hw/omap_gpmc: Add comment about FIFOTHRESHOLDSTATUS bit

Promote the remark about why we handle FIFOTHRESHOLDSTATUS the
way we do from the commit message of de8af7fe0 to a comment in
the code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>

hw/omap_gpmc.c

diff --git a/hw/omap_gpmc.c b/hw/omap_gpmc.c
index 02f0c52..e27b93c 100644
--- a/hw/omap_gpmc.c
+++ b/hw/omap_gpmc.c
@@ -569,6 +569,13 @@
     case 0x1ec:	/* GPMC_PREFETCH_CONTROL */
         return s->prefetch.startengine;
     case 0x1f0:	/* GPMC_PREFETCH_STATUS */
+        /* NB: The OMAP3 TRM is inconsistent about whether the GPMC
+         * FIFOTHRESHOLDSTATUS bit should be set when
+         * FIFOPOINTER > FIFOTHRESHOLD or when it is >= FIFOTHRESHOLD.
+         * Apparently the underlying functional spec from which the TRM was
+         * created states that the behaviour is ">=", and this also
+         * makes more conceptual sense.
+         */
         return (s->prefetch.fifopointer << 24) |
                 ((s->prefetch.fifopointer >=
                   ((s->prefetch.config1 >> 8) & 0x7f) ? 1 : 0) << 16) |