1 files changed, 20 insertions, 1 deletions

diff --git a/drivers/scsi/hpsa.c b/drivers/scsi/hpsa.c
index 410910762fc5..1133b5fda0e7 100644
--- a/drivers/scsi/hpsa.c
+++ b/drivers/scsi/hpsa.c
@@ -3826,7 +3826,26 @@ static __devinit void hpsa_enter_performant_mode(struct ctlr_info *h)
 {
         int i;
         unsigned long register_value;
-        int bft[8] = {5, 6, 8, 10, 12, 20, 28, 35}; /* for scatter/gathers */
+
+        /* This is a bit complicated.  There are 8 registers on
+         * the controller which we write to to tell it 8 different
+         * sizes of commands which there may be.  It's a way of
+         * reducing the DMA done to fetch each command.  Encoded into
+         * each command's tag are 3 bits which communicate to the controller
+         * which of the eight sizes that command fits within.  The size of
+         * each command depends on how many scatter gather entries there are.
+         * Each SG entry requires 16 bytes.  The eight registers are programmed
+         * with the number of 16-byte blocks a command of that size requires.
+         * The smallest command possible requires 5 such 16 byte blocks.
+         * the largest command possible requires MAXSGENTRIES + 4 16-byte
+         * blocks.  Note, this only extends to the SG entries contained
+         * within the command block, and does not extend to chained blocks
+         * of SG elements.   bft[] contains the eight values we write to
+         * the registers.  They are not evenly distributed, but have more
+         * sizes for small commands, and fewer sizes for larger commands.
+         */
+        int bft[8] = {5, 6, 8, 10, 12, 20, 28, MAXSGENTRIES + 4};
+        BUILD_BUG_ON(28 > MAXSGENTRIES + 4);
         /*  5 = 1 s/g entry or 4k
          *  6 = 2 s/g entry or 8k
          *  8 = 4 s/g entry or 16k