How to repropduce the unaligned store problem

    qemu-system-mips -nodefaults -M malta -m 256 \
                     -nographic -serial stdio -monitor null \
                     -bios barebox-flash-image \
                     -net user -net nic,model=rtl8139

    ...

    barebox:/ dhcp

    Ooops, address error on store!

    $ 0   : 00000000 00000000 01010600 697f2a2e
    $ 4   : a0850000 00000000 0000001c a040c1b8
    $ 8   : 00000000 00000002 00000002 00000000
    $12   : 00000000 00000040 00000100 00000001
    $16   : a040bba0 a0850000 a0850000 a0850000
    $20   : 00000000 00000075 00000076 a040ba20
    $24   : 00000002 a080f210
    $28   : 00000000 a03ffce0 fffffffd a0833b8c
    Hi    : 000154f8
    Lo    : 20000000
    epc   : a0833b84
    ra    : a0833b8c
    Status: 00000002
    Cause : 80000414
    Config: 80008482

    ### ERROR ### Please RESET the board ###

The unaligned store instruction is located in the bootp_request() from net/dhcp.c:

a0833b50 <bootp_request>:
..
a0833b7c:       3c020101        lui     v0,0x101
a0833b80:       24420600        addiu   v0,v0,1536 /* 0x1010600 -> v0 */
a0833b84:       0c20024a        jal     a0800928 <get_time_ns>
a0833b88:       ae02002a        sw      v0,42(s0) /* store 0x1010600 to addr s0 + 42 */

This assembler code is generated by mips-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
from this:

    bp = (struct bootp *)payload;
    bp->bp_op = OP_BOOTREQUEST; /* 0x01 */
    bp->bp_htype = HWT_ETHER; /* 0x01 */
    bp->bp_hlen = HWL_ETHER; /* 0x06 */
    bp->bp_hops = 0;

Compiler replaces four 'store byte' instruction by one 'store 32-bit word'
instruction. Alas sometimes this leads to unaligned store situation.

Making struct bootp __packed prevents this optimization and fixes the problem.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Tested-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>