[igt-dev] [PATCH i-g-t 1/1] lib/intel_allocator: Limit last page if not reserved

[Jonathan Cavitt]

We currently limit the last page of the allocator space to avoid a hang
on some platforms when a batch would be pinned in the last page.
However, in the future, we may already be reserving the last page of the
ppgtt in kernel space, so we would not need to limit the last page
because gem_aperture_size would not report its existence in
__intel_allocator_open_full when this is the case.

Check the gtt_size and only limit the last page if it's not already
reserved by kernel space.  This is generalized to if the reported page
count is even.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt at intel.com>
---
 lib/intel_allocator.c | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/lib/intel_allocator.c b/lib/intel_allocator.c
index f0a9b7fb53..fc7836dcee 100644
--- a/lib/intel_allocator.c
+++ b/lib/intel_allocator.c
@@ -53,12 +53,6 @@ static inline const char *reqstr(enum reqtype request_type)
 #define bind_debug(...) {}
 #endif
 
-/*
- * We limit allocator space to avoid hang when batch would be
- * pinned in the last page.
- */
-#define RESERVED 4096
-
 struct allocator {
 	int fd;
 	uint32_t ctx;
@@ -941,11 +935,21 @@ static uint64_t __intel_allocator_open_full(int fd, uint32_t ctx,
 		if (!end) {
 			igt_assert_f(can_report_gtt_size(fd), "Invalid fd\n");
 			gtt_size = gem_aperture_size(fd);
-			if (!gem_uses_full_ppgtt(fd))
+			if (!gem_uses_full_ppgtt(fd)) {
 				gtt_size /= 2;
-			else
-				gtt_size -= RESERVED;
-
+			} else {
+				/*
+				 * We limit allocator space to avoid hang when batch would be
+				 * pinned in the last page.
+				 *
+				 * The last page is reserved when the gtt_size is aligned
+				 * with an odd number of pages.
+				 */
+				uint64_t align = gtt_size % (SZ_4K * 2);
+				align += SZ_4K;
+				align %= (SZ_4K * 2);
+				gtt_size -= align;
+			}
 			req.open.end = gtt_size;
 		}
 
-- 
2.25.1