kernel.early-console is used to specify which output device the
kernel boot log should be written to. the first thing the kernel
does on boot after initialising the bootstrap processor is initialise
the early console, making it useful for debugging problems that
occur early in the boot process. this arg accepts a list of hard-coded
values for output devices, such as tty0 for the display or ttyS0
for the serial port. the exact values supported will depend on the
platform.
once all drivers are loaded, the kernel switches to the device specified
by kernel.console for output. unlike kernel.early-console, this arg
specifies the name of a tty device in /dev/tty. this means that, not
only are more devices supported (any device provided by a tty driver),
but the kernel can also get input from the user using this console too
(not used by the kernel itself, but will be used by the user to interact
with userspace programs, like the shell).
reading from block devices is done using the block cache (bcache).
This cache stores sectors from a block device in pages of memory
marked as 'cached', which will allow them to be reclaimed when
memory pressure is high (TODO).
while block device drivers implement callbacks allowing reading/writing
at block-granularity, the device subsystem uses the block cache to
implement reading/writing at byte-granularity in a driver-agnostic way.
block drivers can disable the block cache for their devices, but this
will require that any clients communicate with the devices at
block-granularity.
also added an offset parameter to device and object read/write functions/callbacks.
generic devices don't have a type (char, block, etc) and are created
by bus drivers to represent the device until a suitable driver is found.
generic devices are registered by the bus driver. once a more suitable
driver is found, that driver will re-register the device under itself.
this allows the object header to be placed anywhere within the larger
object structure.
the object system now also ensures that the object is zero-initialised
during allocation.
