allocator_v4: Treat above 4G resources more natively

We currently have two competing mechanisms to limit the placement of
resources:

 1. the explicit `.limit` field of a resource, and
 2. the IORESOURCE_ABOVE_4G flag.

This makes the resource allocator unnecessarily complex. Ideally, we
would always reduce the `.limit` field if we want to "pin" a specific
resource below 4G. However, as that's not done across the tree yet,
we will use the _absence_ of the IORESOURCE_ABOVE_4G flag as a hint
to implicitly lower the `limit` of a resource. In this patch, this
is done inside the effective_limit() function that hides the flag
from the rest of the allocator.

To automatically place resources above 4G if their limit allows it,
we have to allocate from top down. Hence, we disable the prompt for
RESOURCE_ALLOCATION_TOP_DOWN if resources above 4G are requested.

One implication of the changes is that we act differently when a
cold-plugged device reports a prefetchable resource with 32-bit
limit. Before this change, we would fail to allocate the resource.
After this change, it forces everything on the same root port below
the 4G line.

A possible solution to get completely rid of the IORESOURCE_ABOVE_4G
flag would be rules to place resources of certain devices below 4G.
For instance, the primary VGA device and storage and HID devices
could be made available to a payload that can only address 32 bits.

For now, effective_limit() provides us enough abstraction as if the
`limit` would be the only variable to consider. With this, we get
rid of all the special handling of above 4G resources during phase 2
of the allocator. Which saves us about 20% of the code :D

Change-Id: I4c7fcd1f5146f6cc287bd3aa5582da55bc5d6955
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65413
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/device/Kconfig b/src/device/Kconfig
index af9beb32f5..8d10c17f4e 100644
--- a/src/device/Kconfig
+++ b/src/device/Kconfig
@@ -934,7 +934,7 @@ config RESOURCE_ALLOCATOR_V4
 	  above 4G boundary as well.
 
 config RESOURCE_ALLOCATION_TOP_DOWN
-	bool "Allocate resources from top down"
+	bool "Allocate resources from top down" if !PCIEXP_HOTPLUG_PREFETCH_MEM_ABOVE_4G
 	default y
 	depends on RESOURCE_ALLOCATOR_V4