Mtdoops cannot save the full panic log - openwrt

Question

I add a mtd partition (512kb) to save panic log by mtdoops on an arm based embedded devices(kernel v4.4).
I trigger the panic using "echo c > /proc/sysrq-trigger".
After the device reboot observe the kernel panic logs in the Oops partition by "strings", but the panic log seems to be truncated(limit in 2kb?).
How to change the limit of mtdoops?
# strings /dev/mtd22
]<4>[ 99.089308] r3 : 00000001 r2 : 00000000 r1 : ef717364 r0 : 00000063
<4>[ 99.095905] Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user
<4>[ 99.102416] Control: 10c5787d Table: 6652c06a DAC: 00000055
<0>[ 99.109618] Process ash (pid: 234, stack limit = 0xee672210)
<0>[ 99.115350] Stack: (0xee673ec8 to 0xee674000)
<0>[ 99.121077] 3ec0: 00000002 ee80b200 00000001 00000000 c0301d24 c058a8dc
<0>[ 99.125336] 3ee0: c058a8a0 c0421b98 00000002 c0421b28 ee673f88 b675fbf0 c0301d24 c03df2ec
<0>[ 99.133495] 3f00: 00000000 000006c0 00000000 00000000 00000000 ee673f18 be461b48 0000000b
<0>[ 99.141656] 3f20: eeab0218 0000000a eeab0240 00080000 ee673f60 ee7531f8 00000001 e4b70780
<0>[ 99.149816] 3f40: ef046998 c0355734 00000002 00000002 e4b70780 ee673f88 b675fbf0 c03df9c0
<0>[ 99.157975] 3f60: e4b70780 b675fbf0 00000002 e4b70780 e4b70780 b675fbf0 00000002 c0301d24
<0>[ 99.166134] 3f80: ee672000 c03e009c 00000000 00000000 00000002 00000000 00000000 00000000
<0>[ 99.174294] 3fa0: 00000004 c0301b80 00000000 00000000 00000001 b675fbf0 00000002 00000000
<0>[ 99.182455] 3fc0: 00000000 00000000 00000000 00000004 00000002 be4619cc 00000020 7f087b44
<0>[ 99.190615] 3fe0: be461958 be461944 b67b66ac b67b5b78 60000010 00000001 00000000 00000000
<4>[ 99.198784] [<c058a480>] (sysrq_handle_crash) from [<c058a7dc>] (__handle_sysrq+0x88/0x124)
<4>[ 99.206940] [<c058a7dc>] (__handle_sysrq) from [<c058a8dc>] (write_sysrq_trigger+0x3c/0x4c)
<4>[ 99.215102] [<c058a8dc>] (write_sysrq_trigger) from [<c0421b98>] (proc_reg_write+0x70/0x84)
<4>[ 99.223435] [<c0421b98>] (proc_reg_write) from [<c03df2ec>] (__vfs_write+0x1c/0xd0)
<4>[ 99.231763] [<c03df2ec>] (__vfs_write) from [<c03df9c0>] (vfs_write+0xa8/0x130)
<4>[ 99.239401] [<c03df9c0>] (vfs_write) from [<c03e009c>] (SyS_write+0x40/0x80)
<4>[ 99.246695] [<c03e009c>] (SyS_write) from [<c0301b80>] (ret_fast_syscall+0x0/0x34)
<0>[ 99.253986] Code: e3a03001 e5823000 f57ff04e e3a02000 (e5c23000)
<4>[ 99.268628]
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# strings /dev/mtd22 | wc -c
2057
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