RDMA/umem: Use simpler logic for ib_umem_find_best_pgsz()

The calculation in rdma_find_pg_bit() is fairly complicated, and the
function is never called anywhere else. Inline a simpler version into
ib_umem_find_best_pgsz()

Link: https://lore.kernel.org/r/3-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/umem.c

@@ -39,6 +39,7 @@
 #include <linux/export.h>
 #include <linux/slab.h>
 #include <linux/pagemap.h>
+#include <linux/count_zeros.h>
 #include <rdma/ib_umem_odp.h>
 
 #include "uverbs.h"
@@ -146,7 +147,6 @@ unsigned long ib_umem_find_best_pgsz(struct ib_umem *umem,
 				     unsigned long virt)
 {
 	struct scatterlist *sg;
-	unsigned int best_pg_bit;
 	unsigned long va, pgoff;
 	dma_addr_t mask;
 	int i;
@@ -186,9 +186,14 @@ unsigned long ib_umem_find_best_pgsz(struct ib_umem *umem,
 			mask |= va;
 		pgoff = 0;
 	}
-	best_pg_bit = rdma_find_pg_bit(mask, pgsz_bitmap);
 
-	return BIT_ULL(best_pg_bit);
+	/* The mask accumulates 1's in each position where the VA and physical
+	 * address differ, thus the length of trailing 0 is the largest page
+	 * size that can pass the VA through to the physical.
+	 */
+	if (mask)
+		pgsz_bitmap &= GENMASK(count_trailing_zeros(mask), 0);
+	return rounddown_pow_of_two(pgsz_bitmap);
 }
 EXPORT_SYMBOL(ib_umem_find_best_pgsz);
 

include/rdma/ib_verbs.h

@@ -3335,30 +3335,6 @@ static inline bool rdma_cap_read_inv(struct ib_device *dev, u32 port_num)
 	return rdma_protocol_iwarp(dev, port_num);
 }
 
-/**
- * rdma_find_pg_bit - Find page bit given address and HW supported page sizes
- *
- * @addr: address
- * @pgsz_bitmap: bitmap of HW supported page sizes
- */
-static inline unsigned int rdma_find_pg_bit(unsigned long addr,
-					    unsigned long pgsz_bitmap)
-{
-	unsigned long align;
-	unsigned long pgsz;
-
-	align = addr & -addr;
-
-	/* Find page bit such that addr is aligned to the highest supported
-	 * HW page size
-	 */
-	pgsz = pgsz_bitmap & ~(-align << 1);
-	if (!pgsz)
-		return __ffs(pgsz_bitmap);
-
-	return __fls(pgsz);
-}
-
 /**
  * rdma_core_cap_opa_port - Return whether the RDMA Port is OPA or not.
  * @device: Device