diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index b586a83a9b4c7f12fa60c6c13135c05b60c5de91..eb41391e06e938a5a0607b90defab00f82fd9827 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -679,7 +679,15 @@ static void mmc_idle_cards(struct mmc_host *host)
 }
 
 /*
- * Apply power to the MMC stack.
+ * Apply power to the MMC stack.  This is a two-stage process.
+ * First, we enable power to the card without the clock running.
+ * We then wait a bit for the power to stabilise.  Finally,
+ * enable the bus drivers and clock to the card.
+ *
+ * We must _NOT_ enable the clock prior to power stablising.
+ *
+ * If a host does all the power sequencing itself, ignore the
+ * initial MMC_POWER_UP stage.
  */
 static void mmc_power_up(struct mmc_host *host)
 {