NTB: Document HW errata

Add a comment describing the necessary ordering of modifications to the
NTB Limit and Base registers.

Signed-off-by: Jon Mason <jon.mason@intel.com>

diff --git a/drivers/ntb/ntb_hw.c b/drivers/ntb/ntb_hw.c
index fa97948..391c377 100644 (file)
--- a/drivers/ntb/ntb_hw.c
+++ b/drivers/ntb/ntb_hw.c
@@ -689,6 +689,12 @@ static int ntb_xeon_setup(struct ntb_device *ndev)
                         */
                        writeq(ndev->mw[1].bar_sz + 0x1000, ndev->reg_base +
                               SNB_PBAR4LMT_OFFSET);
+                       /* HW errata on the Limit registers.  They can only be
+                        * written when the base register is 4GB aligned and
+                        * < 32bit.  This should already be the case based on the
+                        * driver defaults, but write the Limit registers first
+                        * just in case.
+                        */
                } else {
                        ndev->limits.max_mw = SNB_MAX_MW;
 
@@ -707,6 +713,12 @@ static int ntb_xeon_setup(struct ntb_device *ndev)
                         * something silly
                         */
                        writeq(0, ndev->reg_base + SNB_PBAR4LMT_OFFSET);
+                       /* HW errata on the Limit registers.  They can only be
+                        * written when the base register is 4GB aligned and
+                        * < 32bit.  This should already be the case based on the
+                        * driver defaults, but write the Limit registers first
+                        * just in case.
+                        */
                }
 
                /* The Xeon errata workaround requires setting SBAR Base