[RFC PATCH 125/162] drm/i915/lmem: Limit block size to 4G

[Matthew Auld]

From: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota at intel.com>

when allocating pages to lmem object of size 4G or greater
we allocate memory blocks from buddy system. In this scenario
buddy sytem can allocate blocks that can have size >= 4G and
these blocks require >32b to represent block size with these
blocks we run into an issue with sg list construction because
sg->length field is only 32b wide.

Hence limit the max allowed block size to less than 4G.

Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
Cc: Matthew Auld <matthew.auld at intel.com>
Cc: CQ Tang <cq.tang at intel.com>
Signed-off-by: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota at intel.com>
---
 drivers/gpu/drm/i915/intel_memory_region.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_memory_region.c b/drivers/gpu/drm/i915/intel_memory_region.c
index 554fdd7735a8..371cd88ff6d8 100644
--- a/drivers/gpu/drm/i915/intel_memory_region.c
+++ b/drivers/gpu/drm/i915/intel_memory_region.c
@@ -101,6 +101,7 @@ __intel_memory_region_get_pages_buddy(struct intel_memory_region *mem,
 				      struct list_head *blocks)
 {
 	unsigned int min_order = 0;
+	unsigned int max_order;
 	unsigned long n_pages;
 
 	GEM_BUG_ON(!IS_ALIGNED(size, mem->mm.chunk_size));
@@ -121,6 +122,16 @@ __intel_memory_region_get_pages_buddy(struct intel_memory_region *mem,
 
 	n_pages = size >> ilog2(mem->mm.chunk_size);
 
+	/*
+	 * When allocating pages for an lmem object of size > 4G
+	 * the memory blocks allocated from buddy system could be
+	 * from sizes greater than 4G requiring > 32b to represent
+	 * block size. But those blocks cannot be used in sg list
+	 * construction(in caller) as sg->length is only 32b wide.
+	 * Hence limiting the block size to 4G.
+	 */
+	max_order = (ilog2(SZ_4G) - 1) - ilog2(mem->mm.chunk_size);
+
 	mutex_lock(&mem->mm_lock);
 
 	do {
@@ -128,7 +139,7 @@ __intel_memory_region_get_pages_buddy(struct intel_memory_region *mem,
 		unsigned int order;
 		bool retry = true;
 retry:
-		order = fls(n_pages) - 1;
+		order = min_t(u32, (fls(n_pages) - 1), max_order);
 		GEM_BUG_ON(order > mem->mm.max_order);
 		GEM_BUG_ON(order < min_order);
 
-- 
2.26.2