[U-Boot] [PATCH 7/7] qemu-x86: add documentaion for the fw_cfg interface

[Bin Meng]

Hi Miao,

nits: please use "x86: qemu" as the tag in the commit title.

On Mon, Dec 28, 2015 at 5:18 PM, Miao Yan <yanmiaobest at gmail.com> wrote:
> Document the usage of 'fw' command
>
> Signed-off-by: Miao Yan <yanmiaobest at gmail.com>
> ---
>  doc/README.x86 | 36 +++++++++++++++++++++++++++++++++---
>  1 file changed, 33 insertions(+), 3 deletions(-)
>
> diff --git a/doc/README.x86 b/doc/README.x86
> index 1271e5e..f39c157 100644
> --- a/doc/README.x86
> +++ b/doc/README.x86
> @@ -295,9 +295,39 @@ show QEMU's VGA console window. Note this will disable QEMU's serial output.
>  If you want to check both consoles, use '-serial stdio'.
>
>  Multicore is also supported by QEMU via '-smp n' where n is the number of cores
> -to instantiate. Currently the default U-Boot built for QEMU supports 2 cores.
> -In order to support more cores, you need add additional cpu nodes in the device
> -tree and change CONFIG_MAX_CPUS accordingly.
> +to instantiate. U-Boot uses fw_cfg interface provided by QEMU to detect certain
> +system information, such as cpu number, so 'n' can be any number allowed by
> +QEMU.

I've tested this. Looks when n > 47, U-Boot mp_init() will fail. I
guess we may need limit U-Boot maximum supported cpu number to some
reasonable number, say 32?

> +
> +The fw_cfg interface in QEMU also provides information about kernel data, initrd
> +,command-line arguments and more. U-Boot supports directly accessing these informtion
> +from fw_cfg interface, this saves the time of loading them from hard disk or
> +network again, through emulated devices. To use it , simply providing them in
> +QEMU command line:
> +
> +$ qemu-system-x86_64 -nographic -bios path/to/u-boot.rom -m 1024 -kernel /path/to/bzImage

Please use qemu-system-i386 for consistency with other instances
mentioned in the doc.

> +    -append 'root=/dev/sda1 console=ttyS0' -initrd /path/to/initrd -smp 8
> +
> +Note: -initrd and -smp are both optional
> +
> +Then start QEMU, in U-Boot command line use the following U-Boot command to setup kernel:
> +
> + => fw
> +fw - qemu firmware interface
> +
> +Usage:
> +fw <command>
> +    - list        : print firmware(s) currently loaded
> +    - cpus        : print online cpu number
> +    - load <addr> : load kernel (if any) to address <addr>
> +
> +=> fw load
> +loading kernel to address 01000000, initrd 015dd010
> +
> +Here the kernel (bzImage) is loaded to 01000000 and initrd is to 0x15dd010. Then, 'zboot'

Looks 01000000 is not kernel (bzImage) address. It is setup data
address. Can we enhance the 'fw load' command to output all available
loaded components address and size?

> +can be used to boot the kernel:
> +
> +=> zboot 01000000 - 015dd010

zboot has 4 parameters, the second parameter is kernel image size,
which can be zero. The last one is initrd image size, which should not
omitted.

My test shows the kernel does not recognize my initramfs.gz, kernel log below:

[    4.328688] Unpacking initramfs...
[    4.331688] Initramfs unpacking failed: junk in compressed archive

>
>  CPU Microcode
>  -------------
> --

Regards,
Bin