NTB: MFV 58b88920: Document HW errata

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

Authored by:	Jon Mason
Obtained from:	Linux (Dual BSD/GPL driver)
Sponsored by:	EMC / Isilon Storage Division

diff --git a/sys/dev/ntb/ntb_hw/ntb_hw.c b/sys/dev/ntb/ntb_hw/ntb_hw.c
index 7f7fa5f82a8bf11111838ccb5c52d41d16df20f7..b45e8258f3c0ea7b35b5787e62923909407edcaa 100644 (file)
--- a/sys/dev/ntb/ntb_hw/ntb_hw.c
+++ b/sys/dev/ntb/ntb_hw/ntb_hw.c
@@ -764,6 +764,11 @@ ntb_setup_xeon(struct ntb_softc *ntb)
         * which may hang the system.  To workaround this use the second memory
         * window to access the interrupt and scratch pad registers on the
         * remote system.
+        *
+        * There is another HW errata on the limit registers -- they can only
+        * be written when the base register is (?)4GB aligned and < 32-bit.
+        * This should already be the case based on the driver defaults, but
+        * write the limit registers first just in case.
         */
        if (HAS_FEATURE(NTB_REGS_THRU_MW))
                /*