Note: it is possible to have a bootmeth that uses a partition or a whole device
directly, but it is more common to use a filesystem.
+Note that some bootmeths are 'global', meaning that they select the bootdev
+themselves. Examples include VBE and EFI boot manager. In this case, they
+provide a `read_bootflow()` method which checks whatever bootdevs it likes, then
+returns the bootflow, if found. Some of these bootmeths may be very slow, if
+they scan a lot of devices.
+
Boot process
------------
which scans for available bootflows, optionally listing each find it finds (-l)
and trying to boot it (-b).
+When global bootmeths are available, these are typically checked before the
+above bootdev scanning.
+
Controlling ordering
--------------------
work. This includes global information about the state of standard boot. See
`struct bootstd_priv` for this structure, accessed with `bootstd_get_priv()`.
-Within the devicetree, if you add bootmeth devices or a system bootdev, they
-should be children of the bootstd device. See `arch/sandbox/dts/test.dts` for
-an example of this.
-
-
-The system bootdev
-------------------
-
-Some bootmeths don't operate on individual bootdevs, but on the whole system.
-For example, the EFI boot manager does its own device scanning and does not
-make use of the bootdev devices. Such bootmeths can make use of the system
-bootdev, typically considered last, after everything else has been tried.
+Within the devicetree, if you add bootmeth devices, they should be children of
+the bootstd device. See `arch/sandbox/dts/test.dts` for an example of this.
.. _`Automatic Devices`:
It is possible to define all the required devices in the devicetree manually,
but it is not necessary. The bootstd uclass includes a `dm_scan_other()`
function which creates the bootstd device if not found. If no bootmeth devices
-are found at all, it creates one for each available bootmeth driver as well as a
-system bootdev.
+are found at all, it creates one for each available bootmeth driver.
If your devicetree has any bootmeth device it must have all of them that you
-want to use, as well as the system bootdev if needed, since no bootmeth devices
-will be created automatically in that case.
+want to use, since no bootmeth devices will be created automatically in that
+case.
Using devicetree
- distro boot from a disk (syslinux)
- distro boot from a network (PXE)
- EFI boot using bootefi
+ - VBE
- EFI boot using boot manager
are going to be used, with `num_devs` set to the number of bootdevs and
`cur_dev` starting at 0.
-Next, the ordering of bootdevs is determined, by `bootmeth_setup_iter_order()`.
+Next, the ordering of bootmeths is determined, by `bootmeth_setup_iter_order()`.
By default the ordering is again by sequence number, i.e. the `/aliases` node,
or failing that the order in the devicetree. But the `bootmeth order` command
or `bootmeths` environment variable can be used to set up an ordering. If that
has been done, the ordering is in `struct bootstd_priv`, so that ordering is
simply copied into the iterator. Either way, the `method_order` array it set up,
-along with `num_methods`. Then `cur_method` is set to 0.
+along with `num_methods`.
+
+Note that global bootmeths are always put at the end of the ordering. If any are
+present, `cur_method` is set to the first one, so that global bootmeths are done
+first. Once all have been used, these bootmeths are dropped from the iteration.
+When there are no global bootmeths, `cur_method` is set to 0.
At this point the iterator is ready to use, with the first bootdev and bootmeth
-selected. All the other fields are 0. This means that the current partition is
-0, which is taken to mean the whole device, since partition numbers start at 1.
-It also means that `max_part` is 0, i.e. the maximum partition number we know
+selected. Most of the other fields are 0. This means that the current partition
+is 0, which is taken to mean the whole device, since partition numbers start at
+1. It also means that `max_part` is 0, i.e. the maximum partition number we know
about is 0, meaning that, as far as we know, there is no partition table on this
bootdev.
incomplete bootflows, but normally an error results in moving onto the next
iteration.
+Note that `bootflow_check()` handles global bootmeths explicitly, but calling
+`bootmeth_get_bootflow()` on each one. The `doing_global` flag indicates when
+the iterator is in that state.
+
The `bootflow_scan_next()` function handles moving onto the next iteration and
checking it. In fact it sits in a loop doing that repeatedly until it finds
something it wants to return.
0 0 2
0 1 0
0 1 1
- 0 1 1
+ 0 1 2
1 0 0
1 0 1
+ ...
======== ======= =======
The maximum value for `method` is `num_methods - 1` so when it exceeds that, it
works its way through all possibilities, moving forward one each time it is
called.
+Note that global bootmeths introduce a subtlety into the above description.
+When `doing_global` is true, the iteration takes place only among the bootmeths,
+i.e. the last column above. The global bootmeths are at the end of the list.
+Assuming that they are entries 3 and 4 in the list, the iteration then looks
+like this:
+
+ ======== ======= ======= =======================================
+ bootdev part method notes
+ ======== ======= ======= =======================================
+ . . 3 doing_global = true, method_count = 5
+ . . 4
+ 0 0 0 doing_global = false, method_count = 3
+ 0 0 1
+ 0 0 2
+ 0 1 0
+ 0 1 1
+ 0 1 2
+ 1 0 0
+ 1 0 1
+ ...
+ ======== ======= ======= =======================================
+
+The changeover of the value of `doing_global` from true to false is handled in
+`iter_incr()` as well.
+
There is no expectation that iteration will actually finish. Quite often a
valid bootflow is found early on. With `bootflow scan -b`, that causes the
bootflow to be immediately booted. Assuming it is successful, the iteration never
passes the iterator to the bootdev method, so that function knows what we are
talking about. At first, the bootflow is set up in the state `BOOTFLOWST_BASE`,
with just the `method` and `dev` intiialised. But the bootdev may fill in more,
-e.g. updating the state, depending on what it finds.
+e.g. updating the state, depending on what it finds. For global bootmeths the
+`bootmeth_get_bootflow()` function is called instead of
+`bootdev_get_bootflow()`.
-Based on what the bootdev responds with, `bootflow_check()` either
+Based on what the bootdev or bootmeth responds with, `bootflow_check()` either
returns a valid bootflow, or a partial one with an error. A partial bootflow
is one that has some fields set up, but did not reach the `BOOTFLOWST_READY`
state. As noted before, if the `BOOTFLOWF_ALL` iterator flag is set, then all
bootflows are returned, even partial ones. This can help with debugging.
So at this point you can see that total control over whether a bootflow can
-be generated from a particular iteration, or not, rests with the bootdev.
-Each one can adopt its own approach.
+be generated from a particular iteration, or not, rests with the bootdev (or
+global bootmeth). Each one can adopt its own approach.
Going down a level, what does the bootdev do in its `get_bootflow()` method?
Let us consider the MMC bootdev. In that case the call to
projects / platform / kernel / u-boot.git / commitdiff