dm: Allow drivers to be marked 'before relocation'

Driver model currently only operates after relocation is complete. In this
state U-Boot typically has a small amount of memory available. In adding
support for driver model prior to relocation we must try to use as little
memory as possible.

In addition, on some machines the memory has not be inited and/or the CPU
is not running at full speed or the data cache is off. These can reduce
execution performance, so the less initialisation that is done before
relocation the better.

An immediately-obvious improvement is to only initialise drivers which are
actually going to be used before relocation. On many boards the only such
driver is a serial UART, so this provides a very large potential benefit.

Allow drivers to mark themselves as 'pre-reloc' which means that they will
be initialised prior to relocation. This can be done either with a driver
flag or with a 'dm,pre-reloc' device tree property.

To support this, the various dm scanning function now take a 'pre_reloc_only'
parameter which indicates that only drivers marked pre-reloc should be
bound.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/driver-model/README.txt

@@ -95,26 +95,24 @@ are provided in test/dm. To run them, try:
 You should see something like this:
 
     <...U-Boot banner...>
-    Running 12 driver model tests
+    Running 14 driver model tests
     Test: dm_test_autobind
     Test: dm_test_autoprobe
     Test: dm_test_children
     Test: dm_test_fdt
+    Test: dm_test_fdt_pre_reloc
     Test: dm_test_gpio
     sandbox_gpio: sb_gpio_get_value: error: offset 4 not reserved
     Test: dm_test_leak
-    Warning: Please add '#define DEBUG' to the top of common/dlmalloc.c
-    Warning: Please add '#define DEBUG' to the top of common/dlmalloc.c
     Test: dm_test_lifecycle
     Test: dm_test_operations
     Test: dm_test_ordering
     Test: dm_test_platdata
+    Test: dm_test_pre_reloc
     Test: dm_test_remove
     Test: dm_test_uclass
     Failures: 0
 
-(You can add '#define DEBUG' as suggested to check for memory leaks)
-
 
 What is going on?
 -----------------
@@ -538,26 +536,35 @@ dealing with this might not be worth it.
 - Implemented a GPIO system, trying to keep it simple
 
 
+Pre-Relocation Support
+----------------------
+
+For pre-relocation we simply call the driver model init function. Only
+drivers marked with DM_FLAG_PRE_RELOC or the device tree
+'u-boot,dm-pre-reloc' flag are initialised prior to relocation. This helps
+to reduce the driver model overhead.
+
+Then post relocation we throw that away and re-init driver model again.
+For drivers which require some sort of continuity between pre- and
+post-relocation devices, we can provide access to the pre-relocation
+device pointers, but this is not currently implemented (the root device
+pointer is saved but not made available through the driver model API).
+
+
 Things to punt for later
 ------------------------
 
 - SPL support - this will have to be present before many drivers can be
 converted, but it seems like we can add it once we are happy with the
 core implementation.
-- Pre-relocation support - similar story
 
-That is not to say that no thinking has gone into these - in fact there
+That is not to say that no thinking has gone into this - in fact there
 is quite a lot there. However, getting these right is non-trivial and
 there is a high cost associated with going down the wrong path.
 
 For SPL, it may be possible to fit in a simplified driver model with only
 bind and probe methods, to reduce size.
 
-For pre-relocation we can simply call the driver model init function. Then
-post relocation we throw that away and re-init driver model again. For drivers
-which require some sort of continuity between pre- and post-relocation
-devices, we can provide access to the pre-relocation device pointers.
-
 Uclasses are statically numbered at compile time. It would be possible to
 change this to dynamic numbering, but then we would require some sort of
 lookup service, perhaps searching by name. This is slightly less efficient