The reserved_mem array is used to store data for the different
reserved memory regions defined in the DT of a device. The array
stores information such as region name, node reference, start-address,
and size of the different reserved memory regions.
The array is currently statically allocated with a size of
MAX_RESERVED_REGIONS(64). This means that any system that specifies a
number of reserved memory regions greater than MAX_RESERVED_REGIONS(64)
will not have enough space to store the information for all the regions.
This can be fixed by making the reserved_mem array a dynamically sized
array which is allocated using memblock_alloc() based on the exact
number of reserved memory regions defined in the DT.
On architectures such as arm64, memblock allocated memory is not
writable until after the page tables have been setup.
This is an issue because the current implementation initializes the
reserved memory regions and stores their information in the array before
the page tables are setup. Hence, dynamically allocating the
reserved_mem array and attempting to write information to it at this
point will fail.
Therefore, the allocation of the reserved_mem array will need to be done
after the page tables have been setup, which means that the reserved
memory regions will also need to wait until after the page tables have
been setup to be stored in the array.
When processing the reserved memory regions defined in the DT, these
regions are marked as reserved by calling memblock_reserve(base, size).
Where: base = base address of the reserved region.
size = the size of the reserved memory region.
Depending on if that region is defined using the "no-map" property,
memblock_mark_nomap(base, size) is also called.
The "no-map" property is used to indicate to the operating system that a
mapping of the specified region must NOT be created. This also means
that no access (including speculative accesses) is allowed on this
region of memory except when it is coming from the device driver that
this region of memory is being reserved for.[1]
Therefore, it is important to call memblock_reserve() and
memblock_mark_nomap() on all the reserved memory regions before the
system sets up the page tables so that the system does not unknowingly
include any of the no-map reserved memory regions in the memory map.
There are two ways to define how/where a reserved memory region is
placed in memory:
i) Statically-placed reserved memory regions
i.e. regions defined with a set start address and size using the
"reg" property in the DT.
ii) Dynamically-placed reserved memory regions.
i.e. regions defined by specifying a range of addresses where they can
be placed in memory using the "alloc_ranges" and "size" properties
in the DT.
The dynamically-placed reserved memory regions get assigned a start
address only at runtime. And this needs to be done before the page
tables are setup so that memblock_reserve() and memblock_mark_nomap()
can be called on the allocated region as explained above.
Since the dynamically allocated reserved_mem array can only be
available after the page tables have been setup, the information for
the dynamically-placed reserved memory regions needs to be stored
somewhere temporarily until the reserved_mem array is available.
Therefore, this series makes use of a temporary static array to store
the information of the dynamically-placed reserved memory regions until
the reserved_mem array is allocated.
Once the reserved_mem array is available, the information is copied over
from the temporary array into the reserved_mem array, and the memory for
the temporary array is freed back to the system.
The information for the statically-placed reserved memory regions does
not need to be stored in a temporary array because their starting
address is already stored in the devicetree.
Once the reserved_mem array is allocated, the information for the
statically-placed reserved memory regions is added to the array.
Note:
Because of the use of a temporary array to store the information of the
dynamically-placed reserved memory regions, there still exists a
limitation of 64 for this particular kind of reserved memory regions.
