asm volatile("nop");}while(*UART0_FR&0x10);
+    /* read it and return */
+    r=(char)(*UART0_DR);
+    /* convert carrige return to newline */
+    return r=='\r'?'\n':r;
+}
--- a/14_raspbootin64/uart.h
+++ b/14_raspbootin64/uart.h
@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2018 bzt (bztsrc@github)
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+void uart_init();
+void uart_send(unsigned int c);
+char uart_getc();
+void uart_puts(char *s);
+void uart_hex(unsigned int d);