[U-Boot] Proposal for an interface to pass a DT to the kernel on MIPS

[Gabor Juhos <juhosg@openwrt.org>]

Hi Everyone!

I'm working on adding device tree support for the ath79 platform. I have
experimental patches for several things already however my main problem
currently is how the DT should be passed to the kernel.

There were a thread [1] about that on the lmo list in 2011 already, but there
were no clear statements about the correct method. I'm using a simple approach
during my tests to resolve this problem.

The bootloader puts a magic value to the 'a0' register and the address of the
FDT blob is stored in the 'a1' register. I have created the attached patch for
U-Boot and that that implements this method. It is against the 'testing' branch
of the MIPS U-Boot Custodian Tree [2].

I would like to hear others' opinion about it, before I would invest too much
time into an unusable solution. :)

Regards,
Gabor

1. http://www.linux-mips.org/archives/linux-mips/2011-06/msg00059.html
2. git://git.denx.de/u-boot-mips.git

3. bootlog with the patched U-Boot:

> U-Boot> setenv bootargs 'console=ttyATH0,115200 init=/etc/preinit'
> U-Boot> setenv serverip 192.168.1.254; setenv ipaddr 192.168.1.1
> U-Boot> tftp 0xa0800000 tl-wr703n.dtb
> dup 1 speed 100
> Using eth0 device
> TFTP from server 192.168.1.254; our IP address is 192.168.1.1
> Filename 'tl-wr703n.dtb'.
> Load address: 0xa0800000
> Loading: #
> done
> Bytes transferred = 1957 (7a5 hex)
> U-Boot> tftp 0xa0900000 openwrt-ath79-uImage-initramfs-lzma.bin
> Using eth0 device
> TFTP from server 192.168.1.254; our IP address is 192.168.1.1
> Filename 'openwrt-ath79-uImage-initramfs-lzma.bin'.
> Load address: 0xa0900000
> Loading: #################################################################
>          #################################################################
>          #################################################################
>          #################################################################
>          #################################################################
>          #################################################################
>          #################################################################
>          ###########################################################
> done
> Bytes transferred = 2627551 (2817df hex)
> U-Boot> bootm 0xa0900000 - 0xa0800000
> ## Booting kernel from Legacy Image at a0900000 ...
>    Image Name:   MIPS OpenWrt Linux-3.8-rc2
>    Created:      2013-01-07  19:54:05 UTC
>    Image Type:   MIPS Linux Kernel Image (lzma compressed)
>    Data Size:    2627487 Bytes = 2.5 MiB
>    Load Address: 80060000
>    Entry Point:  80060000
>    Verifying Checksum ... OK
> ## Flattened Device Tree blob at a0800000
>    Booting using the fdt blob at 0xa0800000
>    Uncompressing Kernel Image ... OK
>    Loading Device Tree to 81f72000, end 81f757a4 ... OK
> 
> Starting kernel ...
> 
> Linux version 3.8.0-rc2 (juhosg at mag2) (gcc version 4.6.4 20121106 (prerelease) (Linaro GCC 4.6-2012.11) ) #456 Mon Jan 7 20:53:57 CET 2013
> ath79: using Device Tree passed by bootloader
> bootconsole [early0] enabled
> CPU revision is: 00019374 (MIPS 24Kc)
> SoC: Atheros AR9330 rev 1
> Clocks: CPU:400.000MHz, DDR:400.000MHz, AHB:200.000MHz, Ref:25.000MHz
> Determined physical RAM map:
>  memory: 02000000 @ 80000000 (usable)
>  memory: 02000000 @ 00000000 (usable)
> Initrd not found or empty - disabling initrd
> Zone ranges:
>   Normal   [mem 0x00000000-0x1fffffff]
> Movable zone start for each node
> Early memory node ranges
>   node   0: [mem 0x00000000-0x01ffffff]
> Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
> Primary data cache 32kB, 4-way, VIPT, cache aliases, linesize 32 bytes
> Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 8128
