Linux ARM on QEMU

Many times we want to develop a generic device driver for ARM processor, but we want first to test it on an emulator.  In this case, the emulator we're gonna use is ARM.

Here's the content of my script to build Linux for generic ARM (Versatile/PB (ARM926EJ-S). According to https://www.kernel.org/doc/Documentation/arm/Booting, Typically the physical RAM does not start at address zero. LOADADDR specifies the address where the kernel image will be located by U-Boot and is stored in the U-Boot header by the mkimage utility. Typically the load address (for placement in memory) is also the start address (for execution). Note that the uImage file is typically just the (self-extracting, compressed) zImage file with the U-Boot wrapper.

ARCH=arm

core=$(nproc)
make ARCH=$ARCH versatile_defcocnfig
make -j${core} ARCH=$ARCH CROSS_COMPILE=arm-linux-gnueabihf- uImage LOADADDR=0x80008000


There is a much easier way: using Yocto.

The following steps would be everything needed to run Linux arm on Qemu:


1.  git clone git://git.yoctoproject.org/poky

2.  cd poky

3.  git checkout -b pyro origin/pyro

4.  Edit ./build/conf/local.conf and add:

    INHERIT += "rm_work"

    Uncomment (and comment out the x32 one):
     MACHINE ?= "qemuarm"

    exit from editor

5.  From shell, type:
 
    . ./oe-init-build-env

    bitbake core-image-sato

or, if we want to build everything:

  bitbake world 

6.  Once a particular image (in this case is qemuarm) has been built, we can
    test it with command:

        runqemu qemuarm


The target files are located in ${yocto_dir}/build.  In my case it is in ${HOME}/poky.  For example, in my case:

./qemuarm-poky-linux-gnueabi/linux-yocto/4.10.17+gitAUTOINC+e92bd55409_3926e38630-r0/deploy-linux-yocto/zImage

Are US tech companies so picky nowadays?

For last few months I have been looking for technical job in my area of expertise and experience.  I know, it is harder to find jobs in low-level or embedded system compared to application software, but I never realize even when I got some interviews, there are so picky.  A small mistake (not necessarily to judge that I will not be able to do the job), that's it.  You're done.  Next applicant, please.

I am still endeavoring to land my future job. Meanwhile, I am brushing my skills and learn some new stuff.  For example, I now know there is an opensource doing dataplane packet processing.  I also now relearning  Linux kernel and driver, Bluetooth and Wifi.

Some videos of My Embedded projects I have done

Couple of projects done during my spare time:

ATMega32 interfacing to MAX7219 7seg via SPI 

PIC18F9520 doing ultrasonic ranging 

I wish I made video for the PIC-based IoT sensors I made (part of Computer Engineering project in my master program; It was able to push temperatur and humidty data to 'remote' server periodically, which then display it on the web)

FizzBuzz Analysis

Recently I was asked this kind of question. As I was told not to worry too much about it and I got many other questions to answer within short time, I did not pay attention to much on it and just scribbled it on a piece of paper, no time to mentally test the algorithm.

At home I realized I did not complete the answer (blame that to the recruiter).  Anyway, the fizz buzz interview question is to ask interviewee to write a code to print number from 1 to 100, but for every multiplication of 3 to print string "FIZZ" (or something), for every multiplication of 5 to print string "BUZZ", and for every multiplication of 3 and 5 (which is another way to say multiplication of 15) to print "FIZZBUZZ".

Here's my working code in C++:

#include <iostream>

using namespace std;

const static char *s1 = "FIZZ";
const static char *s2 = "BUZZ";

int main()
{
    for(int i=1; i<=100; i++)
    {
        if (i% 3*5==0)
        {
            cout << s1 << s2 << endl;
            continue;
        }
        if (i % 3 == 0)
        {
            cout << s1 << endl;
            continue;
        }
        if (i % 5 == 0)
        {
            cout << s2 << endl;
            continue;
        }

        cout << i << endl;
    }
}

No matter whay you do, the optimal solution is always O(n) (unless you're crazy enough to just print manually without loop, such as cout << "1\n2\FIZZ\n4\BUZZ\n...|").

Some silly optimization can be performed in printing the string, though.

