IB/mthca: Fix max_sge value returned by query_device

The mthca driver returns the maximum number of scatter/gather entries
returned by the firmware as the max_sge value when device properties
are queried.  However, the firmware also reports a limit on the
maximum descriptor size allowed, and because mthca takes into account
the worst case send request overhead when checking whether to allow a
QP to be created, the largest number of scatter/gather entries that
can be used with mthca may be limited by the maximum descriptor size
rather than just by the actual s/g entry limit.

This means that applications cannot actually create QPs with
max_send_sge equal to the limit returned by ib_query_device().  Fix
this by checking if the maximum descriptor size imposes a lower limit
and if so returning that lower limit.
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mthca/mthca_main.c

@@ -45,6 +45,7 @@
 #include "mthca_cmd.h"
 #include "mthca_profile.h"
 #include "mthca_memfree.h"
+#include "mthca_wqe.h"
 
 MODULE_AUTHOR("Roland Dreier");
 MODULE_DESCRIPTION("Mellanox InfiniBand HCA low-level driver");
@@ -200,7 +201,18 @@ static int mthca_dev_lim(struct mthca_dev *mdev, struct mthca_dev_lim *dev_lim)
 	mdev->limits.gid_table_len  	= dev_lim->max_gids;
 	mdev->limits.pkey_table_len 	= dev_lim->max_pkeys;
 	mdev->limits.local_ca_ack_delay = dev_lim->local_ca_ack_delay;
-	mdev->limits.max_sg             = dev_lim->max_sg;
+	/*
+	 * Need to allow for worst case send WQE overhead and check
+	 * whether max_desc_sz imposes a lower limit than max_sg; UD
+	 * send has the biggest overhead.
+	 */
+	mdev->limits.max_sg		= min_t(int, dev_lim->max_sg,
+					      (dev_lim->max_desc_sz -
+					       sizeof (struct mthca_next_seg) -
+					       (mthca_is_memfree(mdev) ?
+						sizeof (struct mthca_arbel_ud_seg) :
+						sizeof (struct mthca_tavor_ud_seg))) /
+						sizeof (struct mthca_data_seg));
 	mdev->limits.max_wqes           = dev_lim->max_qp_sz;
 	mdev->limits.max_qp_init_rdma   = dev_lim->max_requester_per_qp;
 	mdev->limits.reserved_qps       = dev_lim->reserved_qps;