[PATCH 3/3] bios: mtrr: redefine the end point of memory ranges

[Ricardo Neri]

Currently the end of an entry partition is calculated as

   entry->end = entry->start + entry->size

However, this leads to memory _ranges_ that are off by one byte at the end.
For instance, if we had a range starting at 0x10 with a size of 0x10, the
range should be [0x10, 0x1f]. Thus, entry->start should be 0x10 and
entry->end should be 0x1f.

Furthermore, Linux determines the size of the MTRR range by looking at the
contents of the MTRR_PHYSMASKn registers. These registers are set in
such a manner that a mask, not a size, determines whether a memory location
belongs to the MTRR memory range [1].

The entry->start and entry->end values are used for two purposes:

  1) Display the memory _ranges_ covered by each of the MTRR registers, and
  2) Determine whether a particular memory range is covered by any of the
     MTRR registers.

For 1), redefining the value of entry->end a) represents better what the
MTRR registers actually mean and b) make the printout of such ranges
similar to what the /proc/iomem file presents, which is used as input for
the MTRR tests.

For 2), the cache_type function needs to be updated for this new definition
of entry->end. Simple algebra shows how the inequalities need to be
adjusted. Let A, A' and B integers where A' = A - 1. If we have

                                  A > B
                             A' + 1 > B

then we can drop the second member of the expression at the left by using a
greater-than sign. This can be done as involved quantities are integers. We
can simply do

                                 A' >= B

[1]. http://www.intel.com/content/www/us/en/processors
     /architectures-software-developer-manuals.html

Signed-off-by: Ricardo Neri <ricardo.neri-calderon at linux.intel.com>
---
 src/bios/mtrr/mtrr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/bios/mtrr/mtrr.c b/src/bios/mtrr/mtrr.c
index 8176803..120b943 100644
--- a/src/bios/mtrr/mtrr.c
+++ b/src/bios/mtrr/mtrr.c
@@ -144,7 +144,7 @@ static int get_mtrrs(void)
 		if (ptr2 && (*ptr2 == 'k'))
 			entry->size *= 1024;
 
-		entry->end = entry->start + entry->size;
+		entry->end = entry->start + entry->size - 1;
 
 		if (strstr(line, "write-back"))
 			entry->type = WRITE_BACK;
@@ -174,7 +174,7 @@ static int cache_types(uint64_t start, uint64_t end)
 	fwts_list_foreach(item, mtrr_list) {
 		entry = fwts_list_data(struct mtrr_entry*, item);
 
-		if (entry->end > start && entry->start <= end)
+		if (entry->end >= start && entry->start <= end)
 			type |= entry->type;
 	}
 
@@ -186,7 +186,7 @@ restart:
 	fwts_list_foreach(item, mtrr_list) {
 		entry = fwts_list_data(struct mtrr_entry*, item);
 
-		if (entry->end > end && entry->start < end) {
+		if (entry->end >= end && entry->start < end) {
 			end = entry->start;
 			if (end < start)
 				end = start;
-- 
2.8.1