Difference between revisions of "Bering-uClibc 5.x - Developer Guide - Appendices - Hardware-Specific Guides"
Davidmbrooke (Talk | contribs) m (Corrected title) |
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There does not seem to be any standard provision for loading an initramfs (initrd) file when booting the Raspberry Pi. | There does not seem to be any standard provision for loading an initramfs (initrd) file when booting the Raspberry Pi. | ||
That means we need to embed the [[Bering-uClibc 5.x]] initrd into the kernel using the <tt>CONFIG_INITRAMFS_SOURCE</tt> configuration variable. | That means we need to embed the [[Bering-uClibc 5.x]] initrd into the kernel using the <tt>CONFIG_INITRAMFS_SOURCE</tt> configuration variable. | ||
+ | |||
+ | [[User:Davidmbrooke|Davidmbrooke]] 10:50, 1 July 2012 (UTC): Actually that's not (or no longer) the case: <code class="filename">config.txt</code> can contain an entry like the following: | ||
+ | ramfsfile initrd.cpio.gz | ||
+ | as described at [http://elinux.org/RPi_config.txt#Boot_options the eLinux Wiki]. | ||
===Boot Disk Layout=== | ===Boot Disk Layout=== |
Revision as of 10:50, 1 July 2012
Hardware-Specific Guides | ||
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Contents
Introduction
This page provides guidelines for Bering-uClibc 5.x developers targeting specific hardware devices.
See also the equivalent page in the User Guide for hardware-specific information relevant to Users as well as Developers.
ARM CPU Architecture
Raspberry Pi Model B
Overview
The Raspberry Pi Model B has a Broadcom BCM2835 "system on a chip" integrated circuit containing an ARM1176JZF-S CPU which implements the ARMv6 CPU architecture standard.
- As described here, the ARM1176JZF-S is an ARM1176JZ-S with the addition of a Vector Floating-Point (VFP) coprocessor. This means it can execute floating-point arithmetic in hardware rather than (only) in software.
A PDF data sheet for the BCM2835 can be downloaded from here. The BCM2835 contains a variety of devices and many of these seem to be identified with the string BCM2708 rather than BCM2835, judging by the example boot log from an Alpha board here
Linux Kernel Source
Many of the device drivers for the on-board peripherals are not currently (July 2012) included in the mainstream Linux source tree. The Raspberry Pi foundation have forked the Linux 3.1.9 kernel source and added the necessary drivers and a specific bcm2708 "platform". See the Raspberry Pi Git Hub repository
Another Git repository has these modifications merged with the latest Linux 3.2.x kernel. See Chris Boot's page here and his "bootc" Git Hub repository
TODO: Need to decide how to include the necessary drivers within the Bering-uClibc 5.x build system.
Linux Kernel Configuration
TODO - Which configuration settings are required / desirable.
There does not seem to be any standard provision for loading an initramfs (initrd) file when booting the Raspberry Pi. That means we need to embed the Bering-uClibc 5.x initrd into the kernel using the CONFIG_INITRAMFS_SOURCE configuration variable.
Davidmbrooke 10:50, 1 July 2012 (UTC): Actually that's not (or no longer) the case: config.txt
can contain an entry like the following:
ramfsfile initrd.cpio.gz
as described at the eLinux Wiki.
Boot Disk Layout
The Raspberry Pi must boot from its SD card and some vital files must be present in the root directory of the boot partition.
The latest versions of these files can be downloaded from the raspberrypi/firmware page on GitHub.
The first disk partition on the SD card must be formatted as a VFAT filesystem. It works OK if the partition type is 0xb (W95 FAT32). This partition does not have to to be marked as "bootable".
The critical files are:
- A file called
start.elf
which contains the GPU binary driver - A file called
bootcode.bin
which contains the second stage bootloader - A file called
loader.bin
which contains the third stage bootloader - A file called
kernel.img
which contains the Linux kernel- This is a standard uncompressed Linux kernel
Image
file- A standard compressed
zImage
file has also been reported to work OK - need to confirm
- A standard compressed
- Old versions of the Raspberry Pi firmware demanded a special prefix to be added to the kernel file
- This is a standard uncompressed Linux kernel
Some additional files are optional but are referenced if present:
- A text file called
config.txt
- This is comprehensively documented at the eLinux Wiki which states:
The Raspberry Pi config.txt file is read by the GPU before the ARM core is initialised. It can be used to set various system configuration parameters.
- A text file called
cmdline.txt
containing the kernel command line
It seems customary to put the files relating to a particular Linux distribution on a second disk partition.
We probably therefore need a second VFAT disk partition for leaf.cfg
, all the .lrp
Package files etc.
Note that the Raspberry Pi references the first disk partition as /dev/mmcblk0p1
and the second disk partition as /dev/mmcblk0p2
.
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