Setup the stack for an init task

Bootstrap an init task which holds at least the initial stack that is to
be used when setting up the registers (such as `sp` and `tp`).

In order to guarantee that the stack and the rest of the registers are
set up correctly, this commit also provides a raw implementation of a
`printk` function.

Moreover, this commit also adds an argument that must be passed to
`start_kernel`, which is the pointer to the embedded `fdt` blob. This
argument will be used by later work so to fetch, at least, the base
address for the initial ram disk. This was also used to test that the
stack was working as expected.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/README.md b/README.md
index 8ce947f..a7f8799 100644
--- a/README.md
+++ b/README.md
@@ -104,6 +104,19 @@ $ make gdb GDB_EXTRA_FLAGS="-tui"
 
 And now you have started a GDB session with a nice TUI interface.
 
+## Requirements
+
+We do not want to support a myriad of different scenarios, but we want to keep
+things simple. Hence, here there are some limitations/requirements:
+
+- You need a recent enough OpenSBI running on your firmware. I have tested this
+  on a QEMU which has OpenSBI v1.5, but any firmware that implements a Runtime
+  SBI version of 2.0 should be fine.
+- You are supposed to pass an `initrd` always. This kernel will not try to
+  magically come up with a made up file system or try to fetch something from an
+  existing one. An `initrd` is already provided for you on the default `make`
+  target, and that's what you are supposed to be passing to the kernel.
+
 ## Special thanks to
 
 SUSE for organizing [Hack Week 24](https://hackweek.opensuse.org/24/projects).