#include <iostream>

using namespace std;

const static string s = "FizzBuzz";

int main()

{

    for(int i=1; i<=100; i++)

    {

        if (i%15==0)

        {

            cout << s << endl;

            continue;

        }

        if (i % 3 == 0)

        {

            cout << s.substr(0,3) << endl;

            continue;

        }

        if (i % 5 == 0)

        {

            cout << s.substr(4) << endl;

            continue;

        }

        cout << i << endl;

    }

}

Another thought is to eliminate mod 15.  For example:

#include <iostream>

using namespace std;

const static string s = "FizzBuzz";

int main()

{

    int m;

    for(int i=1; i<=100; i++)

    {

        m = 0;

        if (i%3==0)

        {

            cout << s.substr(0,3);

            m++;

        }

        if (i % 5 == 0)

        {

            cout << s.substr(4);

            m++;

        }

        if (m > 0)

            cout << endl;

        else

            cout << i << endl;

    }

}

When I tested it (upper limit set to 1000 and perform "time <program>", the last algorithm shows some speedup.

The last few lines can be optimized to be:

        if (m == 0)

            cout << i;

        cout << endl;

Another tiny optimization is instead of doing post increment (i++), we do preincrement (++i).  This saves tiny bit (no copy to extra variable internally).

Last, we can write it down in Assembly if we want.  The following is suitable for embedded device:

main:

leal 4(%esp), %ecx

andl    $-16, %esp

pushl -4(%ecx)

pushl %ebp

movl %esp, %ebp

pushl %edi

pushl %esi

pushl %ebx

pushl %ecx

subl $8, %esp

movl $1, %ebx         ; EBX=1

movl $3, %esi         ; ESI=3

movl $5, %edi         ; EDI=5

jmp CHECK_FIZZ

FOR_I_NEXT:

movl %ebx, %eax       ; eax stores current value of i

cltd

idivl %edi             ; i / 5, result in eax and mod in edx

testl %edx, %edx       ; i % 5

jne PRINT_I          ; i % 5 != 0 -> jump to PRINT_I

CHECK_FUZZ:

subl $8, %esp

pushl stdout

pushl 'f'

call _IO_putc

popl %edx

popl %ecx

pushl stdout

pushl 'i'

call _IO_putc

popl %eax

popl %edx

pushl stdout

pushl 'z'

call _IO_putc

popl %ecx

popl %eax

pushl stdout

pushl 'z'

call _IO_putc

addl $16, %esp

PRINT_CR:

subl   $8, %esp

pushl stdout

pushl '\n'

call _IO_putc

incl %ebx

addl $16, %esp

cmpl $101, %ebx           ; i == 101 ?

je MAIN_EXIT            ; if (i ==101) goto MAIN_EXIT

CHECK_FIZZ:

movl %ebx, %eax

cltd

idivl %esi

testl %edx, %edx           ; i % 3 == 0

jne FOR_I_NEXT

subl $8, %esp             ; if (i%3==0):

pushl stdout

pushl 'b'

call _IO_putc

popl %eax

popl %edx

pushl stdout

pushl 'u'

call _IO_putc

popl %ecx

popl %eax

pushl stdout

pushl 'z'

call _IO_putc

popl %eax

popl %edx

pushl stdout

pushl 'z'

call _IO_putc

movl %ebx, %eax

cltd

idivl %edi

addl $16, %esp

testl %edx, %edx

jne PRINT_CR

jmp CHECK_FUZZ

PRINT_I:

pushl %eax

pushl %ebx

pushl $.LC0

pushl $1

call __printf_chk

addl $16, %esp

jmp PRINT_CR

MAIN_EXIT:

xorl   %eax, %eax

leal -16(%ebp), %esp

popl %ecx

popl %ebx

popl %esi

popl %edi

popl %ebp

leal -4(%ecx), %esp

ret

Onion Omega2+ Terminal on browser

After I installed the web-based Terminal on my Omega2, I was able to login to the terminal via web-browser. Cool!

My Omega2+ OpenWRT Card

Finally, after waiting for almost 10 months, my order of Omega2+ from Onion arrived yesterday.
Will post later about the getting started experience.  For now, only pictures.