MDM Not Able To Load Linux Kernel Images Larger Than 32MB?

Question

Hi All,
I've been building linux initramfs images for use with a Kintex7 (KC705) running a MicroBlaze processor and have been running into an issue with loading the images using the MicroBlaze Debug Module. It seems the system won’t boot if the image is larger than 32 MB. Unfortunately, my use case requires all application code to be contained in the kernel image and I won't have local non-volatile to boot from.
Some key factors:
Petalinux 2014.4
FPGA image built using Vivado 2014.4 on Windows 7
1GB RAM on KC705
Loading images using TCL script on XMD on Windows 7
As an example, I built an image that would fit within the 32 byte "limit" and booted it with no issues:
Image Type: MicroBlaze Linux Kernel Image (uncompressed)
Data Size: 31035828 Bytes = 30308.43 kB = 29.60 MB
MicroBlaze Processor Configuration :
Version............................9.4
Optimization.......................Performance
Interconnect.......................AXI-LE
MMU Type...........................Full_MMU
No of PC Breakpoints...............1
No of Read Addr/Data Watchpoints...0
No of Write Addr/Data Watchpoints..0
Instruction Cache Support..........on
Instruction Cache Base Address.....0x80000000
Instruction Cache High Address.....0xbfffffff
Data Cache Support.................on
Data Cache Base Address............0x80000000
Data Cache High Address............0xbfffffff
Exceptions Support................on
FPU Support.......................off
Hard Divider Support...............on
Hard Multiplier Support............on - (Mul64)
Barrel Shifter Support.............on
MSR clr/set Instruction Support....on
Compare Instruction Support........on
PVR Supported......................on
PVR Configuration Type.............Full
Data Cache Write-back Support......off
Fault Tolerance Support............off
Stack Protection Support...........off
Connected to "mb" target. id = 0
Starting GDB server for "mb" target (id = 0) at TCP port no 1234
System Reset .... DONE
Downloading Program -- simpleImage.mb
section, .text: 0x80000000-0x802a2b27
section, .init.text: 0x80359000-0x803727a3
section, .init.ivt: 0x803749c0-0x803749e7
section, .note.gnu.build-id: 0x00000000-0x00000023
section, __fdt_blob: 0x802a2b28-0x802aab27
section, .rodata: 0x802ab000-0x8032085f
section, __ksymtab: 0x80320860-0x803254f7
section, __ksymtab_gpl: 0x803254f8-0x80327f17
section, __ksymtab_strings: 0x80327f18-0x803380f6
section, __param: 0x803380f8-0x80338417
section, __modver: 0x80338418-0x80338fff
section, __ex_table: 0x80339000-0x8033a59f
section, .sdata2: 0x8033a5a0-0x8033afff
section, .data: 0x8033b000-0x8035809f
section, .init.data: 0x803727a4-0x803749bf
section, .init.setup: 0x803749e8-0x80374cdb
section, .initcall.init: 0x80374cdc-0x80374fa3
section, .con_initcall.init: 0x80374fa4-0x80374fab
section, .init.ramfs: 0x80374fac-0x81d991b3
section, .bss: 0x81d9a000-0x81dabb7b
Then, I added a 10MB random file (created using dd from /dev/urandom) to the same rootfs and tried booting again unsuccessfully:
Image Type: MicroBlaze Linux Kernel Image (uncompressed)
Data Size: 41521588 Bytes = 40548.43 kB = 39.60 MB
MicroBlaze Processor Configuration :
Version............................9.4
Optimization.......................Performance
Interconnect.......................AXI-LE
MMU Type...........................Full_MMU
No of PC Breakpoints...............1
No of Read Addr/Data Watchpoints...0
No of Write Addr/Data Watchpoints..0
Instruction Cache Support..........on
Instruction Cache Base Address.....0x80000000
Instruction Cache High Address.....0xbfffffff
Data Cache Support.................on
Data Cache Base Address............0x80000000
Data Cache High Address............0xbfffffff
Exceptions Support................on
FPU Support.......................off
Hard Divider Support...............on
Hard Multiplier Support............on - (Mul64)
Barrel Shifter Support.............on
MSR clr/set Instruction Support....on
Compare Instruction Support........on
PVR Supported......................on
PVR Configuration Type.............Full
Data Cache Write-back Support......off
Fault Tolerance Support............off
Stack Protection Support...........off
Connected to "mb" target. id = 0
Starting GDB server for "mb" target (id = 0) at TCP port no 1234
System Reset .... DONE
Downloading Program -- simpleImage.mb
section, .text: 0x80000000-0x802a2b27
section, .init.text: 0x80359000-0x803727a3
section, .init.ivt: 0x803749c0-0x803749e7
section, .note.gnu.build-id: 0x00000000-0x00000023
section, __fdt_blob: 0x802a2b28-0x802aab27
section, .rodata: 0x802ab000-0x8032085f
section, __ksymtab: 0x80320860-0x803254f7
section, __ksymtab_gpl: 0x803254f8-0x80327f17
section, __ksymtab_strings: 0x80327f18-0x803380f6
section, __param: 0x803380f8-0x80338417
section, __modver: 0x80338418-0x80338fff
section, __ex_table: 0x80339000-0x8033a59f
section, .sdata2: 0x8033a5a0-0x8033afff
section, .data: 0x8033b000-0x8035809f
section, .init.data: 0x803727a4-0x803749bf
section, .init.setup: 0x803749e8-0x80374cdb
section, .initcall.init: 0x80374cdc-0x80374fa3
section, .con_initcall.init: 0x80374fa4-0x80374fab
section, .init.ramfs: 0x80374fac-0x827991b3
section, .bss: 0x8279a000-0x827abb7b
I've also tried the same test with a plain Petalinux image and none of my application code or other customizations to rule out a configuration issue. Additionally, I've tried updating Vivado to 2015.2 and rebuilding the FPGA image, also with no success.
FYI, here is the TCL script used to load the images:
fpga -f system_top.bit
connect mb mdm
after 2000
dow simpleImage.mb
con
disconnect 0
I've done a bunch of research on initramfs and haven't seen anything that points to a size limit (except as it relates to RAM size). Anyone have an idea what's going on?
Thanks!
-Nick

Answer 1

Some additional information:
When the system fails to boot, there is no error message displayed either on the XMD window or the linux console output. The console remains blank.
I have been able to add random files to the ramdisk that do not push the size over the 32MB boundary and the system successfully boots.