> Kernel command line: console=ttyATH0,115200 init=/etc/preinit rootfstype=squashfs,yaffs,jffs2 noinitrd
> PID hash table entries: 128 (order: -3, 512 bytes)
> Dentry cache hash table entries: 4096 (order: 2, 16384 bytes)
> Inode-cache hash table entries: 2048 (order: 1, 8192 bytes)
> __ex_table already sorted, skipping sort
> Writing ErrCtl register=00000000
> Readback ErrCtl register=00000000
> Memory: 26784k/32768k available (2682k kernel code, 5984k reserved, 822k data, 1548k init, 0k highmem)
> SLUB: Genslabs=9, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
> NR_IRQS:51
> Calibrating delay loop... 265.42 BogoMIPS (lpj=1327104)
> pid_max: default: 32768 minimum: 301
> Mount-cache hash table entries: 512
> NET: Registered protocol family 16
> ATH79: using Flattened Device Tree
> bio: create slab <bio-0> at 0
> cfg80211: Calling CRDA to update world regulatory domain
> Switching to clocksource MIPS
> NET: Registered protocol family 2
> TCP established hash table entries: 512 (order: 0, 4096 bytes)
> TCP bind hash table entries: 512 (order: -1, 2048 bytes)
> TCP: Hash tables configured (established 512 bind 512)
> TCP: reno registered
> UDP hash table entries: 256 (order: 0, 4096 bytes)
> UDP-Lite hash table entries: 256 (order: 0, 4096 bytes)
> NET: Registered protocol family 1
> squashfs: version 4.0 (2009/01/31) Phillip Lougher
> jffs2: version 2.2. (NAND) (SUMMARY)  ?? 2001-2006 Red Hat, Inc.
> msgmni has been set to 52
> io scheduler noop registered
> io scheduler deadline registered (default)
> Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
> 18020000.uart: ttyATH0 at MMIO 0x18020000 (irq = 11) is a AR933X UART
> console [ttyATH0] enabled, bootconsole disabled
> console [ttyATH0] enabled, bootconsole disabled
> ieee80211 phy0: Atheros AR9330 Rev:0 mem=0xb8100000, irq=2
> TCP: cubic registered
> NET: Registered protocol family 17
> 8021q: 802.1Q VLAN Support v1.8
> Freeing unused kernel memory: 1548k freed
> input: buttons.3 as /devices/buttons.3/input/input0
> - preinit -
> Press the [f] key and hit [enter] to enter failsafe mode
> - regular preinit -
> - init -
> SCSI subsystem initialized
> usbcore: registered new interface driver usbfs
> usbcore: registered new interface driver hub
> usbcore: registered new device driver usb
> PPP generic driver version 2.4.2
> ip_tables: (C) 2000-2006 Netfilter Core Team
> NET: Registered protocol family 24
> ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
> nf_conntrack version 0.5.0 (442 buckets, 1768 max)
> ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver
> Initializing USB Mass Storage driver...
> usbcore: registered new interface driver usb-storage
> USB Mass Storage support registered.
> usbcore: registered new interface driver ums-alauda
> usbcore: registered new interface driver ums-cypress
> usbcore: registered new interface driver ums-datafab
> usbcore: registered new interface driver ums-freecom
> usbcore: registered new interface driver ums-isd200
> usbcore: registered new interface driver ums-jumpshot
> usbcore: registered new interface driver ums-karma
> usbcore: registered new interface driver ums-sddr09
> usbcore: registered new interface driver ums-sddr55
> usbcore: registered new interface driver ums-usbat
> 
> Please press Enter to activate this console.
> 
> 
> BusyBox v1.19.4 (2013-01-06 20:43:13 CET) built-in shell (ash)
> Enter 'help' for a list of built-in commands.
> 
>   _______                     ________        __
>  |       |.-----.-----.-----.|  |  |  |.----.|  |_
>  |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
>  |_______||   __|_____|__|__||________||__|  |____|
>           |__| W I R E L E S S   F R E E D O M
>  -----------------------------------------------------
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>  -----------------------------------------------------
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