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Is there any way to boot an Android phone* from a bus-powered USB drive**? If so, what are the steps to achieve this?

* E.g. one with USB OTG functionality.

** E.g. a flash drive.

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1 Answer 1

up vote 10 down vote accepted
+50

Please clarify what is the intended goal and why?

Android phones have their own boot-loaders and cannot be overridden by other means.

It is not like a PC's BIOS where you can switch the ordering of boot to boot from certain devices such as Network PXE, USB, Primary/Secondary H.D.D...

Edit:

After the comments below, and in relation to the OP's question

Is there any way to boot an Android phone (E.g. one with USB OTG functionality.) via way of a bus-powered USB drive

The generic boot-loader (*which resides on the chip-set) has no knowledge of USB etc, as the lk (Little Kernel) is more concerned about trapping keystrokes in order to chain-load into recovery or to boot directly into Android environment (When holding Vol+Down key in this instance) - in pseudo-code (this is from the context/aspect of lk, and also, the memory addresses pertaining to how to read the partitions are hard-coded into this lk so it will know how to process the logic!)

The lk kernel is the de-facto standard by Qualcomm for MSM chipsets (Snapdragon) and adopted by manufacturers such as Sony, Motorola, LG, Samsung and can be found in the AOSP source under bootable/bootloader.

if (Is Volume Down key pressed?) then

  • chain-load kernel from /recovery partition into particular address in memory and jump to it and start execution, in bringing up the recovery environment

else

  • chain-load kernel from /system partition into particular address in memory and jump to it and start execution in bringing up the Android environment.

end if.

As the kernel within lk is pretty limited, considering that the binary image of the kernel is burned into the chip and therefore no way of modifying it. And also should be mentioned that lk contains the fastboot protocol in preparation for flashing /boot, /recovery, /system and /data partitions. There are two sequences to boot, primary boot and secondary boot as it is:

  • Primary Boot -> lk (depending on outcome of logic)
  • Go into Secondary Boot -> /boot or /recovery

Side note: Samsung is fond of the PBL/SBL (Which is Primary Boot Loader and Secondary Boot Loader respectively) in their jargon when it comes to modding. Thing about Samsung, is that, in some handsets, PBL and SBL may be encrypted (Samsung Wave GT-S8500 is one such example, where porting Android to it was nearly impossible to do because of the DRM within the boot loaders which was a nightmare to deal with and made modding it extremely difficult, nonetheless, it is sort of working via an exploit in the FOTA code!)

This is why there are no extra facilities such as OTG functionality or anything else such as serial communications, reading from SDCard, graphics etc as it would make the lk kernel bigger than is intended. In other words, it is the smallest possible size of kernel that is designated to do just the above pseudo-code happen.

Also, another way of looking at it is this, and this is dependent on the Android version - the USB OTG functionality is fully brought up within the Android environment, i.e when the familiar home screen appears, then OTG's functionality is enabled. Unfortunately not the case when looking at it from lk's perspective.

If you're curious, here's the Qualcomm entry on the above lk which is a part of the tiny C source that has ARM assembly included and found in JellyBean's AOSP source in bootable/bootloader/legacy/usbloader/main.c

int boot_linux_from_flash(void)
{
    boot_img_hdr *hdr = (void*) raw_header;
    unsigned n;
    ptentry *p;
    unsigned offset = 0;
    const char *cmdline;

    if((p = flash_find_ptn("boot")) == 0) {
        cprintf("NO BOOT PARTITION\n");
        return -1;
    }

    if(flash_read(p, offset, raw_header, 2048)) {
        cprintf("CANNOT READ BOOT IMAGE HEADER\n");
        return -1;
    }
    offset += 2048;

    if(memcmp(hdr->magic, BOOT_MAGIC, BOOT_MAGIC_SIZE)) {
        cprintf("INVALID BOOT IMAGE HEADER\n");
        return -1;
    }

    n = (hdr->kernel_size + (FLASH_PAGE_SIZE - 1)) & (~(FLASH_PAGE_SIZE - 1));
    if(flash_read(p, offset, (void*) hdr->kernel_addr, n)) {
        cprintf("CANNOT READ KERNEL IMAGE\n");
        return -1;
    }
    offset += n;

    n = (hdr->ramdisk_size + (FLASH_PAGE_SIZE - 1)) & (~(FLASH_PAGE_SIZE - 1));
    if(flash_read(p, offset, (void*) hdr->ramdisk_addr, n)) {
        cprintf("CANNOT READ RAMDISK IMAGE\n");
        return -1;
    }
    offset += n;

    dprintf("\nkernel  @ %x (%d bytes)\n", hdr->kernel_addr, hdr->kernel_size);
    dprintf("ramdisk @ %x (%d bytes)\n\n\n", hdr->ramdisk_addr, hdr->ramdisk_size);

    if(hdr->cmdline[0]) {
        cmdline = (char*) hdr->cmdline;
    } else {
        cmdline = board_cmdline();
        if(cmdline == 0) {
            cmdline = "mem=50M console=null";
        }
    }
    cprintf("cmdline = '%s'\n", cmdline);

    cprintf("\nBooting Linux\n");

    create_atags(ADDR_TAGS, cmdline,
                 hdr->ramdisk_addr, hdr->ramdisk_size);

    boot_linux(hdr->kernel_addr);
    return 0;
}
share|improve this answer
    
Chicken/egg issue here: I wanted an answer to my question in order to narrow down use cases based upon feasibility; you are asking me to give use cases first :) So, I can only clarify my goal(s) vaguely for now. One might be to achieve sth like full disk encryption by booting from a hardware-encrypted USB drive (Lok-It/dataShur/etc),so that entering a passcode on the drive obviates the need to enter a decryption password on the Android device. Ideally this could be done such that, once the phone is booted, the drive could be removed, leaving the phone still running fine until next reboot. –  sampablokuper Mar 11 '13 at 9:05
    
Right... Interesting - never heard of such case like that - anyway - why? Food for thought, where would you enter a such passcode? Android ICS upwards have the capability to encrypt the entire volume IIRC - Have you not looked into that? –  t0mm13b Mar 11 '13 at 14:23
    
The passcode would be entered using the keypad built in to the drive. (If you don't know what I mean by this, look up the drives I mentioned.) And yes, I've looked into Android's built-in encryption, but (a) it is not without drawbacks (see, e.g. security.stackexchange.com/q/10529; v.gd/6hOcmd), (b) it does not work on all phones, even those that have ICS+ ROMs available from the manufacturers (e.g. some Xperia models), and (c) there are other potential use cases for which being able to boot a phone/tablet from a USB mass storage device would be desirable. –  sampablokuper Mar 11 '13 at 15:35
    
To be quite frank, that is not achievable, for a start there is no such smartphone bootloader that will simply , from high level perspective, "pause" until a passcode is entered! What you're asking for is above and beyond this forum and requires specialized, if not, niche arena of custom bootloaders to achieve this! For a start - the generic bootloader, lk (its in AOSP under bootable/bootloader) is adopted as de-facto by Qualcomm for their chipsets which is used by the likes of Sony, LG, Motorola, to name but a few... just saying, the question is not constructive! –  t0mm13b Mar 11 '13 at 16:38
2  
In short - there is zero way of doing that, you seem to be forgetting that the emphasis on my comments in relation to the bootloader and the fact that smartphones do not have BIOS's either.... just saying. –  t0mm13b Mar 13 '13 at 0:37

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