1 files changed, 11 insertions, 2 deletions

diff --git a/rsnix/src/memory.rs b/rsnix/src/memory.rs
index 2a32da8..0bac169 100644
--- a/rsnix/src/memory.rs
+++ b/rsnix/src/memory.rs
@@ -1200,6 +1200,13 @@ pub fn alloc_pages(count: usize) -> Option<usize> {
 /// chipsets is hostile to PCI masters and shows up as a controller
 /// "System Error" (SE) halt.  Page-aligned like `alloc_pages`.
 pub fn alloc_pages_above(count: usize, min_addr: usize) -> Option<usize> {
+    alloc_pages_range(count, min_addr, usize::MAX)
+}
+
+/// Allocate `count` contiguous physical pages inside `[min_addr, max_addr)`.
+/// The upper bound matters for 32-bit DMA engines (EHCI): a buffer above
+/// 4 GiB is unreachable even though the kernel can address it.
+pub fn alloc_pages_range(count: usize, min_addr: usize, max_addr: usize) -> Option<usize> {
     if count == 0 {
         return None;
     }
@@ -1209,13 +1216,15 @@ pub fn alloc_pages_above(count: usize, min_addr: usize) -> Option<usize> {
         }
         let span = SPAN_PAGES;
         let min_page = min_addr.div_ceil(PAGE_SIZE).max(BASE_PAGE);
-        if min_page >= BASE_PAGE + span {
+        let max_page = (max_addr / PAGE_SIZE).min(BASE_PAGE + span);
+        if min_page >= max_page {
             return None;
         }
         let start_idx = min_page - BASE_PAGE;
+        let end_idx = max_page - BASE_PAGE;
         let mut run_start = 0usize;
         let mut run_len = 0usize;
-        for idx in start_idx..span {
+        for idx in start_idx..end_idx {
             let free = (*BITMAP.add(idx / 8) >> (idx % 8)) & 1 == 0;
             if free {
                 if run_len == 0 {